Ring puckering of the pyrrolidine ring of poly ( l-proline) form I as studied by variable-temperature high-resolution 13C NMR spectroscopy

Abstract

A number of studies have been made on the structural role of proline residues in proteins and naturally occurring cyclic peptides because the flexibility of the proline ring influences the degree of backbone order in proteins [l]. Hence, information on the nature of the side-chain conformation and the mobility in poly(r_-proline) is of much interest. It is well known that poly(L-proline) takes two kinds of specific main-chain forms, form I and form II, in the crystalline state [2-81. In form I the imide group is in the cis configuration and in form II it is in the trans conformation. In our previous work 191, we measured high-resolution 13C NMR spectra of poly(L-proline) form II in the solid state over a wide range of temperatures and found the existence of two kinds of energetically stable conformations for the pyrrolidine ring undergoing no chemical exchange. The aim of this work is to determine the sidechain conformation and the molecular motions of poly(L-proline) form I in the crystalline state