Conformational study of the cyclic hexapeptide L-Ala-L-Pro-Gly-L-Val-Gly-L-Val, by nuclear magnetic resonance spectroscopy

Abstract

The cyclohexapeptide [graphic omitted] has been synthesized and its conformational features have been derived from n.m.r. parameters in chloroform. The nuclear Overhauser effect (n.O.e) has been used to distinguish between the two glycine and valine residues in the molecule. ¹³C Spin–lattice relaxation times (T₁) have been measured and an average T₁ value of 189 ms with a range of 158–202 ms for the α-carbon atoms has been obtained, which is indicative of a relatively rigid molecule. The n.O.e. and the geminal coupling constants [²J] and ³J(NH–αCH) have been utilized to obtain an estimation of the torison angles ψ and ϕ. Two β-turns, formed between the Ala¹ NH and the Val⁴(CO (type II′) and the Val⁴ NH and the Ala¹ CO (type II), are proposed. The conformational properties of this cyclic molecule are discussed in relation to the conformational features of its linear counterpart.