cis-Amide Bond  
  Cis form is rarely found in native proteins, for in trans there are fewer steric collisions.

1. Cis-peptide is about 1000 times less stable than the trans-form.

2. When the second residue is Proline, the cis is only about four times less stable than the trans-form.
Cis-trans isomerization of proline peptides is a slow process and it is a frequently the rate-limiting step in folding for those proteins that have been trapped in a folding "wrong- formed isomer" intermediate.

Example:

staphylococcal nuclease, the Lys-Pro bond isomerizes within 17 sec.

Chymotrypsin Inhibitor 2, the trans->cis Trp-Pro isomerization is 2.5 sec.(JMB, 1997,269, 611-622, A. Fersht)
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