title: GPS, the slope of Laurdan generalized polarization spectra, in the study of phospholipid lateral organization and Escherichia coli lipid phases.
abstract: 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.
authors.affiliation: Department of Biochemistry, Centro de Investigación y de Estudios Avanzados del I.P.N. (CINVESTAV), P.O. Box 14.740, 07000 México D.F., Mexico.
authors.forename: Jesús B
authors.initials: JB
authors.lastname: Velázquez
authors.forename: Marta S
authors.initials: MS
authors.lastname: Fernández
mesh_terms: D003198:Computer Simulation; D004926:Escherichia coli / Q000378:metabolism*; D007848:Laurates / Q000737:chemistry; D008081:Liposomes / Q000737:chemistry*; D008560:Membrane Fluidity; D008956:Models, Chemical; D008968:Molecular Conformation; D044367:Phase Transition; D010743:Phospholipids / Q000737:chemistry*; D013050:Spectrometry, Fluorescence / Q000379:methods*; D013696:Temperature
chemical_list: D007848:Laurates; D008081:Liposomes; D010743:Phospholipids