High resolution nuclear magnetic resonance studies of nigeran

Abstract

Polymer motion in solution can be studied by 13CNMR relaxation methods, which provide information about the correlation time for C-H vectors. 13C-Relaxation and Nuclear Overhauser Enhancement (NOE) data may frequently be combined to determine the dipole-dipole relaxation contribution. An alternative method is proposed based on a comparison of the proton spin-lattice relaxation rates of the centre proton resonances of an unlabelled molecule with the relaxation rates of the 13C satellites (from 13C labelled molecules). Selectively labelled nigeran which is an alternating 1 → 3 and 1 → 4 α- d -glucan has been investigated. The discussion in terms of the occurrence of different motions for each of the two units of the polymer requires an unambiguous assignment of the two anomeric carbons. For this reason a detailed assignment of the 1H and 13C Nuclear Magnetic Resonance (NMR) spectra of nigeran in dimethylsulphoxide-d 6 is described, based on T 1 and NOE measurements in addition to selective homonuclear and heteronuclear spin decoupling experiments. These values are correlated with a conformation estimated by HSEA hard-spheres calculation. The measurements of the relaxation parameters for labelled and unlabelled compounds which provide an alternative determination of the 13C- 1H dipole-dipole relaxation contribution in a macromolecule agree well with 13C-{ 1H} NOE experiments.