The 13C-NMR Spectra of Hyaluronate and Chondroitin Sulphates. Further Evidence on an Alkali-Induced Conformation Change

Abstract

Complete assignments are given for the ¹³C NMR spectra of hyaluronate and chondroitin in deuterium oxide solution at 50.32 MHz. The assignments published earlier for chondroitin 6-sulphate and chondroitin 4-sulphate were largely confirmed but were found to need some revision in detail. Our conclusions for hyaluronate and the chondroitin sulphates were confirmed by off-resonance experiments based on the proton NMR assignments. The spectra for hyaluronate show line narrowing and chemical shift changes from neutral to alkaline solution which are consistent with, and clearer than, the effects reported earlier for the proton spectra. As before no such changes occur for the chondroitin sulphates. The suggested interpretation is in terms of a conformation change for hyaluronate which originates mainly in altered interaction energies across the 1 → 3 linkage with the results that motional freedom is enhanced above that of the parent hyaluronate and even above the chondroitin sulphates. This, and the evidence from a temperature effect, suggests that an additional potential energy minimum is made favourable in alkali so that the overall amplitudes of the bond oscillations are increased.