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The circular dichroism spectra of four β‐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 β‐turns have been observed for peptides 1 and 2 respectively, in the solid state, while the Pro‐D‐Ala sequence adopts a Type II β‐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 β‐turn is characterized by a CD spectrum having two positive CD bands at ˜ 230 nm and ˜ 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‐β‐turn structures in polar solvents, but exhibits a class B spectrum in cyclohexane suggesting a population of Type I β‐turns.

The three-dimensional structure of a protein is the assembly of diferent scna strutural elements, such as a-helices, 3-pleated sheets, and -turns.