Two cis‐prolines in the Bence‐Jones protein Rei and the cis‐pro‐bend

The stabilities of the cisand transforms of the X-Pro peptide group differ only slightly by -2.0 to 2.0 kcal/mol-’ for linear polypeptides [ 11. In small cyclic peptides, the ring closure enforces the formation of cis-peptide groups [2,3]. It is surprising, therefore, that there are only few reports of X-Pro cis-peptide groups found in globular protein molecules. Furthermore, most of these are questioned by the authors due to the uncertainty in the interpretation of a Fourier map calculated with phases obtained from isomorphous replacement [4,5a,Sb,6-81. During the course of the constrained crystallographic refinement of the crystal structure of the Bence-Jones Protein Rei [9a,9b] we found evidence for two X-Pro cis-peptide groups and were able to confirm this by difference Fourier methods.

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