Response of phosphatidylcholine in the gel and liquid-crystalline states to membrane surface charges.

The influence of membrane surface charge on the conformation of the choline head group of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) was investigated in the gel and liquid-crystalline states by using 2H NMR spectroscopy of specifically choline-deuterated DMPC. The surface charge was made progressively more negative through admixture of various proportions of 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol (DMPG). All membrane compositions showed nearly identical gel- to liquid-crystalline-phase transitions centered about 24 degrees C. The gel-state 2H NMR spectra from all three choline head-group deutero-labeling positions (alpha, beta, and gamma) decreased in intensity and broadened relative to the liquid-crystalline-state spectra. These effects were not so severe that they masked the overriding influence of surface charge on the choline head-group conformation as reflected in the 2H NMR spectra. Thus, in both the liquid-crystalline and gel states, the presence of negative surface charge caused the quadrupole splitting from DMPC-alpha-d2 to increase while causing that from DMPC-beta-d2 and DMPC-gamma-d9 to decrease. These effects were progressive with increasing density of negative surface charge. Correlation plots of the quadrupole splittings obtained, under otherwise identical conditions, from different deutero-labeling positions were linear over most of the range of surface charge densities, in both the liquid-crystalline and gel states, for all three correlations (alpha-beta, beta-gamma, and alpha-gamma). At extreme surface charge densities, the alpha-beta and alpha-gamma correlations showed biphasic behavior in that, at high surface charge densities, the change in the quadrupole splittings from DMPC-alpha-d2 became less pronounced.(ABSTRACT TRUNCATED AT 250 WORDS)