Conformational Properties of the Amino Acid Residues L-Cysteine, L-Serine and L-Cystine.

Abstract
The correlation between torsion angles of the main chain and the orientation of the side chain in the solid phase has been investigated for the amino acids. L-cysteine, L-serine and L-cystine. Information has been obtained from the Cambridge Structural Database. In the crystal structures of cysteine and serine the torsion angle N-C.alpha.-C''-N (.psi.) often close to 0 or 180.degree.. When this is the case, N-C.alpha.-C.beta.-O.gamma./S.gamma. (.chi.1) is almost exclusively in the vicinity of 60.degree. (g+). When .psi. is in the interval 30-150.degree., only trans (t) or gauche- (g-) is observed. Thus, .chi.1 is strongly dependent on the conformation of the main chain for theses residues. Force-field energy calculations agree reasonably well with the experimental results. For L-cystine, g- is more frequently observed than g+ at .chi.1, .chi.2 and .chi.3 (disulfide bond) are both around +/- 85.degree.. The sign sequence for the five torsion angles along the disulfide bridge (.chi.1, .chi.2, .chi.3, .chi.2'', .chi.1'') are: cyclic peptides, --+-- or other; non-cyclic with twofold axis, +---+ or +++++; other non-cyclic, ----- or +++++.
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