Pulsed field gradient spin-echo NMR measurement of water diffusion coefficient in thickening and gelling agents: guar galactomannan solutions and pectin gels.

The Pulsed Field Gradient Spin-Echo (PFG-SE) method was used to study the diffusional properties of water in guar galactomannan solutions and in highly methyl-esterified (HDE) pectin solutions and gels. The diffusion coefficient of water in the non-gelling galactomannan system was the same than that of pure water and independent on polymer concentration, despite of a strong increase in viscosity. In the two-component gelling system, pectin-sucrose, the diffusion coefficient of water was lower than in the pure state, and decreasing as a function of the concentration of both constituents. A normalized diffusion coefficient, Dgel/Dsuc, was defined to discriminate the effect of the gelation process on water diffusion, and it showed a clear dependence on the extent of network formation. Unrestricted diffusion was evidenced in all cases.

1H and 23Na NMR relaxation times study of pectin solutions and gels.

1H and 23Na longitudinal and transverse relaxation times have been measured to examine the concentration dependence of the dynamic behaviour of pectin solutions and gels. T1 and T2 relaxation times were measured in HDE pectin solutions and gels prepared with different cosolutes. A lowering of T1 and T2 values was observed in relation to a better efficiency of the intermolecular forces between polymer molecules and/or the formation of more extended junction zones. The T1/T2 ratio was also exploited indicating that even bulk water experiences anisotropic motions due to the gel formation. A correspondence of NMR results with previous reported data of rheological measurements of pectin gels prepared with the same cosolutes was found.

NMR investigation of the dynamic behaviour of guar galactomannan water solutions.

Proton longitudinal and transverse relaxation times have been measured to examine the concentration dependence of the dynamic behaviour of guar galactomannan aqueous solutions. Two concentration regimes have been evidenced from our results, with a limiting concentration separating the two regimens at about 3% guar concentration. The T1/T2 ratio indicates that even bulk water experiences anisotropic motions over the limiting concentration. A comparison is made with T1/T2 ratios evaluated in glucose solutions which do not exhibit any deviation from isotropic behaviour of bulk water molecules. The results are discussed taking into account topological entanglement interactions between polysaccharide molecules, according to the current model for the solution behaviour of guar galactomannan.

NMR study of seed hydration with deuterated water: dependence of proton signals on hydration level.

Proton NMR signals in seeds are shown to depend on hydration level. In fact at low water amount, as it occurs in many native seeds, protons can have a restricted mobility and are not detectable. A NMR method for measuring the dependence of proton signals on hydration is reported. The method also allows the separation of the contributions of water and non-water protons in a low-resolution NMR experiment. It is based on successive hydrations (with deuterated water) - desiccation steps and on the analysis of the transverse magnetization decay curves.

Phase fluctuation in phospholipid membranes revealed by Laurdan fluorescence.

The organization of lipids surrounding membrane proteins can influence their properties. We have used 6-dodecanoyl-2-dimethylaminonaphthalene (Laurdan) to study phase coexistence and phase interconversion in membrane model systems. The fluorescence properties of Laurdan provide a unique possibility to study lipid domains because of the different excitation and emission spectra of this probe in the gel and in the liquid-crystalline phase. The difference in excitation spectra allows photoselection of Laurdan molecules in one of the two phases. Using the difference in emission spectra it is then possible to observe interconversion between the two phases. We have performed experiments in dipalmitoyl-phosphatidylcholine (DPPC) vesicles at different temperatures, in particular in the region of the phase transition, where phase coexistence and interconversion between phases is likely to be maximal. We have also studied vesicles of different lipids and mixtures dilauroyl-phosphatidylcholine (DLPC), DPPC, and 50% DLPC in DPPC. Both steady-state fluorescence intensity and polarization data have been collected. To quantitate phase coexistence and interconversion we have introduced the concept of "generalized polarization." We have also performed time-resolved experiments to directly prove the interconversion process. We have found that in DLPC-DPPC mixtures, at 20 degrees C, phase interconversion occurs in approximately 30-40 ns.