Low-temperature 2H NMR spectroscopy of phospholipid bilayers containing docosahexaenoyl (22:6 omega 3) chains

Polyunsaturated fatty acids are widely distributed components of biological membranes and are believed to be involved in many biological functions. However, the mechanisms by which they act on a molecular level are not understood. To further investigate the unique properties of omega 3 polyunsaturated phospholipid bilayers, deuterium nuclear magnetic resonance (2H NMR) studies have been made of the liquid-crystalline (L alpha) and gel phases of a homologous series of mixed-chain phosphatidylcholines containing docosahexaenoic acid: (per-2H-n:0)(22:6)PC, where n = 12, 14, 16, and 18. The moments of the 2H NMR lineshapes have been evaluated, and from these the warming and cooling main phase transition temperatures were determined. The transition temperatures of the mixed-chain series were found to be significantly lower than those of the corresponding lipids in the disaturated series, di(per-2H-n:0)PC, with hystereses ranging from 2 to 14 degrees C. Distinct effects of the docosahexaenoyl chain on bilayer order were found, though these effects varied across the mixed-chain series. In evaluating the moment data, an empirical method for normalizing the moments with respect to differences in temperature was applied, in addition to using the reduced temperature method. For the systems studied here, the method of normalization had no significant effect on the interpretation of the moment data.