Sulfur cycle oxidation states

Like sulfur, nitrogen has stable compounds in a wide range of oxidation states and many of them are foimd in the atmosphere. Again, both gaseous and particulate forms exist as do a large number of water-soluble compounds. Table 7-5 lists the gaseous forms. The nitrogen cycle is discussed in Chapter 12. 

For both polluted and remote conditions, therefore, the cycling of sulfur from low oxidation state gas to sulfate particles and then back to the surface in rain takes place on a time scale of a few days. 

Fig. 10. Hypothetical reaction cycle for D. gigas hydrogenase, based on the EPR and redox properties of the nickel (Table II). Only the nickel center and one [4Fe-4S] cluster are shown. Step 1 enzyme, in the activated conformation and Ni(II) oxidation state, causes heterolytic cleavage of H2 to produce a Ni(II) hydride and a proton which might be associated with a ligand to the nickel or another base in the vicinity of the metal site. Step 2 intramolecular electron transfer to the iron-sulfur cluster produces a protonated Ni(I) site (giving the Ni-C signal). An alternative formulation of this species would be Ni(III) - H2. Step 3 reoxidation of the iron-sulfur cluster and release of a proton. Step 4 reoxidation of Ni and release of the other proton.

The electronic structure of sulfur is such that a variety of oxidation states are readily obtainable. It can be said that a sulfur cycle exists in nature, as noted in Fig. 1. 

Figure 3 Speculative model for the hydrogenase enzyme cycle such as that from D. gigas. The highest oxidation states of the enzyme are at the top, and each step down corresponds to a one-electron reduction. Some hydrons that are transferred to sites in the protein are not shown. Redox states of the iron-sulfur clusters are omitted.

Sulfur, in its reduced oxidation states, has a complex chemistry due to the formation of polysulfides and their facile interconversions. Pearson estimated the HS/HS" potential as 1.08 V by a thermochemical cycle,... 

In its fully oxidized state, sulfur exists as sulfate. Sulfate is the second most abundant anion in seawater, and the SO4- in marine environments represents a large, slowly cycled sulfur reservoir. 

Tetrahydrofolate, a carrier of activated one-carbon units, plays an important role in the metabolism of amino acids and nucleotides. This coenzyme carries one-carbon units at three oxidation states, which are interconvertible most reduced—methyl intermediate—methylene and most oxidized—formyl, formimino, and methenyl. The major donor of activated methyl groups is -adenosylmethionine, which is synthesized by the transfer of an adenosyl group from ATP to the sulfur atom of methionine. -Adenosylhomocysteine is formed when the activated methyl group is transferred to an acceptor. It is hydrolyzed to adenosine and homocysteine, the latter of which is then methylated to methionine to complete the activated methyl cycle. 

The Earth s sulfur cycle (Figure 22) transfers enormous amounts of this biologically important element through various reservoirs each year. Sulfur has a wide range of oxidation states and shows the ability to form a large number of oxides and oxyanions, many of which are found in the environment. It also has the potential to form polymeric species with a significant number of... 

Importantly, it also occurs naturally in several oxidation states and is, therefore, redox sensitive. Methylation and hydride formation are important, and sulfur and iron compounds play an important role in the cycling of selenium. Microbiological volatilization of organic selenium, particularly dimethyl selenide, is known to be an important factor in the loss of selenium from some selenium-rich soils and waters (Frankenberger and Arshad, 2001 Oremland, 1994). Phytoplankton can also promote the production of gaseous selenium compounds in the marine environment (Amouroux et aL, 2001). 

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