Organosilicon chemistry

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Organosilicon_chemistry

 

Polydimethylsiloxane (PDMS) is the principal component of silicones.

Organosilicon chemistry is the study of organometallic compounds containing carbon–silicon bonds, to which they are called organosilicon compounds. Most organosilicon compounds are similar to the ordinary organic compounds, being colourless, flammable, hydrophobic, and stable to air. Silicon carbide is an inorganic compound.

History

See also: Organometallic chemistry

In 1863 Charles Friedel and James Crafts made the first organochlorosilane compound. The same year they also described a «polysilicic acid ether» in the preparation of ethyl- and methyl-o-silicic acid. Extensive research in the field of organosilicon compounds was pioneered in the beginning of 20th century by Frederic S. Kipping. He also had coined the term "silicone" (resembling ketones, this is errorneous though) in relation to these materials in 1904. In recognition of Kipping's achievements the Dow Chemical Company had established an award in 1960s that is given for significant contributions into the silicon chemistry. In his works Kipping was noted for using Grignard reagents to make alkylsilanes and arylsilanes and the preparation of silicone oligomers and polymers for the first time.

In 1945 Eugene G. Rochow has also made a significant contribution into the organosilicon chemistry by first describing Müller-Rochow process.

Occurrence and applications

Organosilicon compounds are widely encountered in commercial products. Most common are antifoamers, caulks (sealant), adhesives, and coatings made from silicones. Other important uses include agricultural and plant control adjuvants commonly used in conjunction with herbicides and fungicides.

Silicone caulk, commercial sealants, are mainly composed of organosilicon compounds mixed with hardener.

Biology and medicine

Carbon–silicon bonds are absent in biology, however enzymes have been used to artificially create carbon-silicon bonds in living microbes. Silicates, on the other hand, have known existence in diatoms. Silafluofen is an organosilicon compound that functions as a pyrethroid insecticide. Several organosilicon compounds have been investigated as pharmaceuticals.

Bonding

In the great majority of organosilicon compounds, Si is tetravalent with tetrahedral molecular geometry. Compared to carbon–carbon bonds, carbon–silicon bonds are longer and weaker. The C–Si bond is somewhat polarised towards carbon due to carbon's greater electronegativity (C 2.55 vs Si 1.90). The strength of the Si-O bond is strikingly high, and this feature is exploited in many reactions such as the Sakurai reaction, the Brook rearrangement, the Fleming–Tamao oxidation, and the Peterson olefination. Another manifestation is the β-silicon effect describes the stabilizing effect of a β-silicon atom on a carbocation with many implications for reactivity.

Properties Relevant to OrganoSi Chemistry

Bond	Bond length (pm)	Approx. bond strength (kJ/mol)
C-C	154	334
Si-Si	234	196
C-Si	186	314
C-H	110	414
Si-H	146	314
C-O	145	355
Si-O	159	460

Electronegativities 

Relevant to OrganoSi Chemistry

C	Si	H	O
2.5	1.8	2.1	3.4

Preparation

The first organosilicon compound, tetraethylsilane, was prepared by Charles Friedel and James Crafts in 1863 by reaction of tetrachlorosilane with diethylzinc.

The bulk of organosilicon compounds derive from organosilicon chlorides (CH
₃)
_4-xSiCl
_x. These chlorides are produced by the "Direct process", which entails the reaction of methyl chloride with a silicon-copper alloy. The main and most sought-after product is dimethyldichlorosilane:

2 CH
₃Cl + Si → (CH
₃)
₂SiCl
₂

A variety of other products are obtained, including trimethylsilyl chloride and methyltrichlorosilane. About 1 million tons of organosilicon compounds are prepared annually by this route. The method can also be used for phenyl chlorosilanes.

Hydrosilylation

Main article: Hydrosilylation

Another major method for the formation of Si-C bonds is hydrosilylation (also called hydrosilation). In this process, compounds with Si-H bonds (hydrosilanes) add to unsaturated substrates. Commercially, the main substrates are alkenes. Other unsaturated functional groups — alkynes, imines, ketones, and aldehydes — also participate, but these reactions are of little economic value.

Idealized mechanism for metal-catalysed hydrosilylation of an alkene

Hydrosilylation requires metal catalysts, especially those based on platinum group metals.

In the related silylmetalation, a metal replaces the hydrogen atom.

Cleavage of Si-Si bonds

Hexamethyldisilane reacts with methyl lithium to give trimethylsilyl lithium:

(CH₃)₆Si₂ + CH₃Li → (CH₃)₃SiLi + (CH₃)₄Si

Similarly, tris(trimethylsilyl)silyl lithium is derived from tetrakis(trimethylsilyl)silane:

((CH₃)₃Si)₄Si + CH₃Li → ((CH₃)₃Si)₃SiLi + (CH₃)₄Si

Functional groups

Silicon is a component of many functional groups. Most of these are analogous to organic compounds. The overarching exception is the rarity of multiple bonds to silicon, as reflected in the double bond rule.

Silanols, siloxides, and siloxanes

Silanols are analogues of alcohols. They are generally prepared by hydrolysis of silyl chlorides:

R
₃SiCl + H₂O → R
₃SiOH + HCl

Less frequently silanols are prepared by oxidation of silyl hydrides, a reaction that uses a metal catalyst:

2 R
₃SiH + O
₂ → 2 R
₃SiOH

Many silanols have been isolated including (CH
₃)
₃SiOH and (C
₆H
₅)
₃SiOH. They are about 500x more acidic than the corresponding alcohols. Siloxides are the deprotonated derivatives of silanols:

R
₃SiOH + NaOH → R
₃SiONa + H₂O

Silanols tend to dehydrate to give siloxanes:

2 R
₃SiOH → R
₃Si-O-SiR
₃ + H₂O

Polymers with repeating siloxane linkages are called silicones. Compounds with an Si=O double bond called silanones are extremely unstable.

Silyl ethers

Silyl ethers have the connectivity Si-O-C. They are typically prepared by the reaction of alcohols with silyl chlorides:

(CH
₃)
₃SiCl + ROH → (CH
₃)
₃Si-O-R + HCl

Silyl ethers are extensively used as protective groups for alcohols.

Exploiting the strength of the Si-F bond, fluoride sources such as tetra-n-butylammonium fluoride (TBAF) are used in deprotection of silyl ethers:

(CH
₃)
₃Si-O-R + F⁻
+ H₂O → (CH
₃)
₃Si-F + H-O-R + OH⁻

Silyl chlorides

Main article: Chlorosilane

Organosilyl chlorides are important commodity chemicals. They are mainly used to produce silicone polymers as described above. Especially important silyl chlorides are dimethyldichlorosilane (Me
₂SiCl
₂), methyltrichlorosilane (MeSiCl
₃), and trimethylsilyl chloride (Me
₃SiCl) are all produced by direct process. More specialized derivatives that find commercial applications include dichloromethylphenylsilane, trichloro(chloromethyl)silane, trichloro(dichlorophenyl)silane, trichloroethylsilane, and phenyltrichlorosilane.

Although proportionately a minor outlet, organosilicon compounds are widely used in organic synthesis. Notably trimethylsilyl chloride Me
₃SiCl is the main silylating agent. One classic method called the Flood reaction for the synthesis of this compound class is by heating hexaalkyldisiloxanes R
₃SiOSiR
₃ with concentrated sulfuric acid and a sodium halide.

Silyl hydrides

Main article: Hydrosilane

 

Tris(trimethylsilyl)silane is a well-investigated hydrosilane.

The silicon to hydrogen bond is longer than the C–H bond (148 compared to 105 pm) and weaker (299 compared to 338 kJ/mol). Hydrogen is more electronegative than silicon hence the naming convention of silyl hydrides. Commonly the presence of the hydride is not mentioned in the name of the compound. Triethylsilane has the formula Et
₃SiH. Phenylsilane is PhSiH
₃. The parent compound SiH
₄ is called silane.

Silenes

Organosilicon compounds, unlike their carbon counterparts, do not have a rich double bond chemistry. Compounds with silene Si=C bonds (also known as alkylidenesilanes) are laboratory curiosities such as the silicon benzene analogue silabenzene. In 1967, Gusel'nikov and Flowers provided the first evidence for silenes from pyrolysis of dimethylsilacyclobutane. The first stable (kinetically shielded) silene was reported in 1981 by Brook.

Disilenes have Si=Si double bonds and disilynes are silicon analogues of an alkyne. The first Silyne (with a silicon to carbon triple bond) was reported in 2010.

Siloles

Chemical structure of silole

Siloles, also called silacyclopentadienes, are members of a larger class of compounds called metalloles. They are the silicon analogs of cyclopentadienes and are of current academic interest due to their electroluminescence and other electronic properties. Siloles are efficient in electron transport. They owe their low lying LUMO to a favorable interaction between the antibonding sigma silicon orbital with an antibonding pi orbital of the butadiene fragment.

Pentacoordinated silicon

See also: Hypervalent molecule § Pentacoordinated silicon

Unlike carbon, silicon compounds can be coordinated to five atoms as well in a group of compounds ranging from so-called silatranes, such as phenylsilatrane, to a uniquely stable pentaorganosilicate:

The stability of hypervalent silicon is the basis of the Hiyama coupling, a coupling reaction used in certain specialized organic synthetic applications. The reaction begins with the activation of Si-C bond by fluoride:

R-SiR'
₃ + R"-X + F⁻
→ R-R" + R'
₃SiF + X⁻

Various reactions

Certain allyl silanes can be prepared from allylic esters such as 1 and monosilylcopper compounds, which are formed in situ by the reaction of the disilylzinc compound 2, with Copper Iodide, in:

In this reaction type, silicon polarity is reversed in a chemical bond with zinc and a formal allylic substitution on the benzoyloxy group takes place.

Environmental effects

Organosilicon compounds affect bee (and other insect) immune expression, making them more susceptible to viral infection.

Judeo-Christian ethics

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Judeo-Christian_ethics

 

This article is about the 20th-century American concept of common values shared between the two religions. For the term "Judeo-Christian", grouping Judaism and Christianity, see Judeo-Christian. For the intersection in general of Judaism with Christianity, see Christianity and Judaism.

 

A monument at the Texas State Capitol depicting the Ten Commandments. The U.S. Supreme Court let it stand in Van Orden v. Perry (2005).

The idea that a common Judaeo-Christian ethics or Judeo-Christian values underpins American politics, law and morals has been part of the "American civil religion" since the 1940s. In recent years, the phrase has been associated with American conservatism, but the concept—though not always the exact phrase—has frequently featured in the rhetoric of leaders across the political spectrum, including that of Franklin D. Roosevelt and Lyndon B. Johnson.

Ethical value system

See also: Christian ethics and Jewish ethics

The current American use of "Judeo-Christian" — to refer to a value system common to Jews and Christians — first appeared in print in a book review by the English writer George Orwell in 1939, with the phrase "the Judaeo-Christian scheme of morals." Orwell's usage of the term followed at least a decade of efforts by Jewish and Christian leaders, through such groups as the U.S. National Conference of Christians and Jews (founded in 1927), to emphasize common ground. The term continued to gain currency in the 1940s. In part, it was a way of countering antisemitism with the idea that the foundation of morals and law in the United States was a shared one between Jews and Christians.

Franklin D. Roosevelt

The first inaugural address of Franklin D. Roosevelt (FDR), in 1933, the famous speech in which FDR declared that "the only thing we have to fear is fear itself", had numerous religious references, which was widely commented upon at the time. Although it did not use the term "Judeo-Christian", it has come to be seen by scholars as in tune with the emerging view of a Judeo-Christian tradition. Historian Mary Stuckey emphasizes "Roosevelt's use of the shared values grounded in the Judeo-Christian tradition" as a way to unify the American nation, and justify his own role as its chief policymaker.

In the speech, FDR attacked the bankers and promised a reform in an echo of the gospels: "The money changers have fled from their high seats in the temple of our civilization. We may now restore that temple to the ancient truths. The measure of the restoration lies in the extent to which we apply social values more noble than mere monetary profit." Houck and Nocasian, examining the flood of responses to the First Inaugural, and commenting on this passage, argue:

The nation's overwhelmingly Judeo-Christian response to the address thus had both textual and extratextual warrants. For those inclined to see the Divine Hand of Providence at work, Roosevelt's miraculous escape [from assassination] in Miami was a sign—perhaps The Sign—that God had sent another Washington or Lincoln at the appointed hour. ... Many others could not resist the subject position that Roosevelt ... had cultivated throughout the address—that of savior. After all, it was Christ who had expelled the moneychangers from the Temple. ... [Many listeners saw] a composite sign that their new president had a godly mandate to lead.

Gary Scott Smith stresses that Roosevelt believed his welfare programs were "wholly in accord with the social teachings of Christianity." He saw the achievement of social justice through government action as morally superior to the old laissez-faire approach. He proclaimed, "The thing we are seeking is justice," as guided by the precept of "Do unto your neighbor as you would be done by." Roosevelt saw the moral issue as religiosity versus anti-religion. According to Smith, "He pleaded with Protestants, Catholics, and Jews to transcend their sectarian creeds and 'unite in good works' whenever they could 'find common cause.'"

Atalia Omer and Jason A. Springs point to Roosevelt's 1939 State of the Union address, which called upon Americans to "defend, not their homes alone, but the tenets of faith and humanity on with which their churches, their governments and their very civilization are founded." They state that, "This familiar rhetoric invoked a conception of the sanctity of the United States' Judeo-Christian values as a basis for war."

Timothy Wyatt notes that in the coming of World War II Roosevelt's isolationist opponents said he was calling for a "holy war." Wyatt says:

Often in his Fireside Chats or speeches to the houses of Congress, FDR argued for the entrance of America into the war by using both blatant and subtle religious rhetoric. Roosevelt portrayed the conflict in the light of good versus evil, the religious against the irreligious. In doing so, he pitted the Christian ideals of democracy against the atheism of National Socialism.

Lyndon Johnson

Biographer Randall B. Woods has argued that President Lyndon B. Johnson effectively used appeals to the Judeo-Christian ethical tradition to garner support for the civil rights law of 1965. Woods writes that Johnson undermined the Southern filibuster against the bill:

LBJ wrapped white America in a moral straight jacket. How could individuals who fervently, continuously, and overwhelmingly identified themselves with a merciful and just God continue to condone racial discrimination, police brutality, and segregation? Where in the Judeo-Christian ethic was there justification for killing young girls in a church in Alabama, denying an equal education to black children, barring fathers and mothers from competing for jobs that would feed and clothe their families? Was Jim Crow to be America's response to "Godless Communism"?

Woods went on to assess the role of Judeo-Christian ethics among the nation's political elite:

Johnson's decision to define civil rights as a moral issue, and to wield the nation's self-professed Judeo-Christian ethic as a sword in its behalf, constituted something of a watershed in twentieth-century political history. All presidents were fond of invoking the deity, and some conservatives like Dwight Eisenhower had flirted with employing Judeo-Christian teachings to justify their actions, but modern-day liberals, both politicians and the intellectuals who challenged and nourished them, had shunned spiritual witness. Most liberal intellectuals were secular humanists. Academics in particular had historically been deeply distrustful of organized religion, which they identified with small-mindedness, bigotry, and anti-intellectualism. Like his role model, FDR, Johnson equated liberal values with religious values, insisting freedom and social justice served the ends of both god and man. And he was not loath to say so.

Woods notes that Johnson's religiosity ran deep: "At 15 he joined the Disciples of Christ, or Christian, church and would forever believe that it was the duty of the rich to care for the poor, the strong to assist the weak, and the educated to speak for the inarticulate."

History

1930s and 1940s

Promoting the concept of the United States as a Judeo-Christian nation first became a political program in the 1940s, in response to the growth of anti-Semitism in America. The rise of Nazi anti-semitism in the 1930s led concerned Protestants, Catholics, and Jews to take steps to increase understanding and tolerance.

In this effort, precursors of the National Conference of Christians and Jews created teams consisting of a priest, a rabbi, and a minister, to run programs across the country, and fashion a more pluralistic America, no longer defined as a Christian land, but "one nurtured by three ennobling traditions: Protestantism, Catholicism and Judaism. ... The phrase 'Judeo-Christian' entered the contemporary lexicon as the standard liberal term for the idea that Western values rest on a religious consensus that included Jews."

In the 1930s, "In the face of worldwide antisemitic efforts to stigmatize and destroy Judaism, influential Christians and Jews in America labored to uphold it, pushing Judaism from the margins of American religious life towards its very center." During World War II, Jewish chaplains worked with Catholic priests and Protestant ministers to promote goodwill, addressing servicemen who, "in many cases had never seen, much less heard a Rabbi speak before." At funerals for the unknown soldier, rabbis stood alongside the other chaplains and recited prayers in Hebrew. In a much publicized wartime tragedy, the sinking of the Dorchester, the ship's multi-faith chaplains gave up their lifebelts to evacuating seamen and stood together "arm in arm in prayer" as the ship went down. A 1948 postage stamp commemorated their heroism with the words: "interfaith in action."

1950s, 1960s, and 1970s

In December 1952 President Dwight Eisenhower, speaking extemporaneously a month before his inauguration, said, in what may be the first direct public reference by a U.S. president to the Judeo-Christian concept:

[The Founding Fathers said] 'we hold that all men are endowed by their Creator ... ' In other words, our form of government has no sense unless it is founded in a deeply felt religious faith, and I don't care what it is. With us of course it is the Judeo-Christian concept, but it must be a religion with all men created equal.

By the 1950s, many conservatives emphasized the Judeo-Christian roots of their values. In 1958, economist Elgin Groseclose claimed that it was ideas "drawn from Judeo-Christian Scriptures that have made possible the economic strength and industrial power of this country."

Senator Barry Goldwater noted that conservatives "believed the communist projection of man as a producing, consuming animal to be used and discarded was antithetical to all the Judeo-Christian understandings which are the foundations upon which the Republic stands."

Belief in the superiority of Western Judeo-Christian traditions led conservatives to downplay the aspirations of the Third World to free themselves from colonial rule.

The emergence of the "Christian right" as a political force and part of the conservative coalition dates from the 1970s. According to Cambridge University historian Andrew Preston, the emergence of "conservative ecumenism." bringing together Catholics, Mormons, and conservative Protestants into the religious right coalition, was facilitated "by the rise of a Judeo-Christian ethic." These groups "began to mobilize together on cultural-political issues such as abortion and the proposed Equal Rights Amendment for women." As Wilcox and Robinson conclude:

The Christian Right is an attempt to restore Judeo-Christian values to a country that is in deep moral decline. ... [They] believe that society suffers from the lack of a firm basis of Judeo-Christian values and they seek to write laws that embody those values.

1980s and 1990s

By the 1980s and 1990s, favorable references to "Judeo-Christian values" were common, and the term was used by conservative Christians.

President Ronald Reagan frequently emphasized Judeo-Christian values as necessary ingredients in the fight against Communism. He argued that the Bible contains "all the answers to the problems that face us." Reagan disapproved of the growth of secularism and emphasized the need to take the idea of sin seriously. Tom Freiling, a Christian publisher and head of a conservative PAC, stated in his 2003 book, Reagan's God and Country, that "Reagan's core religious beliefs were always steeped in traditional Judeo-Christian heritage." Religion—and the Judeo-Christian concept—was a major theme in Reagan's rhetoric by 1980.

President Bill Clinton during his 1992 presidential campaign, likewise emphasized the role of religion in society, and in his personal life, having made references to the Judeo-Christian tradition.

The term became especially significant in American politics, and, promoting "Judeo-Christian values" in the culture wars, usage surged in the 1990s.

James Dobson, a prominent evangelical Christian, said the Judeo-Christian tradition includes the right to display numerous historical documents in Kentucky schools, after they were banned by a federal judge in May 2000 because they were "conveying a very specific governmental endorsement of religion".

Since 9/11

According to Hartmann et al., usage shifted between 2001 and 2005, with the mainstream media using the term less, in order to characterize America as multicultural. The study finds the term is now most likely to be used by liberals in connection with discussions of Muslim and Islamic inclusion in America, and renewed debate about the separation of church and state.

In 2012, the book Kosher Jesus by Orthodox rabbi Shmuley Boteach was published. In it, Boteach concludes by writing, as to Judeo-Christian values, that "the hyphen between Jewish and Christian values is Jesus himself."

In U.S. law

In the case of Marsh v. Chambers, 463 U.S. 783 (1983), the Supreme Court of the United States held that a state legislature could constitutionally have a paid chaplain to conduct legislative prayers "in the Judeo-Christian tradition." In Simpson v. Chesterfield County Board of Supervisors, the Fourth Circuit Court of Appeals held that the Supreme Court's holding in the Marsh case meant that the "Chesterfield County could constitutionally exclude Cynthia Simpson, a Wiccan priestess, from leading its legislative prayers, because her faith was not 'in the Judeo-Christian tradition.'" Chesterfield County's board included Jewish, Christian, and Muslim clergy in its invited list.

Several legal disputes, especially in Alabama, have challenged the public display of the Ten Commandments. See:

• Glassroth v. Moore for Alabama
• Green v. Haskell County Board of Commissioners, for Oklahoma
• McCreary County v. American Civil Liberties Union, for Kentucky
• Pleasant Grove City v. Summum, for Utah
• Stone v. Graham for Kentucky
• Van Orden v. Perry for Texas

Responses

Some theologians warn against the uncritical use of "Judeo-Christian" entirely, arguing that it can license mischief, such as opposition to secular humanism with scant regard to modern Jewish, Catholic, or Christian traditions, including the liberal strains of different faiths, such as Reform Judaism and liberal Protestant Christianity.

Two notable books addressed the relations between contemporary Judaism and Christianity. Abba Hillel Silver's Where Judaism Differs and Leo Baeck's Judaism and Christianity were both motivated by an impulse to clarify Judaism's distinctiveness "in a world where the term Judeo-Christian had obscured critical differences between the two faiths."

Reacting against the blurring of theological distinctions, Rabbi Eliezer Berkovits wrote that "Judaism is Judaism because it rejects Christianity, and Christianity is Christianity because it rejects Judaism."

Theologian and author Arthur A. Cohen, in The Myth of the Judeo-Christian Tradition, questioned the theological validity of the Judeo-Christian concept and suggested that it was essentially an invention of American politics, while Jacob Neusner, in Jews and Christians: The Myth of a Common Tradition, writes, "The two faiths stand for different people talking about different things to different people."

Law professor Stephen M. Feldman, looking at the period before 1950, chiefly in Europe, sees the concept of a Judeo-Christian tradition as supersessionism, which he characterizes as "dangerous Christian dogma (at least from a Jewish perspective)", and as a "myth" which "insidiously obscures the real and significant differences between Judaism and Christianity."

Abrahamic religion

Advocates of the term "Abrahamic religion" since the second half of the 20th century have proposed an inclusivism that widens the "Judeo-Christian" concept to include Islam as well. The rationale for the term "Abrahamic" is that Islam, like Judaism and Christianity, traces its origins to the figure of Abraham, whom Islam regards as a prophet. Advocates of this umbrella term consider it the "exploration of something positive" in the sense of a "spiritual bond" between Jews, Christians, and Muslims.

Australia

Australian historian Tony Taylor points out that Australia has borrowed the "Judeo-Christian" theme from American conservative discourse.

Jim Berryman, another Australian historian, argues that from the 1890s to the present, rhetoric upholding Australia's traditional attachment to Western civilisation emphasizes three themes: the core British heritage; Australia's Judeo-Christian belief system; and the rational principles of the Enlightenment. These themes have been expressed mostly on the Australian center-right political spectrum, and most prominently among conservative-leaning commentators.

Selenium in biology

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Selenium_in_biology 

 

Selenocysteine is the main organic molecule involving selenium in humans.

Although it is toxic in large doses, selenium is an essential micronutrient for animals. In plants, it sometimes occurs in toxic amounts as forage, e.g. locoweed. Selenium is a component of the amino acids selenocysteine and selenomethionine. In humans, selenium is a trace element nutrient that functions as cofactor for glutathione peroxidases and certain forms of thioredoxin reductase. Selenium-containing proteins are produced from inorganic selenium via the intermediacy of selenophosphate (PSeO₃³⁻).

Se-containing biomolecules

Selenium is an essential micronutrient in mammals, but is also recognized as toxic in excess. Selenium exerts its biological functions through selenoproteins, which contain the amino acid selenocysteine. Twenty-five selenoproteins are encoded in the human genome.

Glutathione peroxidase

The glutathione peroxidase family of enzymes (abbreviated GSH-Px) catalyze reduction of hydrogen peroxide and organic hydroperoxides:

2GSH + H₂O₂ → GSSG + 2 H₂O

The two H atoms are donated by thiols in a process that begins with oxidation of a selenol side chain in GSH-Px. The organoselenium compound ebselen is a drug used to supplement the action of GSH-Px. It functions as a catalyst for the destruction of hydrogen peroxide.

A related selenium-containing enzyme in some plants and in animals (thioredoxin reductase) generates reduced thioredoxin, a dithiol that serves as an electron source for peroxidases and also the important reducing enzyme ribonucleotide reductase that makes DNA precursors from RNA precursors.

Deiodinases

Selenium also plays a role in the functioning of the thyroid gland. It participates as a cofactor for the three thyroid hormone deiodinases. These enzymes activate and then deactivate various thyroid hormones and their metabolites. It may inhibit Hashimoto's disease, an auto-immune disease in which the body's own thyroid cells are attacked by the immune system. A reduction of 21% on TPO antibodies was reported with the dietary intake of 0.2 mg of selenium.

Formate dehydrogenase

Some microorganisms utilize selenium in formate dehydrogenase. Formate is produced in large amounts in the hepatic (liver cells) mitochondria of embryonic cells and in cancer cells by the folate cycle.

Formate is reversibly oxidized by the enzyme formate dehydrogenase:

HCO₂⁻ → CO₂ + H⁺ + 2 e⁻

Thioredoxin reductase

Thioredoxin reductase uses a cysteine-selenocysteine pair to reduce the disulfide in thioredoxin. The selenocysteine is arranged in an unusual Sec-His-Glu catalytic triad, which tunes its pKa.

Indicator plants

Certain species of plants are considered indicators of high selenium content of the soil, since they require high levels of selenium to thrive. The main selenium indicator plants are Astragalus species (including some locoweeds), prince's plume (Stanleya sp.), woody asters (Xylorhiza sp.), and false goldenweed (Oonopsis sp.)

Medical use of synthetic selenium compounds

The substance loosely called selenium sulfide (with the approximate formula SeS₂) is the active ingredient in some anti-dandruff shampoos. The selenium compound kills the scalp fungus Malassezia, which causes shedding of dry skin fragments. The ingredient is also used in body lotions to treat Tinea versicolor due to infection by a different species of Malassezia fungus.

Several clinical trials have assessed the use of selenium supplements in critically ill adults; however, the effectiveness and potential benefits of selenium supplementation in this context is not well understood.

Detection in biological fluids

Selenium may be measured in blood, plasma, serum or urine to monitor excessive environmental or occupational exposure, confirm a diagnosis of poisoning in hospitalized victims or to assist in a forensic investigation in a case of fatal overdosage. Some analytical techniques are capable of distinguishing organic from inorganic forms of the element. Both organic and inorganic forms of selenium are largely converted to monosaccharide conjugates (selenosugars) in the body prior to being eliminated in the urine. Cancer patients receiving daily oral doses of selenothionine may achieve very high plasma and urine selenium concentrations.

Toxicity

Although selenium is an essential trace element, it is toxic if taken in excess. Exceeding the Tolerable Upper Intake Level of 400 micrograms per day can lead to selenosis. This 400 microgram (µg) Tolerable Upper Intake Level is based primarily on a 1986 study of five Chinese patients who exhibited overt signs of selenosis and a follow up study on the same five people in 1992. The 1992 study actually found the maximum safe dietary Se intake to be approximately 800 micrograms per day (15 micrograms per kilogram body weight), but suggested 400 micrograms per day to not only avoid toxicity, but also to avoid creating an imbalance of nutrients in the diet and to account for data from other countries. In China, people who ingested corn grown in extremely selenium-rich stony coal (carbonaceous shale) have suffered from selenium toxicity. This coal was shown to have selenium content as high as 9.1%, the highest concentration in coal ever recorded in literature.

Symptoms of selenosis include a garlic odor on the breath, gastrointestinal disorders, hair loss, sloughing of nails, fatigue, irritability, and neurological damage. Extreme cases of selenosis can result in cirrhosis of the liver, pulmonary edema, and death. Elemental selenium and most metallic selenides have relatively low toxicities because of their low bioavailability. By contrast, selenates and selenites are very toxic, having an oxidant mode of action similar to that of arsenic trioxide. The chronic toxic dose of selenite for humans is about 2400 to 3000 micrograms of selenium per day for a long time. Hydrogen selenide is an extremely toxic, corrosive gas. Selenium also occurs in organic compounds, such as dimethyl selenide, selenomethionine, selenocysteine and methylselenocysteine, all of which have high bioavailability and are toxic in large doses.

Selenium poisoning of water systems may result whenever new agricultural runoff courses through normally dry, undeveloped lands. This process leaches natural soluble selenium compounds (such as selenates) into the water, which may then be concentrated in new "wetlands" as the water evaporates. High selenium levels produced in this fashion have been found to have caused certain congenital disorders in wetland birds.

Relationship between survival of juvenile salmon and concentration of selenium in their tissues after 90 days (Chinook salmon: Hamilton et al. 1990) or 45 days (Atlantic salmon: Poston et al. 1976) exposure to dietary selenium. The 10% lethality level (LC10=1.84 µg/g) was derived by applying the biphasic model of Brain and Cousens (1989) to only the Chinook salmon data. The Chinook salmon data comprise two series of dietary treatments, combined here because the effects on survival are indistinguishable.

In fish and other wildlife, low levels of selenium cause deficiency while high levels cause toxicity. For example, in salmon, the optimal concentration of selenium in the fish tissue (whole body) is about 1 microgram selenium per gram of tissue (dry weight). At levels much below that concentration, young salmon die from selenium deficiency; much above that level they die from toxic excess.

Deficiency

Main article: Selenium deficiency

Selenium deficiency can occur in patients with severely compromised intestinal function, those undergoing total parenteral nutrition, and in those of advanced age (over 90). Also, people dependent on food grown from selenium-deficient soil are at risk. Although New Zealand has low levels of selenium in its soil, adverse health effects have not been detected.

Selenium deficiency as defined by low (<60% of normal) selenoenzyme activity levels in brain and endocrine tissues only occurs when a low selenium status is linked with an additional stress, such as high exposures to mercury or as a result of increased oxidant stress due to vitamin E deficiency.

Selenium interacts with other nutrients, such as iodide and vitamin E. The interaction is observed in the etiology of many deficiency diseases in animals, and pure selenium deficiency is rare. The effect of selenium deficiency on health remains uncertain, particularly in relation to Kashin-Beck disease.

Dietary recommendations

The US Institute of Medicine (IOM) updated Estimated Average Requirements (EARs) and Recommended Dietary Allowances (RDAs) for selenium in 2000. If there is not sufficient information to establish EARs and RDAs, an estimate designated Adequate Intake (AI) is used instead. The current EAR for selenium for people ages 14 and up is 45 μg/day. The RDA is 55 μg/day. RDAs are higher than EARs so as to identify amounts that will cover people with higher than average requirements. RDA for pregnancy is 60 μg/day. RDA for lactation is 70 μg/day. For children ages 1–13 years the RDA increases with age from 20 to 40 μg/day. As for safety, the IOM sets Tolerable upper intake levels (ULs) for vitamins and minerals when evidence is sufficient. In the case of selenium the UL is 400 μg/day. Collectively the EARs, RDAs, AIs and ULs are referred to as Dietary Reference Intakes (DRIs).

The European Food Safety Authority (EFSA) refers to the collective set of information as Dietary Reference Values, with Population Reference Intake (PRI) instead of RDA, and Average Requirement instead of EAR. AI and UL defined the same as in United States. For women and men ages 15 and older the AI is set at 70 μg/day. AI for pregnancy is 70 μg/day, for lactation 85 μg/day. For children ages 1–14 years the AIs increase with age from 15 to 55 μg/day. These AIs are higher than the U.S. RDAs. The European Food Safety Authority reviewed the same safety question and set its UL at 300 μg/day, which is lower than the U.S. value.

In the United States, selenium deficiency is not common. A federal survey of food consumption determined that for women and men over the age of 19, average consumption from foods and beverages was 89 and 125 μg/day, respectively. For women and men of all ages fewer than 3% consumed less than the EAR.

Labeling

For US food and dietary supplement labeling purposes the amount in a serving is expressed as a percent of Daily Value (%DV). For selenium labeling purposes 100% of the Daily Value was 70 μg, but as of May 27, 2016 it was revised to 55 μg. A table of the old and new adult daily values is provided at Reference Daily Intake.

Food sources

Dietary selenium comes from nuts, cereals, meat, mushrooms, fish, and eggs. Brazil nuts are the richest ordinary dietary source and could cause selenium toxicity if consumed regularly – though the actual concentration of selenium (as with any plant-based food sources, such as another selenium-accumulating "paradise nut" Lecythis, belonging to the same family Lecythidaceae) is soil-dependent and may vary significantly by geographic location. In descending order of concentration, high levels are also found in kidney, tuna, crab, and lobster.

The human body's content of selenium is believed to be in the 13–20 milligram range.

Human health

Selenium in cancer

Selenium at nutritional levels is required for cell homeostasis, playing a role as an anti-oxidant through selenoproteins, thus, act chemo-preventive against cancer. In contrast, supra-nutritional levels act as pro-oxidant toxic in tumour cells.

Selenium has bimodal biological action depending on the concentration. At low nutritional doses, selenium acts as an antioxidant through selenoproteins, scavenging ROS, supporting cell survival and growth; while, at supra-nutritional higher pharmacological doses, selenium acts as a pro-oxidant generating ROS and inducing cell death. In cancer, studies have been conducted mostly on the benefits of selenium intake in reducing the risk of cancer incidence at the nutritional level; however, fewer studies have explored the effects of supra-nutritional or pharmacological doses of selenium on cancer.

"Although an inverse association between selenium exposure and the risk of some types of cancer was found in some observational studies, this cannot be taken as evidence of a causal relation, and these results should be interpreted with caution... Conflicting results including inverse, null and direct associations have been reported for some cancer types... RCTs assessing the effects of selenium supplementation on cancer risk have yielded inconsistent results... To date, no convincing evidence suggests that selenium supplements can prevent cancer in humans."

Selenium in anti-tumour immunity

To date, many studies have been conducted on the benefits of selenium intake in reducing the risk of cancer incidence at the nutritional level, indicating that likely selenium functions as an immunostimulator, i.e. reversing the immunosuppression in tumour microenvironment towards antitumour immunity by activating immune cells (e.g. M1 macrophages and CD8+ T-lymphocytes, the elevated number of neutrophils and activated cytotoxic NK cells) and releasing pro-inflammatory cytokines such as IFNγ and TNFα.

Selenium can either play a pro-oxidant role inducing ROS, activating the Akt–NF–кB pathway or play an antioxidant role through selenoprotein synthesis such as TXNRD relocating to the nucleus and activating NF-кB, resulting in further activation of leukocytes and pro-inflammatory cytokine genes.

HIV/AIDS

AIDS appears to involve a slow and progressive decline in levels of selenium in the body. Whether this decline in selenium levels is a direct result of the replication of HIV or related more generally to the overall malabsorption of nutrients by AIDS patients remains debated. Observational studies have found an association between decreased selenium levels and poorer outcomes in patients with HIV, though these studies were mostly done prior to the currently effective treatments with highly active antiretroviral therapy (HAART). Currently there is inadequate evidence to recommend routine selenium supplementation for HIV patients, and further research is recommended.

Mortality

Selenium supplementation has no effect on overall mortality.

Tuberculosis

As with other types of supplementation, there is no good evidence selenium supplementation helps in the treatment of tuberculosis.

Diabetes

A meta-analysis of four RCTs concluded that there is no support for selenium supplementation for prevention of type 2 diabetes mellitus in Caucasians.

Human reproductive system

Abnormally high or low levels of dietary selenium can have an adverse effect on sperm quality, with a consequent lowering of fertility.

COVID-19

During the COVID-19 pandemic, some studies tried to correlate between selenium plasma level and severity of COVID-19 cases. One study done on 33 patients concluded that low plasma selenium levels were correlated with a high mortality rate among COVID-19 patients. However, the median age of deaths in this study was 89 years old; in contrast, survivors' median age was 69 years old, and the study stated that the causality remains unknown. On the other hand, another study revealed that the mean selenium plasma level was within the normal range among all included COVID-19 patients; however, the mean selenium plasma level was elevated among severe cases of COVID-19. This study concluded that there was a significant elevation of selenium serum level among severe cases compared to non-severe cases of COVID-19, and could be correlated with the disease severity.

Mercury poisoning

Selenium has a protective effect towards mercury toxicity. Mercury binds to selenium with high affinity, so this metal can inhibit selenium-dependent enzymes. However, increased selenium intake can preserve the enzyme activities, reducing the adverse effects caused by mercury exposition.

Evolution in biology and biosynthetic considerations

Selenium is incorporated into several prokaryotic selenoprotein families in bacteria, archaea, and eukaryotes as selenocysteine, where selenoprotein peroxiredoxins protect bacterial and eukaryotic cells against oxidative damage. Selenoprotein families of GSH-Px and the deiodinases of eukaryotic cells seem to have a bacterial phylogenetic origin. The selenocysteine-containing form occurs in species as diverse as green algae, diatoms, sea urchin, fish and chicken. Selenium enzymes are involved in utilization of the small reducing molecules glutathione and thioredoxin.

Trace elements involved in GSH-Px and superoxide dismutase enzymes activities, i.e. selenium, vanadium, magnesium, copper, and zinc, may have been lacking in some terrestrial mineral-deficient areas. Marine organisms retained and sometimes expanded their seleno-proteomes, whereas the seleno-proteomes of some terrestrial organisms were reduced or completely lost. These findings suggest that aquatic life supports selenium utilization, whereas terrestrial habitats lead to reduced use of this trace element. Marine fishes and vertebrate thyroid glands have the highest concentration of selenium and iodine. From about 500 Mya, freshwater and terrestrial plants slowly optimized the production of "new" endogenous antioxidants such as ascorbic acid (Vitamin C), polyphenols (including flavonoids), tocopherols, etc. A few of these appeared more recently, in the last 50–200 million years, in fruits and flowers of angiosperm plants. In fact, the angiosperms (the dominant type of plant today) and most of their antioxidant pigments evolved during the late Jurassic period.

About 200 Mya, new selenoproteins were developed as mammalian GSH-Px enzymes.