in your element Under sulfur’s spell Thomas Rauchfuss marvels at the diversity of sulfur reactivity. Although it poisons most industrial catalysts, it adopts many forms in nature and takes on a variety of biological roles — including that of a biocatalyst. hat is sulfur? This is a tricky gold surfaces. Self-assembled monolayers question, because many species are crucial in nanotechnology because they Wwith the formula Sx are known. provide a responsive interface between mobile The most stable at room temperature is the phases (gases and liquids) and an electrically crown-shaped ring S8, but elemental sulfur conductive device. also contains small amounts of the bright- The role of metal–sulfur bonds in yellow S7 and tiny amounts of other rings. On biocatalysis is a popular area of research. heating, sulfur readily converts to a metastable One of the challenges is to elucidate the one-dimensional elastomer, which quickly mechanism of methanogenesis — the source degrades at room temperature back to the S8 of the majority of Earth’s natural gas and form. The tendency of sulfur to form rings a contributor to climate change. The final and chains (to catenate) is its most distinctive stage of methanogenesis involves scission of property. Only at high temperatures does a methyl–sulfur bond in coenzyme-M (first sulfur form S2, the analogue of O2. isolated from the sewers of Urbana, my home The catenating tendencies of sulfur are town). The allure of this area is heightened by also evident in its anions — the polysulfides / JPL UNIVERSITY OF ARIZONA © NASA the discovery that methanogenesis can run in with formula S 2–. They arise by simply adding reverse, through a nickel-catalysed reaction x Io, a moon of Jupiter, has diverse colours owing to small amounts of reducing agents to elemental of methane2. the presence of various sulfur species. sulfur. These chains of sulfur atoms, which Transformations relevant to ‘bioenergy’ resemble alkanes in their conformation, can are effected by Fe–S clusters, including the be extended or further shortened by redox cystine rigidifies proteins in a manner conversions of CO2 to CO, of protons to H2, reactions, a process that is exploited in the similar to the crosslinking provided by the and of N2 to NH3 (ref. 3). These processes all sodium–sulfur battery. The anionic chains S–S bonds in vulcanized rubber. Otherwise, involve the coordinated movement of protons exist in dynamic equilibrium with radicals the thiol group is a favoured site for post- and electrons — a mechanistic theme of •– such as S3 — the blue chromophore in lapis transcriptional modifications. Sulfur is immense significance. The active sites of the lazuli. The anions that terminate these chains featured in many cofactors and vitamins, for relevant enzymes consist of metal clusters can be capped with alkylating agents, protons example, thiamine, biotin and lipoic acid. where sulfide ligands help ‘glue’ together the and metal cations1. With metals, one obtains Elucidation of their biosyntheses has been a metals. Because changes in electron count compounds with such improbable formulae as trove of novel mechanisms. barely affect their structures, metal–sulfide 2– 2– PtS15 and Fe2S12 . As sulfur has occurred in proteins for clusters are able to quickly accept and relay Sulfur is readily oxidized, notably to millennia, fossil fuels often contain a few per electrons, as required for efficient catalysis. dioxide and trioxide species. Depending on cent by weight of organosulfur compounds, Apparently the presence of the soft sulfur their environment, microorganisms subsist most problematically in the form of ligands also enables the metal centres in these on the hydrolysed derivatives of these oxides thiophenes. A continuing focus in petroleum catalysts to bind and thereby activate weakly through anaerobic respiration. Sulfur dioxide, refining is the removal of this sulfur, especially basic substrates such as H2, CO2 and N2 — an electrophilic, bent analogue of carbon in view of the 5 ppm allowed limit of sulfur ligands normally reserved for organometallic dioxide, is produced on a massive, even in many diesel fuels. This technology, called compounds. Our ability to master these weather-changing, scale by volcanic eruptions. hydro-desulfurization, relies on modified transformations could be key to our future. ❐ Further oxidation gives the trioxide precursor molybdenum sulfide catalysts to yield a low- to sulfuric acid, produced annually on the sulfur product and hydrogen sulfide waste. THOMAS RAUCHFUSS is at the Department scale of about 140 megatons. Oxidation of the waste produces vast amounts of Chemistry, University of Illinois, 600 South Sulfur is featured in two coded amino of sulfur, which beckons for new applications. Mathews Avenue, Urbana, Illinois 61801, USA. acids, methionine, which is fairly boring Thiolates exhibit particular affinity for e-mail: [email protected] to the sulfur chemist, and cysteine, which metal ions; in enzyme active sites metals are References has spectacularly diverse roles. By forming commonly anchored via cysteinyl residues. 1. Devillanova, F. A. (ed.) Handbook of Chalcogen Chemistry: New S–S bonds, the conversion of cysteine to The resilience of the metal–thiolate bond is Perspectives in Sulfur, Selenium and Tellurium (Royal Society of exploited in the role of thiols in the formation Chemistry, 2006). 2. Thauer, R. K. Angew. Chem. Int. Ed. 49, 6712–6713 (2010). of self-assembled monolayers. These materials 3. Fontecilla-Camps, J. C., Amara, P., Cavazza, C., Nicolet, Y. & are formed by simply contacting thiols with Volbeda, A. Nature 460, 814–822 (2009). Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As 648 NATURE CHEMISTRY | VOL 3 | AUGUST 2011 | www.nature.com/naturechemistry © 2011 Macmillan Publishers Limited. All rights reserved.