Remember that the cis and trans configuration of the amino acids in polypeptides is due to the parial double bond character of the peptide bonds. Therefore, the trans configuration is much preferred in ordinary peptide bonds so as to avoid steric strain due to having the bulky side chains in the same side.
At equilibrium, the ratio of trans to cis isomers for the X-Pro peptide bond is about 1:1, whereas the ratio is 1000:1 in favor of the trans isomer for the ordinary peptide bonds.
How do you account for unusual stability of a cis X-Pro?
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