Structure and dynamics of the branched polysaccharide scleroglucan in dilute solutions studied by 1D and 2D NMR spectroscopy

Abstract

Variable temperature and magnetic field dependent 13C NMR relaxation measurements ( T 1 and NOE) were carried out for the branched polysaccharide scleroglucan in DMSO-d 6 dilute solutions. The relaxation data of the backbone carbons were analyzed quantitatively by using the bimodal time-correlation functions developed by Dejean, Lauprêtre, and Monnerie (DLM), which offered the best quantitative description of the segmental motion of the carbohydrate chains. Simple internal rotations of the free hydroxymethyl groups of the backbone rings about the exocyclic C-5–C-6 bonds superimposed on segmental motion have been described as a diffusion process of restricted amplitude. Multiple internal rotations involving the exocyclic hydroxymethyl group of the ring at the branched point and that of the side chain ring, as well as the side chain ring itself were described by employing a composite TCF for side chain motions superimposed on polymer segmental motions. These time-correlation functions and their Fourier transform pairs, the spectral densities, offered the best interpretation of the relaxation data of the exocyclic free hydroxymethyl groups, and the pendent d-glucorynosyl ring. Finally, the 1H- and 13C NMR spectra of the neutral scleroglucan in DMSO-d 6 were analyzed by employing a series of 2D NMR experiments. The 500 MHz 1H NMR spectrum of scleroglucan at 283 K show severe peak overlaps, which do not allow the complete assignment of all signals in the carbon spectrum.