Interactions between locust bean gum and cellulose characterized by 13C n.m.r. spectroscopy

Abstract

Molecular interactions between locust bean gum (LBG) and cellulose crystallite surfaces appear to involve most mannosyl residues of the mannan backbone, not just the small proportion contained in long segments which lack galactosyl residues. This conclusion is based on: (1) relative strengths of 13C n.m.r. signals at 102.2 ppm in the cross-polarization (CP) spectrum and 101.3 ppm in the single-pulse excitation (SPE) spectrum, assigned to mannosyl C-1 in bound and non-bound segments of LBG, respectively; (2) displacement of a 13C n.m.r. signal assigned to mannosyl C-4 to 81 ppm, indicating a change of conformation in the mannan backbone relative to a gel phase; (3) similarities between proton spin relaxation time constants for cellulose and LBG, indicating proton spin diffusion between polymers in close contact; (4) broadening of CP n.m.r. signals at 83.9 and 85.0 ppm, assigned to C-4 of cellulose chains exposed on crystallite surfaces. The galactosyl C-1 signal appears at 99.9 ppm in the SPE n.m.r. spectrum of the LBG-cellulose complex but shows a poor response to CP n.m.r., indicating that galactosyl residues are not fixed in rigid conformations relative to the mannan backbones.