The potentiality for tm elevation of dipalmitoylphosphatidylcholine liposomes

GPS, the slope of laurdan generalized polarization spectra, in the study of phospholipid lateral organization and Escherichia coli lipid phases

The dependence of Laurdan generalized polarization (GP) on the excitation or emission wavelengths has been employed, at a descriptive level, to estimate lipid membrane physical state, including the coexistence of phases. In this paper, we introduce GPS, a quantitative, simplified estimation of the GP spectrum slope, and present a novel approach to assessing phase states through a graphical representation of its temperature dependence. The thermotropic profile of GPS allows the detection of the main phase transition of liposomes from model phosphoglycerides and renders a clear identification of T(c), a temperature that is unique for each phospholipid studied, marking the apparent limit between coexistence of phases and liquid crystalline state. Since at this temperature GPS is equal to zero, the tenet that the absence of wavelength effect on generalized polarization always means pure gel phase, can be called into question. Interestingly, GPS allows the discrimination between the thermotropic behavior of vesicles of lipid extracts from Escherichia coli grown at 30, 37, 42 or 45 degrees C, consistent with the remodeling in phospholipid acyl chain composition induced by changes in culture temperature. Yet in all cases, GPS reports liquid crystalline state at a temperature equal to the growth temperature of the bacteria from which each extract was obtained.

Determination of the pKa of membrane-bound N,N-dimethylsphingosine using deuterium NMR spectroscopy

Deuterium nuclear magnetic resonance (2H-NMR) spectroscopy was applied to determine the pKa of the protein kinase C (PKC) inhibitor, N,N-dimethylsphingosine (DMS), when bound to lipid bilayers composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). The quadrupolar splittings from the deuterium labels at the alpha- and the beta-choline positions of the headgroup of POPC responded to the presence of DMS in a manner indicative of an accumulation of cationic charges near the surface plane occupied by the phospholipid phosphate group. Both quadrupolar splittings varied linearly with the amount of added DMS at pH 7.0. Conversely, at pH 10.0 DMS had virtually no influence on either quadrupole splitting, an effect attributed to titration of the dimethyl amino group of DMS to its neutral form. A DMS titration curve was obtained by quantifying the change in the quadrupolar splittings as a function of pH. The pKa of membrane-bound DMS was extracted from this 2H-NMR data by simulating the quadrupole splitting-titration curve for different values of the pKa, yielding a pKa of 8.8 after non-linear least squares fitting.