Structure and Conformation of a Cyclic Tripeptide

Abstract

MANY cyclic peptides have interesting biological functions and the details of their molecular structure and conformation have been the subject of extensive investigations. Cyclic dipeptides such as diketopiperazine have been synthesised and shown to occur with the peptide units in the cis configuration^1,2. In the case of a tripeptide, cyclisation can take place only if all three units are in the cis configuration³. In cyclic peptides with four units also, cis peptides are found^4,5. As the number of the peptide units increases, the more stable trans configuration is generally more common^6,7. We report here the main results of our X-ray crystallographic investigations of the cyclic tripeptides L-Pro-L-Pro-L-Pro and L-Pro-L-Pro-L-Hyp (hereafter called CTP 1 and CTP 2, respectively). CTP 1 was synthesised by Rothe et al.⁸ and its derivatives have been prepared by Blout and his collaborators⁹.