H-2 NMR-STUDIES OF ISOMERIC OMEGA-3 AND OMEGA-6 POLYUNSATURATED PHOSPHOLIPID-MEMBRANES

The properties of aqueous multilamellar dispersions of [H-2(31)] 16:0-alpha 18:3 PC (1-[H-2(31)]palmitoyl-2-cis,cis,cis-octa-9,12,15-trienoylphosphatidylcholine) and of [H-2(31)] 16:0-gamma 18:3 PC (1-[H-2(31))]palmitoyl-2-cis,cis,cis-octa-6,9,12-trienoylphosphatidylcholine) were compared by broadline H-2 NMR spectroscopy. These isomeric phospholipids differ only in the location of the unsaturations in the sn-2 chain. The (alpha 18:3 chain has double bonds at Delta 9, 12, and 15 positions whereas in the gamma 18:3 chain they are at positions Delta 6, 9, and 12. Moment analysis of spectra recorded as a function of temperature reveals dramatically distinct phase behavior for the two isomers. The gel to liquid crystalline transition for [H-2(31)] 16:0-alpha 18:3 PC membranes exhibits broad hysteresis which is characterized by a mid point temperature of -9 degrees C and -20 degrees C on heating and cooling, respectively. In contrast, the phase transition of [H-2(31)]16:0-gamma 18:3 PC membranes does not exhibit hysteresis and occurs over a lower temperature range centred on -27 degrees C. Appreciably different molecular ordering also exists within the membranes in the liquid crystalline state. Average order parameters (S) over bar(CD) are smaller in [H-2(31)]16:0-alpha 18:3 PC than in [H-2(31)]16:0-gamma 18:3 PC by 10% at the same temperature and by 20% at equal reduced temperature. Smoothed order parameter profiles generated from depaked spectra clarify the nature of the difference. There is an elevation in the order of the plateau region of approximately constant order in the upper portion (C2-C7) of the 16:0 sn-1 chain in [H-2(31)]16:0-gamma 18:3 PC, while in the lower portion (C8-C16) of the chain the profiles gradually converge as order decreases toward the terminal methyl. Our results emphasize that the position of double bonds, not just their number, must be included in attempts to explain the unique biological role of PUFA (polyunsaturated fatty acids).