Multinuclear NMR Investigation of Phosphatidylcholine Organogels^†

A multinuclear NMR investigation of organogels formed by soybean lecithin and by a series of synthetic phosphatidylcholines in cyclohexane in the presence of a small amount of water is presented. The NMR measurements are based on ¹H, ¹³C, and ³¹P dynamic parameters and the line width. To study the gelation process, measurements are carried out with samples at different amounts of added water. Both for proton and phosphorus resonances, the onset of the gel formation is clearly evidenced by a broadening of the line width. In the first set of measurements soybean lecithin is studied. It is shown that as water is being added, the line widths of the different protons of lecithin become broader, each to a different extent. Particularly significant is the stiffening of the geminal protons at the sn-1 position of the glycerol backbone. ³¹P NMR T₂ measurements allow the distinction between gel-forming and nongel-forming solvents. The NMR line width broadening is also present in regions in which rheology data show no high viscosity, e.g., at high water content and/or at low lecithin concentration. This is thought to indicate that a considerable molecular stiffening of the glycerol moiety and of the phosphate is present even in the absence of a high viscosity macroscopic gel structure. To study the influence of the molecular structure on the dynamics of gel formation, studies have been 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 ³¹P NMR T₂ values are quite small, while the line widths, both on protons as well as on phosphorus, appear to be much more sensitive to differences in the molecular architecture. Accordingly, this study allows one to draw a quite general picture of lecithin gels in which the molecular structure is linked to the dynamic parameters during gel formation, which are in turn linked to the macroscopic physical properties such as viscosity and phase transition temperature. By comparison of all these data, it appears DOPC is the closest model to natural lecithin. Even in this case, however, caution is required, since local motions in the glycerol moiety are more hindered in DOPC than in soybean lecithin.