Lipid solvation of the aqueous form of the myelin proteolipid apoprotein: evidence and characterization of two lipid populations by fluorescence polarization, differential calorimetry, and sucrose gradient centrifugation

Minimal surface tension, squeeze-out and transition temperatures of binary mixtures of dipalmitoylphosphatidylcholine and unsaturated phospholipids

Fluorescence polarization (FP) measurements and surface tension (ST) experiments were performed to determine the gel-to-liquid-crystal transition or melting temperature of phospholipid mixtures. The FP-temperature diagrams showed main transition temperatures of 41 degrees C for dipalmitoylphosphatidylcholine (DPPC). The 7:3 and 9:1 binary mixtures of DPPC and phosphatidylinositol (PI), phosphatidylglycerol (PG) and phosphatidylcholine (PC) had main transition temperatures of, respectively, 32-36 degrees C and 37-39 degrees C. The minimal surface tension of DPPC monolayers increased rapidly at 40 degrees C, suggesting that this was the transition temperature for the melting of these monolayers. This value was in close accordance with the main transition temperature of DPPC, observed with the fluorescence polarization measurements. Melting temperatures of monolayers were higher for almost all mixtures than the temperatures at which the transition started, indicating preferential squeeze out of the unsaturated component and enrichment of the monolayer with DPPC. However, neither the 7:3 DPPC/PC nor the DPPC/PG mixtures could withstand high surface pressures at temperatures above 30 degrees C, whereas monolayers of DPPC/PG (9:1) became fluid at temperatures above 35 degrees C. Preferential squeeze-out of the unsaturated phospholipid was especially effective in both the 7:3 and 9:1 DPPC/PI mixtures. These monolayers started to melt at 39-40 degrees C, which is above their main transition temperatures of, respectively, 32 and 37 degrees C, and which approximate the melting temperature of DPPC. Preferential squeeze-out is essential for an artificial lung surfactant. The estimation of this phenomenon by determining the monolayer melting temperatures is therefore useful for distinguishing between mixtures which are effective surfactants at body temperature and those which are less effective.

Comparative study of myelin proteolipid apoprotein solvation by multilayer membranes of synthetic DPPC and biological lipid extract from bovine brain. An FT-IR investigation

The interaction between the aqueous form of the myelin proteolipid apoprotein (PLA) and model membranes prepared with either synthetic dipalmitoylphosphatidyl choline (DPPC) or biological lipids extracted from bovine brain (BE) has been investigated by Fourier-Transform IR spectroscopy. IR spectra obtained with lyophilized samples of PLA demonstrated 2 main peaks (amide I and amide II) culminating at 1656 cm-1 and 1545 cm-1, which we assigned to helical conformation. When PLA was solvated in DPPC or BE membranes, both the amide I and amide II features remained located at 1655 cm-1 and 1545 cm-1, although their half-width significantly decreased, demonstrating that the lipid environment favoured alpha helix structures. However differences between both mixtures were detected by measuring the amide I and amide II half-widths as a function of the L:P molar ratio. Moreover, analysis of the 1545/1515 peak intensity ratio brought evidence of different localization and/or molecular arrangement of the protein segments containing tyrosine residues, depending on the lipid composition of the membrane. According to previously published models, these data suggest that recombinants prepared with PLA and BE multilayers better mimic the biological membrane than do DPPC-PLA mixtures.