Characterization of leucine side-chain reorientation in collagen-fibrils by solid-state 2H NMR.

Abstract

Quadrupole-echo PMR spectroscopy was used to study the molecular dynamics of the leucine side chain in [chick] collagen fibrils labeled with [2H10]leucine. X-ray crystallographic studies of leucine and small leucyl-containing peptides and proteins previously showed that the amino acid side chain exists predominantly in only 2 of the 9 possible conformations. PMR spectra of polycrystalline D,L [2H10]leucine obtained from -45.degree. to +100.degree. C showed that interconversion of the 2 conformations did not take place on the PMR timescale in this temperature range. Experimental lineshapes observed for [2H10]leucine-labeled collagen fibrils from -85.degree. - +30.degree. C were simulated by using a model in which the side chain hops at various rates between the 2 predominant conformations found by the X-ray studies. A small difference between calculated and observed linewidths above the freezing point of water can be accounted for by backbone reorientation or by the presence of a small percentage of other side-chain conformations. The 2 predominant X-ray conformations not only exist in the fibrils as the preferred orientations but interconvert at rates that are proportional to temperature over the range -85- +30.degree. C. Apparently, there is a mobile contact region between collagen molecules.