A conformational study of peptides with the general structure Ac-L-Xaa-Pro-D-Xaa-L-Xaa-NH_2: spectroscopic evidence for a peptide with significant β-turn character in water and in dimethyl sulfoxide

Several tetrapeptides, Ac-Val-Pro-D-Ser-His-NH_2, in particular, show significant type II β-turn character in water and in dimethyl sulfoxide. Evidence for this turn population is provided by 2D-rotating frame nuclear Overhauser effect (ROESY) spectroscopy, ^1H NMR amide temperature coefficients, and circular dichroism (CD) studies. To further investigate which residues specifically contribute to the integrity of the turn, studies on 10 tetrapeptides, having the general sequence AC-LXaa-Pro-D-Xaa-L-Xaa-NH_2, are described. The results show the effects of sequence variations on the type II β-turn forming propensity of these peptides in solution. Conclusions from these studies indicate that a cooperative effect between a sterically hindered, β-branched amino acid at the (i) position and a small, non-β-branched D-amino acid at the (i+2) position promotes turn formation. Implications for use of these sequences as structural nucleation elements in de novo protein design are discussed.