Molecular mobility and microscopic structure changes in κ-carrageenan solutions studied by gradient NMR

Changes in the molecular mobility of kappa-carrageenan were observed by the pulsed field gradient stimulated echo (PGSTE) and Carr-Purcell-Meiboom-Gill (CPMG) methods for elucidating the molecular aspect of the sol-to-gel transition. The echo signal intensity of kappa-carrageenan without a gradient, I-kap(0), decreased steeply near the sol-to-gel temperature (T-sg), suggesting that kappa-carrageenan chains formed aggregates and a network structure. Below T-sg, the spin-spin relaxation time T-2 and the diffusion coefficient of kappa-carrageenan (D-kap) increased with decreasing temperature, indicating that the solute kappa-carrageenan chains have a lower molecular weight M-w than chains involved in the aggregation. The diffusion coefficient of pullulan (D-pul) added as a probe molecule in kappa-carrageenan solutions was measured, and the characteristic hydrodynamic screening length, xi, was then estimated from the degree of diffusion restriction. Below a certain temperature, D-kap reached a higher value than that of D-pul, suggesting that the M-w of solute kappa-carrageenan became lower than that of pullulan. GPC measurements confirmed the presence of kappa-carrageenan chains with a lower M-w than that of pullulan. A simple physical model of the structural change in kappa-carrageenan solution was proposed with a bimodal distribution of kappa-carrageenan with higher and lower M-w than the pullulan probe. The higher M-w chains form the gel network restricting the probe's diffusion, and the lower M-w chains increase the effective viscosity. The concentration of the high M-w solute kappa-carrageenan chains in 1%, 2% and 4% kappa-carrageenan solutions was estimated from I-kap(0) and the total kappa-carrageenan concentration, and the relation with pullulan diffusion was studied. (c) 2013 Elsevier Ltd. All rights reserved.