| Abstract: | A practical method is described for determining some characteristics of the spectrum of proton mobilities in a hydrated system from the frequency dependence of the nuclear magnetic resonance (NMR) relaxation processes. The technique is applied to water in association with agarose and gelatin. The results for agarose are consistent with the hypothesis that a fraction of the protons is distributed over states of reduced mobility and exchanges rapidly with the remaining fraction which is attributed to water in the normal state. No variation in the characteristics of the modified fraction could be detected for water concentrations in the range 1.2-50 g H2O/g agarose. Within the modified fraction, higher mobilities are more common than low mobilities; at 1.2 g H2O/g agarose, not more than 10% of the proton population has mobilities more than 100 times smaller than normal. The modified proton fraction is tentatively identified with agarose hydroxyl protons and possibly water molecules bound to the polymer. Proton states with mobilities intermediate between water and ice have also been detected in hydrated gelatin. As in agarose, higher mobilities are the most common. In contrast to agarose, the characteristics of the modified proton states are markedly dependent on water concentration. They are tentatively attributed to gelatin protons coupled for spinlattice relaxation with those of the bulk phase by exchange and spin diffusion. |
