Torsion angle, chain conformation geometry

The conformations of the probucol molecule in the two forms were found to be quite different. In Form II, the CSCSC chain is extended, and the molecular symmetry approximates Cjv This molecular symmetry is lost in the structure of Form I, where now the torsional angles around the two CS bonds deviate significantly from 180°. Steric crowding of the phenolic groups by the t-butyl groups was evident from deviations from trigonal geometry at two phenolic carbons in both... 

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. 

A statistical survey of crystal structures of nucleic acid constituents has led to the proposal that a full description of nucleoside molecular geometry can be given in terms of four parameters the phase angle of pseudorotation, P, the puckering amplitude, the conformation of the side-chain, 5, and the glycosidic torsional angle, X these allow the variability of bond and torsion angles to be described. ... 

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