Comparison of the conformational dynamics of the (1 → 4)‐ and (1 → 6)‐linked α‐D‐glucans using 13C‐NMR relaxation

Abstract

The conformational dynamics of α- (1 → 4)- and α- (1 → 6)-glucan homooligomers in the nanosecond time domain have been compared by measuring the ¹³C-nmr longitudinal relaxation times T₁ for carbons of the terminal and interior sugar residues. Measurements are reported on monomeric glucose and on oligomers containing up to ten glucose residues at room temperature in aqueous solution at concentrations of 3 and 20 g/dL. The carbons of terminal residues display longer relaxation times than do those of interior residues, presumably as a consequence of a greater degree of conformational mobility of the chain ends. The T₁s of the reducing terminal residues of all oligomers are significantly longer than those of the corresponding nonreducing termini, a phenomenon that we associate tentatively with the anomeric equilibrium at the reducing end. Carbons of the reducing terminal residues in the β-anomeric form relax more slowly than their α-anomeric counterparts. At 20 g/dL the mean T₁s for carbons of the terminal and interior residues attain asymptotic behavior with increasing chain length at a chain length of about six residues, and carbons of the α- (1 → 4)-linked maltooligomers relax significantly more slowly than those of the corresponding α-(1 → 6)-linked isomaltooligomers. The T₁s of both glucan series increase with decreasing concentration. This concentration dependence disappears below 3 g/dL, where the T₁s of the two series of homoligomers are no longer distinguishable. This suggests that in dilute aqueous solution at room temperature viscous damping effects predominate over contributions to the T1-sensitive conformational dynamics from structural differences in the glycosidic linkage region. At 3 g/dL the approach to long chain-length asymptotic behavior is more protracted than at 20 g/dL, and the T₁s of carbons of interior oligomeric residues appear to match the corresponding high-polymer behavior at a chain length of eight and greater.