NMR study of the cooperative behavior of thermotropic model polypeptides

Abstract

Using 1H and 13C 1D and 2D NMR spectra, relaxation and PFG NMR measurements, the structural changes of two genetically engineered, and thus unimolecular, elastin-like polypentapeptides [(GVGVP)2-(GEGVP)-(GVGVP)2]n, n = 9 and 15, in D2O and H2O solutions and at two concentrations (5 and 0.5% wt), were studied in the temperature interval 288–330 K, i.e. below, at and above their inverse temperature transition (ITT). According to the extent of extreme signal broadening, the rigidity of both polymers increases at and above ITT. The process of polymer stiffening proceeds primarily from the polymer backbone and is accompanied by coiling of the polymer and aggregation. It is distinctly more effective in the polymer with higher molecular weight, as well as at higher concentration, which indicates intra- and intermolecular cooperativity of the process. According to the measured NHCH dihedral angles and chemical shifts, the conformation of the visible part of both polypeptides does not significantly differ from random coil conformation both below and above the transition temperatures. However, this could be due to fast averaging of more definite conformations. Tighter coiling of the NMR-visible part of the polymer during thermal transition is indicated by a decrease of its hydrodynamic radius, derived from the self-diffusion coefficient measured by PFG NMR. Lower values of HOD T1 relaxation time and lower slope of its increase compared to those of HOD in pure D2O suggest a remaining interaction of the polymer with water above ITT. Copyright © 2006 Society of Chemical Industry