Peptide models for beta-turns. A circular dichroism study.

The circular dichroism spectra of four beta-turn model peptides, Z-Aib-Pro-Aib-Pro-OMe (1), Piv-Pro-Aib- NHMe (2), Piv-Pro-D-Ala- NHMe (3) and Piv-Pro-Val- NHMe (4) have been examined under a wide range of solvent conditions, using methanol, hexafluoroisopropanol and cyclohexane. Type I and Type II beta-turns have been observed for peptides 1 and 2 respectively, in the solid state, while the Pro-D-Ala sequence adopts a Type II beta-turn in a related peptide crystal structure. A class C spectrum is observed for 1 in various solvents suggesting a variant of a Type I (III) structure. The Type II beta-turn is characterized by a CD spectrum having two positive CD bands at approximately 230 nm and approximately 202 nm, a feature observed in Piv-Pro-D-Ala- NHMe in cyclohexane and methanol and for Piv-Pro-Aib- NHMe in methanol. Peptide 2 exhibits solvent dependent CD spectra, which may be rationalized by considering Type II, III and V reverse turn structures. Piv-Pro-Val- NHMe adopts non-beta-turn structures in polar solvents, but exhibits a class B spectrum in cyclohexane suggesting a population of Type I beta-turns.