Sulfate bonding modes

The sulfate ions HSO4 and SOI" form numerous complexes. Sulfate can have the following bonding modes 144... 

C9H19-C6H4-0-(CH2-CH2-0)2-CH2-S03(NP(E0)2- S03). In contrast to sulfate, carboxylate and phosphate homologues no bond cleavage could be observed in APCI(+) mode. The sulfonate could also be ionised in the positive mode resulting in a [M + NH4]+ ion at m/z 420 [15]. 

Phosphosulfates may react with a nucleophile (Nu) in either of the two modes of P-O or S-O bond fission (Figure 2). If water is the nucleophile, both modes of fission result in the same hydrolysis products. Mechanistically, however, the enzymes that catalyze P—O fission may be regarded as phosphatases, while those that catalyze S—O fission are sulfohydrolases. In fact, many hydrolytic enzymes are assumed to be sulfohydrolases without mechanistic proof. The possibility that they might be phosphatases was suggested by Roy by taking account their metal ion dependency (4). Meanwhile, PAPS acts as the sulfate donor to numerous nucleophilic acceptors such as steroids and phenols. In such sulfate transfer reactions, S—O fission must occur. PAPS and APS also are known to act as the key intermediates in the reduction of sulfate to sulfite. Here again, the S—O fission may be the most probable mode. 

The thiosulfate anion is structurally very similar to sulfate and has a range of coordination modes (Figure 25). Two examples of S-bonded thiosulfate complexes are shown in Figure 26 and 27, and the substitution chemistry of [0s02(S203)2] is summarized in Figure 28. Thiosulfates have a diverse range of applications. Both sodium and... 

Proposed mechanism for the catalytic hydrolysis of the sulfate ester bond by the active form of aryl sulfatase-B (AS-B), and the structure (right) of the active centre of the vanadate AS-B complex, otherwise containing the substrate sulfate. The active centre also accommodates a seven-coordinated calcium ion, ligated to three aspartates (one of which is in the Y-mode), an asparagine and two 0X0 anions of the sulfate/vanadate. Structure modified from that provided in ref. 95. 

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