Theoretical analysis of water 1H T2 based on chemical exchange and polysaccharide mobility during gelation

It was demonstrated that macroscopic structural changes of polysaccharide chains, such as random coil–helix transition and aggregation of helices, accompanied with the sol–gel and the gel–sol transition can be monitored by numerically analyzing the temperature dependence of the observed water proton spin–spin relaxation time (T2obs) modified by the chemical exchange between water proton and labile proton on the chain. According to the numerical analysis of T2obs, typical profiles of T2obs against the temperature were simulated with various parameters of the relaxation time (T2i) and the mean residence time (τi) of water and the labile protons on the random coil and the ordered chain as well as the activation energy (Ei,Eex,i) for the motion and the chemical exchange of the respective proton. The difference of structural change of kappa- , iota- , and lambda-carrageenan aqueous systems in the cooling and the heating process was analyzed. In conclusion, (1) the characteristic profile of the temperature dependence of T2obs, commonly observed in the temperature-induced gelling process, is attributable to the variation of fraction of the ordered chain and the relative rate of chemical exchange to the relaxation rate of labile protons on polysaccharide (1/T2p); (2) the loose aggregated or associated structure is detected for iota-carrageenan system; and (3) the thermally stable junction zones formed by well aggregated helices of kappa-carrageenan in the gel state provokes a thermal hysteresis in the temperature-induced structural change (experimentally, in T2obs). It is clarified by the numerical simulation that the aggregated helices formed during further cooling down to the temperature below Tsg results in the T2obs shifts to a longer value in the heating process as compared with that in the cooling process.