MULTINUCLEAR NMR INVESTIGATION OF PHOSPHATIDYLCHOLINE ORGANOGELS

A multinuclear NMR investigation of organogels formed by soybean lecit hin and by a series of synthetic phosphatidylcholines in cyclohexane i n the presence of a small amount of water is presented. The NMR measur ements are based on H-1, C-13, and P-31 dynamic parameters and the lin e width. To study the gelation process, measurements are carried out w ith samples at different amounts of added water. Both for proton and p hosphorus resonances, the onset of the gel formation is clearly eviden ced by a broadening of the line width. In the first set of measurement s soybean lecithin is studied. It is shown that as water is being adde d, the line widths of the different protons of lecithin become broader , each to a different extent. Particularly significant is the stiffeni ng of the geminal protons at the sn-1 position of the glycerol backbon e. P-31 NMR T-2 measurements allow the distinction between gel-forming and nongel-forming solvents. The NMR line width broadening is also pr esent in regions in which rheology data show no high viscosity, e.g., at high water content and/or at low lecithin concentration. This is th ought to indicate that a considerable molecular stiffening of the glyc erol moiety and of the phosphate is present even in the absence of a h igh viscosity macroscopic gel structure. To study the influence of the molecular structure on the dynamics of gel formation, studies have be en extended to synthetic gel-forming phosphatidylcholines, such as 1,2 -dioleoyl-sn-glycero-3-phosphocholine (DOPC) between 281 and 300 K; 1- palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) at 300 K, and 1, 2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) between 313 and 333 K ; 1,2-dilinoleoyl-sn-glycero-3-phosphocholine (DLPC) at 281 K. On all gels, differences in P-31 NMR T-2 values are quite small, while the li ne widths, both on protons as well as on phosphorus, appear to be much more sensitive to differences in the molecular architecture. Accordin gly, this study allows one to draw a quite general picture of lecithin gels in which the molecular structure is linked to the dynamic parame ters during gel formation, which are in turn linked to the macroscopic physical properties such as viscosity and phase transition temperatur e. By comparison of all these data, it appears DOPC is the closest mod el to natural lecithin. Even in this case, however, caution is require d, since local motions in the glycerol moiety are more hindered in DOP C than in soybean lecithin.