NMR study of the rotational dynamics of linear homopolysaccharides in dilute solutions as a function of linkage position and stereochemistry

Variable temperature and magnetic field dependent 13C NMR relaxation measurements (T1, T2, and NOE) were carried out on a series of linear homopolysaccharides: α-(1→3)-d-glucan, β-(1→3)-d-glucan in Me2SO-d6, and α-(1→6)-d-glucan in D2O and Me2SO-d6 dilute solutions. The relaxation data of the backbone carbons were analyzed quantitatively by using a variety of theoretical unimodal and bimodal time-correlation functions in an attempt to describe the main carbohydrate chain dynamics as a function of linkage position and stereochemistry. Among these, the time-correlation function developed by Dejean, Laupretre, and Monnerie (DLM) offered the best quantitative description of the segmental motion of the carbohydrate chains. The internal rotation of the hydroxymethyl groups about the exocyclic C-5–C-6 bonds superimposed on segmental motion has been described as a diffusion process of restricted amplitude. Comparison of the dynamics of polysaccharides has been extended to include amylose and inulin studied previously. On the basis of the calculated correlation times for segmental motion, the flexibility of the carbohydrate chains decreases from inulin and dextran following the order; inulin>dextran>α-(1→3)-d-glucan>β-(1→3)-d-glucan∼amylose, whereas the rate and the amplitude of the internal rotation of the hydroxymethyl groups about the exocyclic C-5–C-6 bonds showed that the restriction of the hydroxymethyl internal rotation decreases from inulin to amylose following the order; inulin>α-(1→3)-d-glucan∼β-(1→3)-d-glucan>amylose. Solvent effects on segmental dynamics and the temperature–frequency superposition of the relaxation data of the three polysaccharides have been discussed as well.