Estimation of the hydrodynamic screening length in κ-carrageenan solutions using NMR diffusion measurements

Abstract

We quantify the hydrodynamic screening length (n) in j-carrageenan solutions by observing restrictions on the diffusion of several pullulan probes, each with a different molecular weight (M w ). The diffusion coefficient of pullulan in a pure solvent (D pull,0 ) and a j-carrageenan solution (D pull ) were measured using a pulsed field gradient stimulated spin-echo 1 H nuclear magnetic resonance. The hydrodynamic radii (R H ) of the pullulan samples were determined using D pull,0 measured at 25 1C and the Stoke-Einstein equation. The echo signal intensity of j-carrageenan without a gradient, I kappa (0), decreased steeply at around the sol-to-gel transition temperature (T sg ), suggesting that the j-carrageenan chains formed helical aggregates at T sg diluting the solute j-carrageenan concentration. The ratio, D pull /D pull,0 , was less than 1, indicating that the mobility of pullulan was restricted by the j-carrageenan chains. Over the entire temperature range, pullulan with larger molecular size yielded more restrictions on the molecular mobility, and D pull /D pull,0 decreased as the M w of pullulan increased. Below T sg , D pull /D pull,0 increased as the temperature decreased. This behavior suggests that a decrease in the restriction degree by the j-carrageenan chains is concomitant with the formation of aggregates. To quantify the restriction degree, n was estimated at various temperatures using the relation D pull /D pull,0 ¼ exp( ÀR H /n). Polymer Journal ( Keywords: diffusion coefficient; gel; gradient NMR; hydrodynamic radius; hydrodynamic screening length; pullulan; k-carrageenan INTRODUCTIONThe diffusion of solutes in a gel matrix has an important role in applications such as the delivery of drugs and the encapsulation of fragrances. 1,2 NMR has proved to be a powerful technique for determining the dynamic and structural properties of food model systems 3 and the gelation process of polysaccharides. 4,5 NMR relaxation times give information about the tumbling motion of molecules in food hydrocolloid systems. The diffusion coefficients determined by pulsed-field-gradient NMR reflect the translational motion of molecules. The diffusion coefficient of solutes in a polymer matrix strongly depends on molecular weight, temperature, viscosity and the network structure of the diffusing matrix. Any factor that reduces the space delineated by the polymer chains will have an effect on the molecular mobility of the solute. In polysaccharide gels, the aggregation of polysaccharide chains can enlarge the diffusing space. Therefore, to understand the diffusion of molecules in a gel, it is important to understand the aggregation behavior.k-Carrageenan is a linear sulfated polysaccharide that is extracted from marine red algae. As shown in Figure 1, k-carrageenan comprises two galactose units: a-(1-3)-D-galactose-4-sulfate and b-(1-4)-3,6-anhydro-D-galactose. It is a gelling agent that has

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

“…The diffusion coefficient of pullulan in a dilute D 2 O solution at 25 • C was measured as described previously (Zhao & Matsukawa, 2012). The hydrodynamic radius R H of pullulan was assumed to be independent of temperature (Nordmeier, 1993;Viebke & Williams, 2000), and the diffusion coefficient of pullulan in dilute D 2 O (D pul,0 ) at all temperatures was calculated from the Stoke-Einstein equation:…”