1H and 13C NMR of Multilamellar Dispersions of Polyunsaturated (22:6) Phospholipids

Abstract

The polyunsaturated fatty acid docosahexaenoic acid (DHA) makes up ∼50% of the lipid chains in the retinal rod outer segment disk membranes and a large fraction of the lipid chains in the membranes of neuronal tissues. There is an extensive literature concerned with the dietary requirements for essential fatty acids and the importance of DHA to human health, but relatively little research has been done on the physical properties of this important molecule. Using 1H and 13C MAS NMR measurements of dispersions of 1-palmitoyl-2-docosahexaenoyl-phosphatidylcholine in excess phosphate buffer, we have unambiguously assigned most of the resonances in both the 1H and 13C NMR spectra. We were able to use cross-polarization spectroscopy to follow the transfer of polarization from specific 1H nuclei not only to their directly bonded 13C but also to those 13C that are in close proximity, even though they are not directly bonded. Cross-peaks in two-dimensional cross-polarization spectra revealed a close association between the choline headgroup and at least part of the DHA chain but not with the palmitate chain. Finally, we examined the dynamics of the different parts of this lipid molecule, using rotating frame spin-lattice relaxation measurements, and found that methylene groups of both chains experience important motions with correlation times in the 10- μs range, with those for the palmitate chain being ∼50% longer than those of the DHA chain. The choline headgroup and the chain terminal groups have significantly shorter correlation times, and that part of the dipolar interaction that is fluctuating at these correlation times is significantly smaller for these groups than it is for the palmitate and DHA chain methylenes.