THE EFFECT OF BACTERIORHODOPSIN ON THE TH ERMOTROPIC BEHAVIOR OF 1,2-DIPALMITOYL-SN-GLYCERO-3-PHOSPHOCHOLINE IN THE RECONSTITUTED PURPLE MEMBRANES

The thermotropic behavior of 1,2-dipalmitoyl-sn-glycero-3-phosphocholi ne in reconstituted membranes containing delipidated bacteriorhodopsin at low lipid-protein ratios (10/1-30/1, by moles) was investigated us ing DSC and freeze-fracture electron microscopy. The existence of two populations pf dipalmitoylphosphalidylcholine has been shown assigned to the <<bound>> and <<trapped>> lipid molecules, which undergo the ge l-to-liquid-cristalline phase transition with lower enthalpy and coope rativity as compared to the protein-less membranes having all lipid mo lecules in the <<free>> state. The phase transition temperature of the <<bound>> lipids slightly differs from that of the <<free>> lipids, w hile the phase transition of the <<trapped>> lipids occurs at the temp erature that is 8 degrees C lower than in the case of the <<free>> lip ids. The analysis of electron micrographs together with DSC data gives the evidence that breakdown of protein particles caused by heat denat uration of bacteriorhodopsin results in increase of the <<trapped>> li pid content.