MULTINUCLEAR AND MAGIC-ANGLE-SPINNING NMR INVESTIGATIONS OF MOLECULAR-ORGANIZATION IN PHOSPHOLIPID TRIGLYCERIDE AQUEOUS DISPERSIONS

The supramolecular organization of multi-bilayers formed by aqueous egg phosphatidylcholine-triolein (PC-TO) mixtures has been investigated using P-31, C-13, and H-1 nuclear magnetic resonance (NMR). For these emulsions, which mimic substrates in the early hydrolytic stages of fat digestion, the NMR spectra obtained with magic-angle spinning (MAS) exhibit resolution comparable to that of sonicated vesicles and integrated peak intensities consistent with their chemical composition. Both P-31 line shapes and MAS sideband patterns from the phosphocholine group indicate that mixing with triolein produces a PC bilayer which remains predominantly liquid crystalline in its organization; nevertheless, anomalous spectral features in MAS spectra may be attributed to additional phases in which the headgroups adopt a different orientation with respect to the bilayer normal, and tight packing enhances phosphorus-phosphorus interactions. C-13 and H-1 line widths monitored as a function of PC mole fraction, spinning speed, and decoupling strength show that the choline headgroups and glycerol backbones are anchored preferentially in the phospholipid-triglyceride assemblies, whereas all acyl chains become very fluid. The average degree of chain order also decreases for the mixed dispersions, as judged from spinning-sideband intensities in H-1 MAS NMR spectra. The absence of proton spin-diffusion effects in the PC-TO multilayers is demonstrated by examination of their spinning sidebands and relaxation times, making it possible to use MAS-assisted two-dimensional NMR to assign overlapped H-1 resonances and to identify proximal interactions between the two constituents. The usefulness of these NMR strategies in mechanistic studies of gastric fat digestion is also discussed.