²H NMR Studies of Positional Isomers of a Monounsaturated Phospholipid Membrane

Department of Physics, Indiana University-Purdue University Indianapolis, 402 N. Blackford St., Indianapolis IN 46202-3273, USA

Isomeric [²H31]16:0-18:1PC (1-[²H31]palmitoyl-2-cis-octadecenoylphosphatidylcholine) memb-ranes in which the double bond is at the [[Delta]]6, [[Delta]]9, [[Delta]]12 or[[Delta]]15 position were compared by solid state ²H NMR. The studies unambiguously demonstrate that the location of unsaturation has tremendous impact upon membrane properties and must be included in attempts to explain the unique biological role of PUFA (polyunsaturated fatty acid). More than 40^oC variation in the temperature of the gel to liquid crystalline phase transition Tm is revealed by moment analysis of spectra. Energy minimized structures obtained with the CHARMm force field for isomers of 16:0-18:1DG (1-palmitoyl-2-cis-octadecenoylgylcerol) qualitatively correlate with the maximum reduction in Tm exhibited when the double bond is in the middle of the chain. They indicate that introduction of unsaturation at the [[Delta]]9 or [[Delta]]12 position causes greater disruption to acyl chain conformation than at the [[Delta]]6 or [[Delta]]15 position. Lineshape analysis also establishes that molecular ordering in liquid crystalline [²H31]16:0-18:1PC membranes changes by over 35%. At equal temperature T the [[Delta]]9 isomer is least ordered and the dependence resembles that seen for Tm , while the variation is inverted and the [[Delta]]12 isomer is most ordered at the same reduced temperature Tred = (T-Tm)/Tm. The order parameter profiles generated from depaked spectra for the 16:0 sn-1 chain are typical of phospholipid membranes, possessing a characteristic plateau region of approximately constant order in the upper portion followed by gradually decreasing order in the lower portion towards the centre of the membrane. A possible increase in length of the plateau is indicated as the unsaturation is moved down the 18:1 sn-2 chain.