in your element Selenium stories Russell Boyd ponders on how selenium — despite close similarities with its neighbours of the chalcogen family, sulfur and tellurium — continues to reveal chemical and biological activities of its own.

lement 34 was discovered in 1817 by preventing cellular damage from radicals Jöns Jacob Berzelius, the ‘father of produced as by-products of oxygen ESwedish chemistry’. While preparing metabolism, and they may also prevent, sulfuric acid he noticed a residue, which or slow, tumour growth by enhancing he first thought was tellurium. Realizing it immune-cell activity. was a new element, he decided to name it In many countries, meat, seafood, after the Greek word for Moon,selènè , in rice, noodles and bread are common a similar manner to tellurium, named two sources of dietary selenium, but ingesting decades earlier by Martin Heinrich Klaproth too little or too much can have serious after the Latin word for Earth,tellus . consequences. The recommended2 dietary In nature, selenium is rarely found in its allowance (55 μg per day for adults elemental form and only occurs in a few according to the Institute of Medicine minerals — for example in sulfide ores such Most of the selenium produced of the National Academy of Sciences in as pyrite, where it partially replaces sulfur. worldwide is isolated from the mud that the US) can be supplied by a single dried It exists as six naturally occurring isotopes forms at the anode during the electrolytic Brazil-nut. Selenium deficiency can cause with mass numbers 74, 76, 77, 78, 80 and refining of copper. Selenium was once heart disease or weaken the immune 82; 80Se and 78Se are the most common, with an essential material in photocopying, system, whereas excessive ingestion leads natural abundances close to 50 and 24%, but has largely been replaced by organic to selenosis (selenium poisoning), with respectively. Selenium is a semi-metallic photoconductors. Combined with symptoms such as discoloration of the element that belongs to the family of bismuth, it has replaced lead in plumbing skin, a garlic odour to the breath and lack chalcogens (group 15). Placed just between brasses since the 1990s to meet lead-free of mental alertness — more than 5 mg per sulfur and tellurium in that column of the environmental standards. Selenium is day can be fatal. The tolerable upper intake periodic table, it resembles both elements now particularly promising for various level set by the Institute of Medicine is in some aspects. In particular, it has similar electronic devices — the grey allotrope, 400 μg per day for adults; it thus cautions allotropic forms and compounds to those of its most stable form, is a semiconductor that many dietary supplements contain

sulfur — for example, red selenium is a Se8 that conducts electricity better in the light 50 to 200 μg per daily dose. macrocycle similar to the sulfur allotrope S8. than in the dark, and serves in photovoltaic Elemental selenium is usually cells. The grey form also converts electric assumed to be harmless, but many of The recommended daily current from a.c. to d.c., which explains its compounds, for example hydrogen why it is a component in rectifiers. The selenide (H2Se) are extremely toxic. This dietary allowance of selenium largest worldwide use of selenium, however, is why the general perception is that can be supplied by a single resides in glass manufacturing, where it is selenium is toxic, even though modern used as a dopant to produce vivid red and science has recognized it as an essential dried Brazil-nut. pink colours. micronutrient. Recent advances indicate One hundred and forty years passed that selenium has the potential to improve Although standard inorganic chemistry between the discovery of selenium human lives in varied ways, including as textbooks often seem to imply that the and its recognition as essential for part of more efficient solar cells2, synthetic chemistry of selenium is not as well cellular function in most mammals. antioxidants3 or nanocluster coatings for developed as that of sulfur, it certainly It is incorporated in proteins through use in orthopaedic applications4. ❐ presents interesting reactivity. Selenic acid the amino acids selenocysteine and References (H2SeO4) resembles sulfuric acid (H2SO4) selenomethionine — in which it as its first proton also fully dissociates in replaces the sulfur atom of cysteine and 1. Combs, G. F. Jr BioFactors 14, 153–159 (2001). 2. Mayer, M. A. Appl. Phys. Lett. 97, 022104 (2010). water, yet it is a more powerful oxidant, methionine, respectively. Selenoproteins, 3. Heverly-Coulson, G. S. J. Phys. Chem. 114, 10706–10711 (2010). capable, for example, of releasing Cl2 from in turn incorporated in enzymes, are 4. Tran, P. A. Int. J. Nanomed. 5, 351–358 (2010). concentrated HCl or of dissolving gold to essential components of several metabolic form gold(iii) selenate. pathways, including thyroid hormone RUSSELL BOYD is at the Department of metabolism, antioxidant defence systems Chemistry, Dalhousie University, Halifax, or immune functions. There is evidence1 Nova Scotia B3H 4R2, Canada. that selenoproteins reduce cancer risk by e-mail: [email protected] Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb 570 NATURE CHEMISTRY | VOL 3 | JULY 2011 | www.nature.com/naturechemistry

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