The antiproton, p , (pronounced p-bar) is the antiparticle of the proton. Antiprotons are stable, but they are typically short-lived, since any collision with a proton will cause both particles to be annihilated in a burst of energy.
The existence of the antiproton with electric charge of −1 e, opposite to the electric charge of +1 e of the proton, was predicted by Paul Dirac in his 1933 Nobel Prize lecture. Dirac received the Nobel Prize for his 1928 publication of his Dirac equation that predicted the existence of positive and negative solutions to Einstein's energy equation () and the existence of the positron, the antimatter analog of the electron, with opposite charge and spin.
In terms of valence quarks, an antiproton consists of two up antiquarks and one down antiquark ( u u d ).
The properties of the antiproton that have been measured all match the
corresponding properties of the proton, with the exception that the
antiproton has electric charge and magnetic moment that are the
opposites of those in the proton, which is to be expected from the
antimatter equivalent of a proton. The questions of how matter is
different from antimatter, and the relevance of antimatter in explaining
how our universe survived the Big Bang, remain open problems—open, in part, due to the relative scarcity of antimatter in today's universe.
Occurrence in nature
Antiprotons have been detected in cosmic rays
beginning in 1979, first by balloon-borne experiments and more recently
by satellite-based detectors. The standard picture for their presence
in cosmic rays is that they are produced in collisions of cosmic ray
protons with atomic nuclei in the interstellar medium, via the reaction, where A represents a nucleus:
p + A → p + p + p + A
The secondary antiprotons ( p ) then propagate through the galaxy, confined by the galactic magnetic fields.
Their energy spectrum is modified by collisions with other atoms in the
interstellar medium, and antiprotons can also be lost by "leaking out"
of the galaxy.
The antiproton cosmic ray energy spectrum is now measured reliably and is consistent with this standard picture of antiproton production by cosmic ray collisions.
These experimental measurements set upper limits on the number of
antiprotons that could be produced in exotic ways, such as from
annihilation of supersymmetricdark matter particles in the galaxy or from the Hawking radiation caused by the evaporation of primordial black holes.
This also provides a lower limit on the antiproton lifetime of about
1–10 million years. Since the galactic storage time of antiprotons is
about 10 million years, an intrinsic decay lifetime would modify the
galactic residence time and distort the spectrum of cosmic ray
antiprotons. This is significantly more stringent than the best
laboratory measurements of the antiproton lifetime:
APEX collaboration at Fermilab: 50000 years for p → μ− + anything
APEX collaboration at Fermilab: 300000 years for p → e− + γ
The magnitude of properties of the antiproton are predicted by CPT symmetry
to be exactly related to those of the proton. In particular, CPT
symmetry predicts the mass and lifetime of the antiproton to be the same
as those of the proton, and the electric charge and magnetic moment of
the antiproton to be opposite in sign and equal in magnitude to those of
the proton. CPT symmetry is a basic consequence of quantum field theory and no violations of it have ever been detected.
List of recent cosmic ray detection experiments
BESS: balloon-borne experiment, flown in 1993, 1995, 1997, 2000, 2002, 2004 (Polar-I) and 2007 (Polar-II).
CAPRICE: balloon-borne experiment, flown in 1994 and 1998.
PAMELA:
satellite experiment to detect cosmic rays and antimatter from space,
launched June 2006. Recent report discovered 28 antiprotons in the South Atlantic Anomaly.
Antiprotons were routinely produced at Fermilab for collider physics operations in the Tevatron,
where they were collided with protons. The use of antiprotons allows
for a higher average energy of collisions between quarks and antiquarks
than would be possible in proton–proton collisions. This is because the
valence quarks in the proton, and the valence antiquarks in the
antiproton, tend to carry the largest fraction of the proton or antiproton's momentum.
Formation of antiprotons requires energy equivalent to a temperature of 10 trillion K (1013 K), and this does not tend to happen naturally. However, at CERN, protons are accelerated in the Proton Synchrotron to an energy of 26 GeV and then smashed into an iridium rod. The protons bounce off the iridium nuclei with enough energy for matter to be created. A range of particles and antiparticles are formed, and the antiprotons are separated off using magnets in vacuum.
Measurements
In July 2011, the ASACUSA experiment at CERN determined the mass of the antiproton to be 1836.1526736(23) times that of the electron. This is the same as the mass of a proton, within the level of certainty of the experiment.
In October 2017, scientists working on the BASE experiment at CERN reported a measurement of the antiproton magnetic moment to a precision of 1.5 parts per billion.
It is consistent with the most precise measurement of the proton
magnetic moment (also made by BASE in 2014), which supports the
hypothesis of CPT symmetry. This measurement represents the first time
that a property of antimatter is known more precisely than the
equivalent property in matter.
In January 2022, by comparing the charge-to-mass ratios between
antiproton and negatively charged hydrogen ion, the BASE experiment has
determined the antiproton's charge-to-mass ratio is identical to the
proton's, down to 16 parts per trillion.
Possible applications
Antiprotons
have been shown within laboratory experiments to have the potential to
treat certain cancers, in a similar method currently used for ion (proton) therapy.
The primary difference between antiproton therapy and proton therapy is
that following ion energy deposition the antiproton annihilates,
depositing additional energy in the cancerous region.
Isotopic labeling (or isotopic labelling) is a technique used to track the passage of an isotope (an atom with a detectable variation in neutron count) through chemical reaction, metabolic pathway, or a biological cell. The reactant
is 'labeled' by replacing one or more specific atoms with their
isotopes. The reactant is then allowed to undergo the reaction. The
position of the isotopes in the products is measured to determine the sequence the isotopic atom followed in the reaction or the cell's metabolic pathway. The nuclides used in isotopic labeling may be stable nuclides or radionuclides. In the latter case, the labeling is called radiolabeling.
An example of the use of isotopic labeling is the study of phenol (C6H5OH) in water by replacing common hydrogen (protium) with deuterium (deuterium labeling). Upon adding phenol to deuterated water (water containing D2O in addition to the usual H2O), the substitution of deuterium for the hydrogen is observed in phenol's hydroxyl group (resulting in C6H5OD),
indicating that phenol readily undergoes hydrogen-exchange reactions
with water. Only the hydroxyl group is affected, indicating that the
other 5 hydrogen atoms do not participate in the exchange reactions.
Isotopic tracer
A carbon-13 label was used to determine the mechanism in the 1,2- to 1,3-didehydrobenzene conversion of the phenyl substituted aryne precursor 1 to acenaphthylene.
An isotopic tracer, (also "isotopic marker" or "isotopic label"), is used in chemistry and biochemistry to help understand chemical reactions and interactions. In this technique, one or more of the atoms of the molecule of interest is substituted for an atom of the same chemical element, but of a different isotope (like a radioactive isotope used in radioactive tracing).
Because the labeled atom has the same number of protons, it will behave
in almost exactly the same way as its unlabeled counterpart and, with
few exceptions, will not interfere with the reaction under
investigation. The difference in the number of neutrons, however, means that it can be detected separately from the other atoms of the same element.
Nuclear magnetic resonance (NMR) and mass spectrometry
(MS) are used to investigate the mechanisms of chemical reactions. NMR
and MS detects isotopic differences, which allows information about the
position of the labeled atoms in the products' structure to be
determined. With information on the positioning of the isotopic atoms in
the products, the reaction pathway the initial metabolites utilize to
convert into the products can be determined. Radioactive isotopes can be
tested using the autoradiographs of gels in gel electrophoresis. The radiation emitted by compounds containing the radioactive isotopes darkens a piece of photographic film, recording the position of the labeled compounds relative to one another in the gel.
Isotope tracers are commonly used in the form of isotope ratios.
By studying the ratio between two isotopes of the same element, we avoid
effects involving the overall abundance of the element, which usually
swamp the much smaller variations in isotopic abundances. Isotopic
tracers are some of the most important tools in geology
because they can be used to understand complex mixing processes in
earth systems. Further discussion of the application of isotopic tracers
in geology is covered under the heading of isotope geochemistry.
Isotopic tracers are usually subdivided into two categories: stable isotope tracers and radiogenic
isotope tracers. Stable isotope tracers involve only non-radiogenic
isotopes and usually are mass-dependent. In theory, any element with two
stable isotopes can be used as an isotopic tracer. However, the most
commonly used stable isotope tracers involve relatively light isotopes,
which readily undergo fractionation in natural systems. See also isotopic signature. A radiogenic isotope tracer involves an isotope produced by radioactive decay,
which is usually in a ratio with a non-radiogenic isotope (whose
abundance in the earth does not vary due to radioactive decay).
Stable isotope labeling
Isotopic
tracing through reactions in the pentose phosphate pathway. The blue
circles indicate a labeled carbon atom, while white circles are an
unlabeled carbon atom.
Stable isotope labeling involves the use of non-radioactive isotopes
that can act as a tracers used to model several chemical and
biochemical systems. The chosen isotope can act as a label on that
compound that can be identified through nuclear magnetic resonance (NMR) and mass spectrometry (MS). Some of the most common stable isotopes are 2H, 13C, and 15N, which can further be produced into NMR solvents, amino acids, nucleic acids, lipids, common metabolites and cell growth media.
The compounds produced using stable isotopes are either specified by
the percentage of labeled isotopes (i.e. 30% uniformly labeled 13C glucose contains a mixture that is 30% labeled with 13 carbon isotope and 70% naturally labeled carbon) or by the specifically labeled carbon positions on the compound (i.e. 1-13C glucose which is labeled at the first carbon position of glucose).
A network of reactions adopted from the glycolysis pathway and the pentose phosphate pathway
is shown in which the labeled carbon isotope rearranges to different
carbon positions throughout the network of reactions. The network starts
with fructose 6-phosphate (F6P), which has 6 carbon atoms with a label 13C at carbon position 1 and 2. 1,2-13C F6P becomes two glyceraldehyde 3-phosphate (G3P), one 2,3-13C T3P and one unlabeled T3P. The 2,3-13C T3P can now be reacted with sedoheptulose 7-phosphate (S7P) to form an unlabeled erythrose 4-phosphate(E4P) and a 5,6-13C F6P. The unlabeled T3P will react with the S7P to synthesize unlabeled products.
The figure demonstrates the use of stable isotope labeling to discover
the carbon atom rearrangement through reactions using position specific
labeled compounds.
Metabolic flux analysis using stable isotope labeling
Determining
the percent of isotope labeling throughout a reaction. If a 50% labeled
and 50% unlabeled metabolite is split in the manner shown, the expected
percent of each outcome can be found. The blue circles indicate a
labeled atom, while a white circle indicates an unlabeled atom.
Metabolic flux analysis (MFA) using stable isotope labeling is an important tool for explaining the flux of certain elements through the metabolic pathways and reactions within a cell.
An isotopic label is fed to the cell, then the cell is allowed to grow
utilizing the labeled feed. For stationary metabolic flux analysis the
cell must reach a steady state
(the isotopes entering and leaving the cell remain constant with time)
or a quasi-steady state (steady state is reached for a given period of
time). The isotope pattern of the output metabolite is determined. The output isotope pattern provides valuable information, which can be used to find the magnitude of flux, rate of conversion from reactants to products, through each reaction.
The figure demonstrates the ability to use different labels to
determine the flux through a certain reaction. Assume the original
metabolite, a three carbon compound, has the ability to either split
into a two carbon metabolite and one carbon metabolite in one reaction
then recombine or remain a three carbon metabolite. If the reaction is
provided with two isotopes of the metabolite in equal proportion, one
completely labeled (blue circles), commonly known as uniformly labeled,
and one completely unlabeled (white circles). The pathway down the left
side of the diagram does not display any change in the metabolites,
while the right side shows the split and recombination. As shown, if the
metabolite only takes the pathway down the left side, it remains in a
50–50 ratio of uniformly labeled to unlabeled metabolite. If the
metabolite only takes the right side new labeling patterns can occur,
all in equal proportion. Other proportions can occur depending on how
much of the original metabolite follows the left side of the pathway
versus the right side of the pathway. Here the proportions are shown for
a situation in which half of the metabolites take the left side and
half the right, but other proportions can occur. These patterns of labeled atoms and unlabeled atoms in one compound represent isotopomers.
By measuring the isotopomer distribution of the differently labeled
metabolites, the flux through each reaction can be determined.
MFA combines the data harvested from isotope labeling with the stoichiometry of each reaction, constraints, and an optimization procedure resolve a flux map. The irreversible reactions provide the thermodynamic constraints needed to find the fluxes. A matrix is constructed that contains the stoichiometry of the reactions. The intracellular fluxes are estimated by using an iterative method
in which simulated fluxes are plugged into the stoichiometric model.
The simulated fluxes are displayed in a flux map, which shows the rate
of reactants being converted to products for each reaction. In most flux maps, the thicker the arrow, the larger the flux value of the reaction.
Isotope labeling measuring techniques
Any technique in measuring the difference between isotopomers can be used. The two primary methods, nuclear magnetic resonance (NMR) and mass spectrometry (MS), have been developed for measuring mass isotopomers in stable isotope labeling.
Proton NMR was the first technique used for 13C-labeling experiments. Using this method, each single protonated carbon position inside a particular metabolite pool can be observed separately from the other positions.
This allows the percentage of isotopomers labeled at that specific
position to be known. The limit to proton NMR is that if there are n carbon atoms in a metabolite, there can only be at most n
different positional enrichment values, which is only a small fraction
of the total isotopomer information. Although the use of proton NMR
labeling is limiting, pure proton NMR experiments are much easier to
evaluate than experiments with more isotopomer information.
In addition to Proton NMR, using 13C NMR
techniques will allow a more detailed view of the distribution of the
isotopomers. A labeled carbon atom will produce different hyperfine
splitting signals depending on the labeling state of its direct
neighbors in the molecule.
A singlet peak emerges if the neighboring carbon atoms are not labeled.
A doublet peak emerges if only one neighboring carbon atom is labeled.
The size of the doublet split depends on the functional group of the
neighboring carbon atom. If two neighboring carbon atoms are labeled, a
doublet of doublets may degenerate into a triplet if the doublet
splittings are equal.
The drawbacks to using NMR techniques for metabolic flux analysis
purposes is that it is different from other NMR applications because it
is a rather specialized discipline. An NMR spectrometer may not be
directly available for all research teams. The optimization of NMR
measurement parameters and proper analysis of peak structures requires a
skilled NMR specialist. Certain metabolites also may require
specialized measurement procedures to obtain additional isotopomer data.
In addition, specially adapted software tools are needed to determine
the precise quantity of peak areas as well as identifying the
decomposition of entangled singlet, doublet, and triplet peaks.
As opposed to nuclear magnetic resonance, mass spectrometry (MS)
is another method that is more applicable and sensitive to metabolic
flux analysis experiments. MS instruments are available in different
variants. Different from two-dimensional nuclear magnetic resonance (2D-NMR), the MS instruments work directly with hydrolysate.
In gas chromatography-mass spectrometry (GC-MS),
the MS is coupled to a gas chromatograph to separate the compounds of
the hydrolysate. The compounds eluting from the GC column are then
ionized and simultaneously fragmented. The benefit in using GC-MS is
that not only are the mass isotopomers of the molecular ion measured but
also the mass isotopomer spectrum of several fragments, which
significantly increases the measured information.
In liquid chromatography-mass spectrometry (LC-MS), the GC is replaced with a liquid chromatograph. The main difference is that chemical derivatization is not necessary. Applications of LC-MS to MFA, however, are rare.
In each case, MS instruments divide a particular isotopomer
distribution by its molecular weight. All isotopomers of a particular
metabolite that contain the same number of labeled carbon atoms are
collected in one peak signal. Because every isotopomer contributes to
exactly one peak in the MS spectrum, the percentage value can then be
calculated for each peak, yielding the mass isotopomer fraction.
For a metabolite with n carbon atoms, n+1 measurements are produced.
After normalization, exactly n informative mass isotopomer quantities
remain.
The drawback to using MS techniques is that for gas
chromatography, the sample must be prepared by chemical derivatization
in order to obtain molecules with charge. There are numerous compounds
used to derivatize samples. N,N-Dimethylformamide dimethyl acetal
(DMFDMA) and N-(tert-butyldimethylsilyl)-N-methyltrifluoroacetamide (MTBSTFA) are two examples of compounds that have been used to derivatize amino acids.
In addition, strong isotope effects observed affect the retention
time of differently labeled isotopomers in the GC column. Overloading
of the GC column also must be prevented.
Lastly, the natural abundance of other atoms than carbon also
leads to a disturbance in the mass isotopomer spectrum. For example,
each oxygen atom in the molecule might also be present as a 17O isotope and as a 18O
isotope. A more significant impact of the natural abundance of isotopes
is the effect of silicon with a natural abundance of the isotopes 29Si and 30Si. Si is used in derivatizing agents for MS techniques.
Radioisotopic labeling is a technique for tracking the passage of a
sample of substance through a system. The substance is "labeled" by
including radionuclides in its chemical composition. When these decay, their presence can be determined by detecting the radiation emitted by them. Radioisotopic labeling is a special case of isotopic labeling.
For these purposes, a particularly useful type of radioactive decay is positron emission. When a positron collides with an electron, it releases two high-energy photons
traveling in diametrically opposite directions. If the positron is
produced within a solid object, it is likely to do this before traveling
more than a millimeter. If both of these photons can be detected, the location of the decay event can be determined very precisely.
Strictly speaking, radioisotopic labeling includes only cases
where radioactivity is artificially introduced by experimenters, but
some natural phenomena allow similar analysis to be performed. In
particular, radiometric dating uses a closely related principle.
Applications
Applications in human mineral nutrition research
The use of stable isotope tracers to study mineral nutrition and metabolism in humans was first reported in the 1960s.
While radioisotopes had been used in human nutrition research for
several decades prior, stable isotopes presented a safer option,
especially in subjects for which there is elevated concern about
radiation exposure, e.g. pregnant and lactating women and children.
Other advantages offered by stable isotopes include the ability to study
elements having no suitable radioisotopes and to study long-term tracer
behavior.
Thus the use of stable isotopes became commonplace with the increasing
availability of isotopically enriched materials and inorganic mass
spectrometers. The use of stable isotopes instead of radioisotopes does
have several drawbacks: larger quantities of tracer are required, having
the potential of perturbing the naturally existing mineral; analytical
sample preparation is more complex and mass spectrometry instrumentation more costly; the presence of tracer in whole bodies or particular tissues cannot be measured externally. Nonetheless, the advantages have prevailed making stable isotopes the standard in human studies.
Most of the minerals that are essential for human health and of
particular interest to nutrition researchers have stable isotopes, some
well-suited as biological tracers because of their low natural
abundance. Iron, zinc, calcium, copper, magnesium, selenium and molybdenum
are among the essential minerals having stable isotopes to which
isotope tracer methods have been applied. Iron, zinc and calcium in
particular have been extensively studied.
Aspects of mineral nutrition/metabolism that are studied include absorption (from the gastrointestinal tract
into the body), distribution, storage, excretion and the kinetics of
these processes. Isotope tracers are administered to subjects orally
(with or without food, or with a mineral supplement) and/or
intravenously. Isotope enrichment is then measured in blood plasma,
erythrocytes, urine and/or feces. Enrichment has also been measured in breast milk
and intestinal contents. Tracer experiment design sometimes differs
between minerals due to differences in their metabolism. For example,
iron absorption is usually determined from incorporation of tracer in
erythrocytes whereas zinc or calcium absorption is measured from tracer
appearance in plasma, urine or feces.
The administration of multiple isotope tracers in a single study is
common, permitting the use of more reliable measurement methods and
simultaneous investigations of multiple aspects of metabolism.
The measurement of mineral absorption from the diet, often conceived of as bioavailability,
is the most common application of isotope tracer methods to nutrition
research. Among the purposes of such studies are the investigations of
how absorption is influenced by type of food (e.g. plant vs animal
source, breast milk vs formula), other components of the diet (e.g. phytate), disease and metabolic disorders (e.g. environmental enteric dysfunction), the reproductive cycle, quantity of mineral in diet, chronic mineral deficiency,
subject age and homeostatic mechanisms. When results from such studies
are available for a mineral, they may serve as a basis for estimations
of the human physiological and dietary requirements of the mineral.
When tracer is administered with food for the purpose of
observing mineral absorption and metabolism, it may be in the form of an
intrinsic or extrinsic label.
An intrinsic label is isotope that has been introduced into the food
during its production, thus enriching the natural mineral content of the
food, whereas extrinsic labeling refers to the addition of tracer
isotope to the food during the study. Because it is a very
time-consuming and expensive approach, intrinsic labeling is not
routinely used. Studies comparing measurements of absorption using
intrinsic and extrinsic labeling of various foods have generally
demonstrated good agreement between the two labeling methods, supporting
the hypothesis that extrinsic and natural minerals are handled
similarly in the human gastrointestinal tract.
Enrichment is quantified from the measurement of isotope ratios,
the ratio of the tracer isotope to a reference isotope, by mass
spectrometry. Multiple definitions and calculations of enrichment have
been adopted by different researchers.
Calculations of enrichment become more complex when multiple tracers
are used simultaneously. Because enriched isotope preparations are never
isotopically pure, i.e. they contain all the element's isotopes in
unnatural abundances, calculations of enrichment of multiple isotope
tracers must account for the perturbation of each isotope ratio by the
presence of the other tracers.
Due to the prevalence of mineral deficiencies and their critical
impact on human health and well-being in resource-poor countries, the International Atomic Energy Agency
has recently published detailed and comprehensive descriptions of
stable isotope methods to facilitate the dissemination of this knowledge
to researchers beyond western academic centers.
Applications in proteomics
In proteomics, the study of the full set of proteins expressed by a genome, identifying diseasesbiomarkers can involve the usage of stable isotope labeling by amino acids in cell culture (SILAC), that provides isotopic labeled forms of amino acid used to estimate protein levels.
In protein recombinant, manipulated proteins are produced in large
quantities and isotope labeling is a tool to test for relevant proteins.
The method used to be about selectively enrich nuclei with 13C or 15N or deplete 1H from them. The recombinant would be expressed in E.coli with media containing 15N-ammonium chloride as a source of nitrogen. The resulting 15N
labeled proteins are then purified by immobilized metal affinity and
their percentage estimated. In order to increase the yield of labeled
proteins and cut down the cost of isotope labeled media, an alternative
procedure primarily increases the cell mass using unlabeled media before
introducing it in a minimal amount of labeled media. Another application of isotope labeling would be in measuring DNA synthesis, that is cell proliferation in vitro. Uses H3-thymidine labeling to compare pattern of synthesis (or sequence) in cells.
Applications for ecosystem process analysis
Isotopic
tracers are used to examine processes in natural systems, especially
terrestrial and aquatic environments. In soil science 15N tracers are used extensively to study nitrogen cycling, whereas 13C and 14C, stable and radioisotopes of carbon respectively, are used for studying turnover of organic compounds and fixation of CO2 by autotrophs. For example, Marsh et al. (2005) used dual labeled (15N- and 14C) urea to demonstrate utilization of the compound by ammonia oxidizers as both an energy source (ammonia oxidation) and carbon source (chemoautotrophic carbon fixation). Deuterated water is also used for tracing the fate and ages of water in a tree or in an ecosystem.
Applications for oceanography
Tracers are also used extensively in oceanography
to study a wide array of processes. The isotopes used are typically
naturally occurring with well-established sources and rates of formation
and decay. However, anthropogenic isotopes may also be used with great
success. The researchers measure the isotopic ratios at different
locations and times to infer information about the physical processes of
the ocean.
Particle transport
The
ocean is an extensive network of particle transport. Thorium isotopes
can help researchers decipher the vertical and horizontal movement of
matter. 234Th has a constant, well-defined production rate in
the ocean and a half-life of 24 days. This naturally occurring isotope
has been shown to vary linearly with depth. Therefore, any changes in
this linear pattern can be attributed to the transport of 234Th
on particles. For example, low isotopic ratios in surface water with
very high values a few meters down would indicate a vertical flux in the
downward direction. Furthermore, the thorium isotope may be traced
within a specific depth to decipher the lateral transport of particles.
Circulation
Circulation within local systems, such as bays, estuaries, and groundwater, may be examined with radium isotopes. 223Ra
has a half-life of 11 days and can occur naturally at specific
locations in rivers and groundwater sources. The isotopic ratio of
radium will then decrease as the water from the source river enters a
bay or estuary. By measuring the amount of 223Ra at a number of different locations, a circulation pattern can be deciphered. This same exact process can also be used to study the movement and discharge of groundwater.
Various isotopes of lead can be used to study circulation on a
global scale. Different oceans (i.e. the Atlantic, Pacific, Indian,
etc.) have different isotopic signatures. This results from differences
in isotopic ratios of sediments and rocks within the different oceans.
Because the different isotopes of lead have half-lives of 50–200 years,
there is not enough time for the isotopic ratios to be homogenized
throughout the whole ocean. Therefore, precise analysis of Pb isotopic
ratios can be used to study the circulation of the different oceans.
Tectonic processes and climate change
Isotopes
with extremely long half-lives and their decay products can be used to
study multi-million year processes, such as tectonics and extreme
climate change. For example, in rubidium–strontium dating, the isotopic ratio of strontium (87Sr/86Sr)
can be analyzed within ice cores to examine changes over the earth's
lifetime. Differences in this ratio within the ice core would indicate
significant alterations in the earth's geochemistry.
Isotopes related to nuclear weapons
The
aforementioned processes can be measured using naturally occurring
isotopes. Nevertheless, anthropogenic isotopes are also extremely useful
for oceanographic measurements. Nuclear weapons tests released a
plethora of uncommon isotopes into the world's oceans. 3H, 129I, and 137Cs can be found dissolved in seawater, while 241Am and 238Pu
are attached to particles. The isotopes dissolved in water are
particularly useful in studying global circulation. For example,
differences in lateral isotopic ratios within an ocean can indicate
strong water fronts or gyres.
Conversely, the isotopes attached to particles can be used to study
mass transport within water columns. For instance, high levels of Am or
Pu can indicate downwelling when observed at great depths, or upwelling when observed at the surface.
Nanoparticles for drug delivery to the brain is a method for transporting drug molecules across the blood–brain barrier (BBB) using nanoparticles.
These drugs cross the BBB and deliver pharmaceuticals to the brain for
therapeutic treatment of neurological disorders. These disorders include
Parkinson's disease, Alzheimer's disease, schizophrenia, depression, and brain tumors. Part of the difficulty in finding cures for these central nervous system (CNS) disorders is that there is yet no truly efficient delivery method for drugs to cross the BBB. Antibiotics, antineoplastic agents, and a variety of CNS-active drugs, especially neuropeptides, are a few examples of molecules that cannot pass the BBB alone.
With the aid of nanoparticle delivery systems, however, studies have
shown that some drugs can now cross the BBB, and even exhibit lower toxicity and decrease adverse effects throughout the body. Toxicity is an important concept for pharmacology
because high toxicity levels in the body could be detrimental to the
patient by affecting other organs and disrupting their function. Further, the BBB is not the only physiological barrier for drug delivery to the brain.
Other biological factors influence how drugs are transported throughout
the body and how they target specific locations for action. Some of
these pathophysiological factors include blood flow alterations, edema and increased intracranial pressure, metabolic perturbations, and altered gene expression and protein synthesis.
Though there exist many obstacles that make developing a robust
delivery system difficult, nanoparticles provide a promising mechanism
for drug transport to the CNS.
Background
The first successful delivery of a drug across the BBB occurred in 1995. The drug used was hexapeptide dalargin, an anti-nociceptive peptide that cannot cross the BBB alone. It was encapsulated in polysorbate 80 coated nanoparticles and intravenously injected. This was a huge breakthrough in the nanoparticle drug delivery field, and it helped advance research and development toward clinical trials
of nanoparticle delivery systems. Nanoparticles range in size from 10 -
1000 nm (or 1 µm) and they can be made from natural or artificial polymers, lipids, dendrimers, and micelles. Most polymers used for nanoparticle drug delivery systems are natural, biocompatible, and biodegradable, which helps prevent contamination in the CNS. Several current methods for drug delivery to the brain include the use of liposomes, prodrugs, and carrier-mediated transporters. Many different delivery methods exist to transport these drugs into the body, such as peroral, intranasal, intravenous,
and intracranial. For nanoparticles, most studies have shown increasing
progression with intravenous delivery. Along with delivery and
transport methods, there are several means of functionalizing, or activating,
the nanoparticle carriers. These means include dissolving or absorbing a
drug throughout the nanoparticle, encapsulating a drug inside the
particle, or attaching a drug on the surface of the particle.
Types of nanoparticles for CNS drug delivery
Lipid-based
Diagram of liposome showing a phospholipid bilayer surrounding an aqueous interior.
One type of nanoparticle involves use of liposomes as drug molecule carriers. The diagram on the right shows a standard liposome. It has a phospholipid bilayer separating the interior from the exterior of the cell.
Liposomes are composed of vesicular bilayers, lamellae, made of biocompatible and biodegradable lipids such as sphingomyelin, phosphatidylcholine, and glycerophospholipids. Cholesterol,
a type of lipid, is also often incorporated in the lipid-nanoparticle
formulation. Cholesterol can increase stability of a liposome and
prevent leakage of a bilayer because its hydroxyl
group can interact with the polar heads of the bilayer phospholipids.
Liposomes have the potential to protect the drug from degradation,
target sites for action, and reduce toxicity and adverse effects. Lipid nanoparticles can be manufactured by high pressure homogenization, a current method used to produce parenteralemulsions.
This process can ultimately form a uniform dispersion of small droplets
in a fluid substance by subdividing particles until the desired
consistency is acquired.
This manufacturing process is already scaled and in use in the food
industry, which therefore makes it more appealing for researchers and
for the drug delivery industry.
Liposomes can also be functionalized by attaching various ligands on the surface to enhance brain-targeted delivery.
Cationic liposomes
Another type of lipid-nanoparticle that can be used for drug delivery to the brain is a cationic liposome. These are lipid molecules that are positively charged. One example of cationic liposomes uses bolaamphiphiles, which contain hydrophilic groups surrounding a hydrophobic
chain to strengthen the boundary of the nano-vesicle containing the
drug. Bolaamphiphile nano-vesicles can cross the BBB, and they allow
controlled release of the drug to target sites. Lipoplexes can also be formed from cationic liposomes and DNA solutions, to yield transfection agents. Cationic liposomes cross the BBB through adsorption mediated endocytosis followed by internalization in the endosomes of the endothelial cells. By transfection of endothelial cells
through the use of lipoplexes, physical alterations in the cells could
be made. These physical changes could potentially improve how some
nanoparticle drug-carriers cross the BBB.
Metallic
Metal
nanoparticles are promising as carriers for drug delivery to the brain.
Common metals used for nanoparticle drug delivery are gold, silver, and
platinum, owing to their biocompatibility. These metallic nanoparticles
are used due to their large surface area to volume ratio, geometric and
chemical tunability, and endogenous antimicrobial properties.
Silver cations released from silver nanoparticles can bind to the
negatively charged cellular membrane of bacteria and increase membrane
permeability, allowing foreign chemicals to enter the intracellular
fluid.
Metal nanoparticles are chemically synthesized using reduction reactions.
For example, drug-conjugated silver nanoparticles are created by
reducing silver nitrate with sodium borohydride in the presence of an
ionic drug compound. The drug binds to the surface of the silver,
stabilizing the nanoparticles and preventing the nanoparticles from
aggregation.
Metallic nanoparticles typically cross the BBB via transcytosis.
Nanoparticle delivery through the BBB can be increased by introducing
peptide conjugates to improve permeability to the central nervous
system. For instance, recent studies have shown an improvement in gold
nanoparticle delivery efficiency by conjugating a peptide that binds to
the transferrin receptors expressed in brain endothelial cells.
Solid lipid
Diagram
displays a solid lipid nanoparticle (SLN). There is only one
phospholipid layer because the interior of the particle is solid.
Molecules such as antibodies, targeting peptides, and drug molecules can
be bound to the surface of the SLN.
Also, solid lipid nanoparticles
(SLNs) are lipid nanoparticles with a solid interior as shown in the
diagram on the right. SLNs can be made by replacing the liquid lipid oil
used in the emulsion process with a solid lipid. In solid lipid
nanoparticles, the drug molecules are dissolved in the particle's solid hydrophobic lipid core, this is called the drug payload, and it is surrounded by an aqueous solution. Many SLNs are developed from triglycerides, fatty acids, and waxes. High-pressure homogenization or micro-emulsification can be used for manufacturing. Further, functionalizing the surface of solid lipid nanoparticles with polyethylene glycol (PEG) can result in increased BBB permeability. Different colloidal carriers such as liposomes, polymeric nanoparticles, and emulsions have reduced stability, shelf life and encapsulation efficacy. Solid lipid nanoparticles
are designed to overcome these shortcomings and have an excellent drug
release and physical stability apart from targeted delivery of drugs.
Nanoemulsions
Another form for nanoparticle delivery systems is oil-in-water emulsions done on a nano-scale. This process uses common biocompatible oils such as triglycerides and fatty acids, and combines them with water and surface-coating surfactants. Oils rich in omega-3 fatty acids especially contain important factors that aid in penetrating the tight junctions of the BBB.
Polymer-based
Other nanoparticles are polymer-based, meaning they are made from a natural polymer such as polylactic acid (PLA), poly D,L-glycolide (PLG),
polylactide-co-glycolide (PLGA), and polycyanoacrylate (PCA).
Some studies have found that polymeric nanoparticles may provide better
results for drug delivery relative to lipid-based nanoparticles because
they may increase the stability of the drugs or proteins being
transported. Polymeric nanoparticles may also contain beneficial controlled release mechanisms.
Polymer Branch
Nanoparticles made from natural polymers that are biodegradable have
the abilities to target specific organs and tissues in the body, to
carry DNA for gene therapy, and to deliver larger molecules such as proteins, peptides, and even genes. To manufacture these polymeric nanoparticles, the drug molecules are first dissolved and then encapsulated
or attached to a polymer nanoparticle matrix. Three different
structures can then be obtained from this process; nanoparticles, nanocapsules
(in which the drug is encapsulated and surrounded by the polymer
matrix), and nanospheres (in which the drug is dispersed throughout the
polymeric matrix in a spherical form).
One of the most important traits for nanoparticle delivery
systems is that they must be biodegradable on the scale of a few days. A few common polymer materials used for drug delivery studies are polybutyl cyanoacrylate (PBCA), poly(isohexyl cyanoacrylate) (PIHCA), polylactic acid (PLA), or polylactide-co-glycolide (PLGA). PBCA undergoes degradation through enzymatic cleavage of its ester bond on the alkyl side chain to produce water-soluble byproducts. PBCA also proves to be the fastest biodegradable material, with studies showing 80% reduction after 24 hours post intravenous therapy injection. PIHCA, however, was recently found to display an even lower degradation rate, which in turn further decreases toxicity.
PIHCA, due to this slight advantage, is currently undergoing phase III
clinical trials for transporting the drug doxorubicin as a treatment for
hepatocellular carcinomas.
Human serum albumin (HSA) and chitosan
are also materials of interest for the generation of nanoparticle
delivery systems. Using albumin nanoparticles for stroke therapy can
overcome numerous limitations. For instance, albumin nanoparticles can
enhance BBB permeability, increase solubility, and increase half-life in
circulation. Patients who have brain cancer overexpress albumin-binding
proteins, such as SPARC and gp60, in their BBB and tumor cells,
naturally increasing the uptake of albumin into the brain. Using this
relationship, researches have formed albumin nanoparticles that
co-encapsulate two anticancer drugs, paclitaxel and fenretinide, modified with low weight molecular protamine (LMWP), a type of cell-penetrating protein, for anti-glioma therapy.
Once injected into the patient's body, the albumin nanoparticles can
cross the BBB more easily, bind to the proteins and penetrate glioma
cells, and then release the contained drugs. This nanoparticle
formulation enhances tumor-targeting delivery efficiency and improves
the solubility issue of hydrophobic drugs. Specifically, cationic bovine serum albumin-conjugated
tanshinone IIA PEGylated nanoparticles injected into a MCAO rat model
decreased the volume of infarction and neuronal apoptosis. Chitosan,
a naturally abundant polysaccharide, is particularly useful due to its
biocompability and lack of toxicity. With its adsorptive and
mucoadhesive properties, chitosan can overcome limitations of internasal
administration to the brain. It has been shown that cationic chitosan
nanoparticles interact with the negatively charged brain endothelium.
Coating these polymeric nanoparticle devices with different
surfactants can also aid BBB crossing and uptake in the brain.
Surfactants such as polysorbate 80, 20, 40, 60, and poloxamer
188, demonstrated positive drug delivery through the blood–brain
barrier, whereas other surfactants did not yield the same results.
It has also been shown that functionalizing the surface of
nanoparticles with polyethylene glycol (PEG), can induce the "stealth
effect", allowing the drug-loaded nanoparticle to circulate throughout
the body for prolonged periods of time. Further, the stealth effect,
caused in part by the hydrophilic and flexible properties of the PEG
chains, facilitates an increase in localizing the drug at target sites
in tissues and organs.
Mechanisms for delivery
Liposomes
A mechanism for liposome transport across the BBB is lipid-mediated free diffusion, a type of facilitated diffusion, or lipid-mediated endocytosis. There exist many lipoprotein receptors which bind lipoproteins to form complexes that in turn transport the liposome nano-delivery system across the BBB. Apolipoprotein E (apoE) is a protein that facilitates transport of lipids and cholesterol. ApoE constituents bind to nanoparticles, and then this complex binds to a low-density lipoprotein receptor (LDLR) in the BBB and allows transport to occur.
This
diagram shows several ways in which transport across the BBB works. For
nanoparticle delivery across the BBB, the most common mechanisms are
receptor-mediated transcytosis and adsorptive transcytosis
Polymeric nanoparticles
The mechanism for the transport of polymer-based nanoparticles across the BBB has been characterized as receptor-mediated endocytosis by the brain capillary endothelial cells. Transcytosis
then occurs to transport the nanoparticles across the tight junction of
endothelial cells and into the brain. Surface coating nanoparticles
with surfactants such as polysorbate 80 or poloxamer 188 was shown to
increase uptake of the drug into the brain also. This mechanism also relies on certain receptors located on the luminal surface of endothelial cells of the BBB. Ligands
coated on the nanoparticle's surface bind to specific receptors to
cause a conformational change. Once bound to these receptors,
transcytosis can commence, and this involves the formation of vesicles
from the plasma membrane pinching off the nanoparticle system after internalization.
Additional receptors identified for receptor-mediated endocytosis of nanoparticle delivery systems are the scavenger receptor class B type I (SR-BI), LDL receptor (LRP1), transferrin receptor, and insulin receptor.
As long as a receptor exists on the endothelial surface of the BBB, any
ligand can be attached to the nanoparticle's surface to functionalize
it so that it can bind and undergo endocytosis.
Another mechanism is adsorption mediated transcytosis, where electrostatic interactions are involved in mediating nanoparticle crossing of the BBB.
Cationic nanoparticles (including cationic liposomes) are of interest
for this mechanism, because their positive charges assist binding on the
brain's endothelial cells. Using TAT-peptides,
a cell-penetrating peptide, to functionalize the surface of cationic
nanoparticles can further improve drug transport into the brain.
Magnetic and Magnetoelectric nanoparticles
In
contrast to the above mechanisms, a delivery with magnetic fields does
not strongly depend on the biochemistry of the brain. In this case,
nanoparticles are literally pulled across the BBB via application of a
magnetic field gradient. The nanoparticles can be pulled in as well as
removed from the brain merely by controlling the direction of the
gradient. For the approach to work, the nanoparticles must have a
non-zero magnetic moment and have a diameter of less than 50 nm. Both
magnetic and magnetoelectric nanoparticles (MENs) satisfy the
requirements. However, it is only the MENs which display a non-zero
magnetoelectric (ME) effect. Due to the ME effect, MENs can provide a
direct access to local intrinsic electric fields at the nanoscale to
enable a two-way communication with the neural network at the
single-neuron level.
MENs, proposed by the research group of Professor Sakhrat Khizroev at
Florida International University (FIU), have been used for targeted drug
delivery and externally controlled release across the BBB to treat HIV
and brain tumors, as well as to wirelessly stimulate neurons deep in the
brain for treatment of neurodegenerative diseases such as Parkinson's
Disease and others.
Focused ultrasound
Studies
have shown that focused ultrasound bursts can noninvasively be used to
disrupt tight junctions in desired locations of BBB, allowing for the
increased passage of particles at that location. This disruption can
last up to four hours after burst administration. Focused ultrasound
works by generating oscillating microbubbles, which physically interact
with the cells of the BBB by oscillating at a frequency which can be
tuned by the ultrasound burst. This physical interaction is believed to
cause cavitation and ultimately the disintegration of the tight junction
complexes
which may explain why this effect lasts for several hours. However, the
energy applied from ultrasound can result in tissue damage.
Fortunately, studies have demonstrated that this risk can be reduced if
preformed microbubbles are first injected before focused ultrasound is
applied, reducing the energy required from the ultrasound.
This technique has applications in the treatment of various diseases.
For example, one study has shown that using focused ultrasound with
oscillating bubbles loaded with a chemotherapeutic drug, carmustine, facilitates the safe treatment of glioblastoma
in an animal model. This drug, like many others, normally requires
large dosages to reach the target brain tissue diffusion from the blood,
leading to systemic toxicity and the possibilities of multiple harmful
side effects manifesting throughout the body. However, focused
ultrasound has the potential to increase the safety and efficacy of drug
delivery to the brain.
Toxicity
A study was performed to assess the toxicity effects of doxorubicin-loaded polymeric nanoparticle systems.
It was found that doses up to 400 mg/kg of PBCA nanoparticles alone did
not cause any toxic effects on the organism. These low toxicity effects
can most likely be attributed to the controlled release and modified biodistribution of the drug due to the traits of the nanoparticle delivery system.
Toxicity is a highly important factor and limit of drug delivery
studies, and a major area of interest in research on nanoparticle
delivery to the brain.
Metal nanoparticles are associated with risks of neurotoxicity and cytotoxicity. These heavy metals generate reactive oxygen species, which causes oxidative stress and damages the cells' mitochondria and endoplasmic reticulum.
This leads to further issues in cellular toxicity, such as damage to
DNA and disruption of cellular pathways. Silver nanoparticles in
particular have a higher degree of toxicity compared to other metal
nanoparticles such as gold or iron.
Silver nanoparticles can circulate through the body and accumulate
easily in multiple organs, as discovered in a study on the silver
nanoparticle distribution in rats.
Traces of silver accumulated in the rats' lungs, spleen, kidney, liver,
and brain after the nanoparticles were injected subcutaneously.
In addition, silver nanoparticles generate more reactive oxygen species
compared to other metals, which leads to an overall larger issue of
toxicity.
Research
In the
early 21st century, extensive research is occurring in the field of
nanoparticle drug delivery systems to the brain. One of the common
diseases being studied in neuroscience is Alzheimer's disease. Many
studies have been done to show how nanoparticles can be used as a
platform to deliver therapeutic drugs to these patients with the
disease. A few Alzheimer's drugs that have been studied especially are rivastigmine, tacrine, quinoline, piperine, and curcumin. PBCA, chitosan,
and PLGA nanoparticles were used as delivery systems for these drugs.
Overall, the results from each drug injection with these nanoparticles
showed remarkable improvements in the effects of the drug relative to
non-nanoparticle delivery systems. This possibly suggests that
nanoparticles could provide a promising solution to how these drugs
could cross the BBB. One factor that still must be considered and
accounted for is nanoparticle accumulation in the body. With long-term
and frequent injections that are often required to treat chronic diseases
such as Alzheimer's disease, polymeric nanoparticles could potentially
build up in the body, causing undesirable effects. This area for concern
would have to be further assessed to analyze these possible effects and
to improve them.
