Determination of the complete order matrix of the alpha methylene segment of potassium palmitate

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Abstract
The orientational order of the methylene segment adjacent to the head group in the lamellar liquid crystalline phase of potassium palmitate/D2O was investigated using multinuclear magnetic resonance. Dipolar and quadrupolar couplings were obtained from the 1H, 13C, and 2H N.M.R. spectra of potassium palmitate-d31 and 1-13C-2,2-H2- potassium palmitate-d29 dispersed in lamellar liquid crystalline phases as a function of temperature. In order to observe 1H-1H dipolar couplings, a spin echo (Π/2-t1/2-Π—t1/2-echo) was used to remove heteronuclear dipolar couplings to the chain deuterons. A refocusing pulse applied simultaneously to the 13C spins allowed observation of the heteronuclear 1H-13C dipolar couplings in the carbon-13 labelled compound. The complete orientational order matrix of the alpha methylene segment was determined from the dipolar and quadrupolar couplings. As the temperature is decreased from 110°C to a temperature just above the gel-liquid crystalline phase transition (45°C), the principal orientation axis of the alpha methylene segment is rotated by 3° towards a configuration in which the first C-C bond is parallel to the bilayer normal. This is in direct agreement with a previous model of lipid-water interaction, in which the decrease in orientational order for the alpha C-D direction was postulated to be a strictly geometric effect arising from electrostatic interactions of the lipid with the water.