The dependence of glucan conformational dynamics on linkage position and stereochemistry

Abstract

The 13C NMR T 1 relaxation times for the (1 → 4)-linked maltooligomers (M i ) and the (1 → 6)-linked isomaltooligomers (IM i ) with i = 2, 4, 6, and 8 were measured in aqueous solution at 22 and 65°C at a concentration (3%) low enough to have removed concentration-dependent effects on the measured T 1 values. Separate T 1 values were measured for each carbon in the residue at the reducing end of the oligosaccharide, in the residue at the non-reducing end, and in the interior, i.e., non-terminal, residue(s). Analogous data for the corresponding high polymers show that at 22°C the relaxation times for the carbons of the interior residues of the oligomers have converged to their high chain length asymptotes at about i = 10. This observation suggests that at room temperature polymeric motions in the frequency domain effective for 13C NMR relaxation at a magnetic field strength of 11.7 T have a “wavelength” of the order of 10 residues. The relaxation times characterizing the two ends of the chain are different, with longer T 1 values for the carbons of the reducing end than for those of the non-reducing end. Carbons of α-anomeric residues at the reducing end have shorter relaxation times than those of the corresponding β-anomeric reducing sugars. Carbons of the interior residues have T 1 values shorter than the carbons of either type of terminal residue. For oligomers of a given dp there is no T 1 difference between oligomers of the M i and IM i series at room temperature. This observation is seemingly at odds with the great differences in the inherent conformational freedom of the (1 → 4)- and (1 → 6)-linkages. At elevated temperatures the orientational relaxation behavior of the two series of oligomers measured by 13C T 1 values show interesting differences, and in the case of the M i series, structure develops in the chain length dependence of the T 1 values.