Disposition of chloroform in phosphatidylcholine membranes: A H-2- and P-31-NMR study

The interaction of chloroform with bilayers of dimyristoylphosphatidylcholine (DMPC) has been studied by deuterium and phosphorus-31 nuclear magnetic resonance (NMR). Orientational order has been measured as a function of temperature at many sites in DMPC, water and chloroform for aqueous multilamellar dispersions of the lipid. At equivalent temperatures above the main phase transition temperature for a molar ratio of DMPC to chloroform of approximately 10 to 1, disordering at several sites in the head group of DMPC is observed, unlike the acyl chains where no disordering is observed. With higher concentrations of chloroform (DMPC/chloroform approximate to 4:1) greater disordering occurs at the same head group sites and is accompanied by disordering of the acyl chains. The pattern of solvent-induced changes in DMPC is similar to that produced by benzyl alcohol and n-alkanols. With H-2-labelled chloroform. the H-2-NMR spectra show two components, one isotropic and the other ordered (Delta v approximate to 1.5 kHz) arising from solute intercalated in the bilayer. In DMPC/water systems at low hydration the ordering of the (H2O)-H-2 in the L(alpha) phase is little affected by chloroform at comparable temperatures whereas at temperatures below the main phase temperature a large disordering of the water is observed. A model of the mode of interaction between chloroform and DMPC is proposed, in which the chloroform is localized principally in an ordered environment in the vicinity of the choline head group at lower temperatures and solute concentrations. Increasing either of these parameters favors the penetration of the chloroform into the center of the bilayer.