Individual chain conformation, geometry

A number of simple diarsines have been structurally investigated and, although individual molecules are similar to those of the corresponding distibines, the diarsines do not show the extended chain structures and consequent thermochromism often found for the latter. The geometry at the two arsenic atoms is pyramidal, though the angles at arsenic can be asymmetric. The substituents occupy anti (gauche) conformations and in some cases the molecules have imposed Cj symmetry. Parameters for four of the compounds are summarized in Table 2. 

An analysis by Chakrabarti [21] of protein structures in the PDB showed that metal ions approach the sulfur of methionine at about 38(5)° from the perpendicular to the C-S-C group. This is similar to values in the range foimd, as just described, for smaU-molecule crystal structures in which the metal ion is presumed to interact with a sulfur lone-pair orbital [18]. It was also found that metal ions approach cysteine residues such that the M- -S-C-C torsion angle is 90° or 180°, and that the conformation of the cysteine side chain is generally affected by the metal ion. The metal ions that readily bind to sulfur in proteins are copper, iron, mercury, and zinc. The geometry of binding of metal ions to methionine or cysteine did not appear to depend on the identity of the individual metal. 

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