N.m.r studies of internal mobility in a cyclic tetrapeptide

The conformation of cyclo(D-Phe-D-Pro-Ala-Pro) is reported. Measurements of spin-lattice relaxation in the rotating frame indicate that this peptide is conformationally less mobile on the microsecond time scale than larger cyclic peptides previously studied. Libration of the Pro-Ala and Pro-Phe peptide bond planes is suggested as the source of the small exchange contributions to 1/T1p.