Sulphur oxoacids

Non-metals — These are often oxidised to the corresponding oxoacid, and nitrogen oxide is formed. For example, sulphur gives sulphuric acid with cold concentrated nitric acid ... 

Ferredoxins (Fds) are widespread in the three domains of life and an abundance of sequence data and structural information are available for Fds isolated from several sources. In particular, the bacterial type Fds are small electron-transfer proteins that posses cubane xFe-yS clusters attached to the protein matrix by Fe ligation of Cys via a conserved consensus ligating sequence. The archaeal type ferredoxins are water-soluble electron acceptors for the acyl-coenzyme A forming 2-oxoacid/ferredoxin oxidoreductase, a key enzyme involved in the central archaeal metabolic pathways. Fds have been distinguished according to the number of iron and inorganic sulphur atoms, 2Fe-2S, 4Fe-4S/3Fe-4S (Fig. Ib-d) and Zn-containing Fds. 

Such oxides may therefore be considered as acid anhydrides. Table above shows a selection of oxoacids with their anhydrides and illustrates the conventional nomenclature. For example, sulphurous and sulphuric acids are display the lower (+4) and higher (+6) oxidation state, respectively, and their anions are called sulphite and sulphate. 

HS03) and sulphite (S032-) ions. The expected sulphurous acid H2S03, however, is present only in very low concentrations. Se02 and Te02 have polymeric structures and give oxoacids salt similar to those from sulphur. 

There are many other oxoacids of sulphur, of which the most important are peroxodisulphte S2Os2-, which has a peroxo (0 0) bond, and compounds with S S bonds including thiosulphate S2032, dithionite S2042- and tetrathionate S4062-. The reaction... 

The proposed catalytic mechanism of the ferredoxin oxidoreductase [32] is shown in Fig. 4, a similar mechanism existing for the analogous citric acid cycle enzyme, 2-oxoglutarate oxidoreductase. In outline, the 2-oxoacid is decarboxylated in a TPP-dependent reaction to give an hydroxyalkyl-TPP. From this, one electron is abstracted and transferred to the enzyme-bound iron-sulphur cluster, generating a free-radical-TPP species. This intermediate can then interact direct with coenzyme-A to form acyl-CoA, the iron-cluster receiving the second electron. In each case, ferredoxin serves to re-oxidise the enzyme s redox centre. 

Oxoacids of Sulphur Sulphur fonus a number of oxoacids, many of the oxoacids of sulphur do not exist free acids, but are known as anions. Oxoacids of sulphur may be classified into following four series ... 

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