Diffusion, relaxation, and chemical exchange in casein gels: A nuclear magnetic resonance study

Abstract

Water in protein/water mixtures can be described in terms of bound water and free water, by exchange between these two states, and by its exchange with appropriate sites on the protein. 1H-NMR diffusion and relaxation measurements provide insights into the mobility of these states. T2 relaxation-time dispersions (i.e., T2 relaxation times at different echo pulse spacings) reveal additional information about exchange. We present a comprehensive set of diffusion and T2 dispersion measurements on casein gels for which the protein/water ratio ranges from 0.25 to 0.5. The combination of these methods, taken in conjunction with concentration dependence, allows a good estimate of the parameters required to fit the data with Luz/Meiboom and Carver/Richards models for relaxation and chemical exchange. We compare the exchange (a) between water and protein and (b) between free water and bound water. Further, we attempt to distinguish chemical site exchange and diffusion/susceptibility exchange.

NMR diffusion measurements under chemical exchange between sites involving a large chemical shift difference

“…All these latter works concern however slow exchange (at least with respect to chemical shift). Although a relatively recent publication (14) deals with fast exchange (the topic of the present work), we were unable to find any explanation of our experimental observations in these previous studies.…”

A 2D NMR method to characterize granular structure of dairy products

“…This confirms the identification of the two signals. T 2 relaxation in model milk systems has been studied recently in detail by Le Dean et al [33] and Gottwald et al [34]. They showed that it is mainly controlled by the concentration of casein micelles and the dominant mechanism is proton exchange between labile protein protons and the exchange of water molecules internal to the casein micelle and in the bulk state.…”

Quantitative characterization of food products by two-dimensional D– and – distribution functions in a static gradient

“…T 2 relaxation in model milk systems has been studied recently in detail by le Dean et al [10] and Gottwald et al [23]. They showed that it is mainly controlled by the concentration of casein micelles and that the dominant mechanisms are proton exchange between labile protein protons and the exchange of water molecules internal to the casein micelle and the in the bulk state.…”