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

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.