Surface Ionization of Dipalmitoylphosphatidylcholine as Detected by the pH Dependence of the Phase Transition Temperature of Liposomes

The effect of hydronium ions on the structure of phospholipid membranes

This work studies the mechanisms by which hydronium ions modulate the structure of phospholipid bilayers.

Lateral organization of mixed, two-phosphatidylcholine liposomes as investigated by GPS, the slope of Laurdan generalized polarization spectra

The effect of the excitation or emission wavelengths on Laurdan generalized polarization (GP) can be evaluated by GPS, a quantitative, simplified determination of the GP spectrum slope, the thermotropic dependence of which allows the assessment of phospholipid lamellar membrane phase, as shown in a recent publication of our laboratory [J.B. Velázquez, M.S. Fernández, Arch. Biochem. Biophys. 455 (2006) 163-174]. In the present work, we applied Laurdan GPS to phase transition studies of mixed, two-phosphatidylcholine liposomes prepared from variable proportions of dimyristoyl- and dipalmitoylphosphatidylcholine (DMPC and DPPC, respectively). We have found that the GPS function reports a clear limit between the gel/liquid-crystalline phase coexistence region and the liquid-crystalline state, not only at a certain temperature T(c) for liposomes of constant composition submitted to temperature scans, but also at a defined mole fraction X(c), for two-component liposomes of variable composition at constant temperature. The T(c) or the X(c) values obtained from GPS vs. temperature or GPS vs. composition plots, respectively, allow the construction of a partial phase diagram for the DMPC-DPPC mixtures, showing the boundary between the two-phase coexisting region and the liquid-crystalline state. Likewise, at the onset of the transition region, i.e., the two-phase coexisting region as detected by GPS, it is possible to determine, although with less precision, a temperature T(o) or a mole fraction X(o) defining a boundary located below but near the limit between the gel and ripple phase, reported in the literature. These GPS results are consistent with the proposal by several authors that a fraction of L(alpha) phospholipids coexists with gel phospholipids in the rippled phase.

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.

Interfacial activation of porcine pancreatic phospholipase A(2) studied with 7-nitrobenz-2-oxa-1,3-diazol-4-yl-labeled lipids

The interfacial activation of porcine pancreatic phospholipase A(2) (PLA(2)) during the hydrolysis of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine liposomes at different temperatures has been monitored by fluorescence changes of the 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) lipid derivatives 1-palmitoyl-2-[6-[(7-nitro-2-1,3-benzoxadiazol-4-yl)amino]dodecanoyl]-sn-glycero-3-phosphocholine (C(12)-NBD-PC) and 12-[(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)]dodecanoic acid (C(12)-NBD-FA) inserted in the substrate vesicles. These long-chain monitors, in contrast to the previously used C(6)-NBD-PC, detect latency times of PLA(2) action, similar to those measured by the classic titrimetric, pH-stat method. Interestingly, hydrolysis of the host vesicles results in a decrease in fluorescence not only of C(12)-NBD-PC, a substrate analog, but also of product derivative C(12)-NBD-FA. Ultrafiltration experiments show that C(12)-NBD-FA does not migrate to the aqueous phase upon hydrolysis of the host liposomes. Besides, in a simulated hydrolysis experiment in which increasing proportions of palmitic acid and 1-palmitoyl-sn-glycero-3-phosphocholine were cosonicated with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, C(12)-NBD-PC fluorescence was insensitive to products, whereas C(12)-NBD-FA did show a decreased emission intensity as in the actual hydrolysis experiments. The phenomenon is triggered above a critical concentration of products (10 mol%) suggesting that cosegregation of NBD-FA (either added as such or generated by hydrolysis of C(12)-NBD-PC) and products may be related to the decrease in fluorescence. Phase separation should create microdomains of increased C(12)-NBD-FA surface density and cause concentration quenching. In addition, and taking into account that the NBD group may be located near the interfacial region, it is possible that in segregating with products, the fluorescent moiety of C(12)-NBD-FA becomes exposed to microenvironments of higher surface polarity, which further decreases its quantum yield.

Phases and phase transitions of the phosphatidylcholines

LIPIDAT (http://www.lipidat.chemistry.ohio-state.edu) is an Internet accessible, computerized relational database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior and molecular structures. Here, a review of the data subset referring to phosphatidylcholines is presented together with an analysis of these data. This subset represents ca. 60% of all LIPIDAT records. It includes data collected over a 43-year period and consists of 12,208 records obtained from 1573 articles in 106 different journals. An analysis of the data in the subset identifies trends in phosphatidylcholine phase behavior reflecting changes in lipid chain length, unsaturation (number, isomeric type and position of double bonds), asymmetry and branching, type of chain-glycerol linkage (ester, ether, amide), position of chain attachment to the glycerol backbone (1,2- vs. 1,3-) and head group modification. Also included is a summary of the data concerning the effect of pressure, pH, stereochemical purity, and different additives such as salts, saccharides, amino acids and alcohols, on phosphatidylcholine phase behavior. Information on the phase behavior of biologically derived phosphatidylcholines is also presented. This review includes 651 references.

Activity of phospholipase A2 on a fluorescent substrate incorporated into non-hydrolyzable phospholipid liposomes

The activity of phospholipase A2 (PLA2) on phospholipid liposomes depends on the physicochemical properties of the aggregated substrate, which are subject to continuous modification by the products released during hydrolysis. We propose here an experimental design that, by means of the incorporation of a fluorescent substrate at very low molar ratio (< or = 1:500) into a nonhydrolizable liposomal matrix of 1,2-dihexadecyl-sn-glycero-3-phosphocholine (DHPC), allows the study of hydrolysis by porcine pancreatic phospholipase A2, in virtual absence of physical perturbations of the lamellar phase, by the released products. We have been able to measure immediate hydrolysis of the fluorescent substrate 1,2-di-[omega(1'-pyreno)-decanoyl]-sn- glycero-3-phosphocholine when the sonicated liposomal matrix is in the gel phase. In the liquid crystalline state, in contrast, hydrolysis is very poor even after 80 min of adding the enzyme. Both in the gel and liquid-crystalline phases, incorporation of unlabeled PLA2 products activates the hydrolysis rate to comparable levels. It appears that the conformation adopted by the substrate immersed in the gel or liquid crystalline matrix is especially important in determining its susceptibility to hydrolysis in the absence of products.