Interactions of water and calcium ions with food components, studied by NMR

Abstract

NMR studies of water oxygen-17 relaxation in aqueous sucrose and lysozyme solutions have been carried out to investigate the interactions of water with sucrose and lysozyme. The effect of sucrose and lysozyme concentration on water oxygen-17 relaxation has been studied in detail. The dependence of relaxation on frequency and pH has also been analysed. The existing model, describing the relaxation of water oxygen-17 in aqueous protein solutions suggested by Halle in 1981, is tested to see whether it gives a true representation. It is found that at low concentrations of sucrose and lysozyme, the experimental data give good agreement with the model. However, at saturated sucrose concentration the agreement is not so good. An extra contribution to the transverse relaxation rate is seen. A possible explanation for the extra contribution to the transverse relaxation rate at high sucrose content is discussed. The effect of ionic charge on oxygen-17 relaxation in lysozyme solutions is also investigated. It is observed that both the ionic charge of lysozyme as well as lysozyme aggregation strongly affect the relaxation of water oxygen-17. A method for analysing the experimental data for water oxygen-17 relaxation in aqueous sucrose and lysozyme solutions using Halle's model is presented and employed to calculate the various parameters of the model. The relaxation and chemical shift of calcium-43 in simple calcium salts, calcium acetate and calcium ascorbate have been studied as a function of concentration and pH. The complexation of calcium to sucrose and to lysozyme has also been investigated. In almost all cases, a significant calcium-43 chemical shift has been detected. The direct measurement of complexation and binding of calcium by relaxation time and chemical shift measurements has been of particular interest.