Polypeptides mutarotation

The semi-empirical conformational energy calculations of poly-cis-5-ethylproline (PC5EP) predict that the helical structure may exist in two conformational forms such as I and II. Experimental results confirmed that in solution two major conformations may be assumed by the poly-cis-5-ethyl-D-proline. However, the calculations for poly-trans-5-ethyl-D-proline indicated that only one form may be allowed.Spectroscopic data (Circular Dichroism, NMR) showed the polypeptide exists in a poly-L-proline form-I-type helix and changes slowly to some intermediate conformation. The slow muta-rotation is partially due to the steric interactions of the ethyl group with the carbonyl group of the amide during the mutarotation. 

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