proline isomerase assay 2012-08-10T08:56:38Z orchard PSI-MI Measurment of the conversion between cis- and trans- peptide bonds formed by the amine group of a proline. Peptide bonds to proline, and to other N-substituted amino acids (such as sarcosine), are able to populate both the cis and trans isomers the cis and trans isomers of the X-Pro peptide bond (where X represents any amino acid) both experience steric clashes with the neighboring substitution and are nearly equal energetically. Hence, the fraction of X-Pro peptide bonds in the cis isomer under unstrained conditions ranges from 10-40%; the fraction depends slightly on the preceding amino acid, with aromatic residues favoring the cis isomer slightly. MI:1236 isomerase assay