NMR studies of the gelation mechanism and molecular dynamics in agar solutions

Changes in the molecular mobility of agar were measured by pulsed-field-gradient stimulated echo (PFG-STE) 1H NMR at various temperatures in order to elucidate the mechanism of gelation in solutions. The echo signal intensity of agar decreased steeply and the diffusion coefficient D of agar increased near the sol-to-gel transition temperature Ts–g, and D decreased with further cooling. These results suggested that the polysaccharide chains in agar aggregated in bundles to form a network at around Ts–g. High molecular weight chains aggregated preferentially in agar, with the soluble, non-aggregated agar (“solute agar”) left in the network forming loose aggregates upon further cooling. Evidence for this behavior was obtained from GPC measurements on the solute agar squeezed from the gel. These loose aggregates readily disassociated on reheating, whereas the aggregated bundles were quite thermally stable, which corresponded well with the thermal stability of the gel strength. The changes in the restrictions on molecular mobility in these solutions were evaluated from measurements of D of a dendrimer added as a probe molecule, which was sensitive to the dilution of the solute agar accompanying gelation. The hydrodynamic shielding length ξ, which was considered to represent the hydrodynamic mesh size created by the solute agar, was calculated from D of the dendrimer, shedding light on the changes in the microscopic environment during gelation.