# Standard dimensions for cis N-methyl peptide unit and flexibility of cis peptide units

Ramnarayan, K and Ramakrishnan, C (1987) Standard dimensions for cis N-methyl peptide unit and flexibility of cis peptide units. In: Journal of Biosciences, 12 (4). pp. 331-347.

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The mean dimensions of the cis N-methyl peptide unit have been arrived at by analysing the crystal structure data on compounds containing such units. These dimensions can be used as standard in conformational studies on cyclic peptides. While the bonds meeting at C are almost coplanar, those meeting at Ν show a slight pyramidal disposition. A comparison of the dimensions of the normal and N-methylated cis peptide units show that there are perceptible differences in the parameters connected with N. In addition, the flexibility of the cis peptide unit has been analysed by studying the distribution of the parameters in different classes of compounds such as cyclic di, tri and higher peptides. The salient features are: (i) The angle $C^\alpha CN$ in cyclic dipeptide and the angle $C^\delta NC^\alpha$ in higher peptides tend to be lower, when the peptide unit is associated with a prolyl residue; (ii) in cyclic tripeptides the internal angles viz., $C^\alpha CN$ and $CNC^\alpha$ are significantly larger thereby increasing the intra-annular space; (iii) the bond $C^\alpha -C$ is distinctly shorter when it occurs in cyclic dipeptides. The results lead to the conclusion that the cis peptide unit takes up a need-based flexibility in its dimension.