Evidence for a novel β‐bend structure with prolines at the corner: 1 H and 13 C NMR study of cyclo(Pro‐Pro‐Gly) 2

The solution conformation of cyclo‐(Pro‐Pro‐Gly) 2 in CDCl 3 and DMSO‐ d 6 was investigated by 1 H and 13 C NMR spectroscopy. In CDCl 3 solution the peptide maintains a conformational homogeneity at 300 K and the single asymmetric conformation observed is characterized by a single Pro‐Pro cis ‐peptide bond and an intramolecularly hydrogen‐bonded β‐bend structure involving CO of one glycine to the NH of the other glycine. The segment Gly‐Pro‐Pro‐Gly responsible for the β‐bend has a Pro‐Pro peptide bond in the trans configuration. However, in DMSO‐ d 6 solution, there is a conformational heterogeneity and the population of the two main conformers present is in the ratio of ca. 80:20. The major component is characterized as having two contiguous cis ‐peptide bonds involving Gly‐Pro and Pro‐Pro peptide bonds, while the minor component has a conformation characterized by a single Pro‐Pro cis ‐peptide bond which is included in the β‐bend structure. The equilibrium between the major and minor components is insensitive to temperature in the range 300‐‐360 K. However, the conformational homogeneity in CDCl 3 solution appears to be disturbed by addition of DMSO‐ d 6 and becomes detectable beyond 20% (v/v) of DMSO‐ d 6 and at 50:50 (v/v) the populations of the major and minor components become almost equal.