Thermodynamics and volume compressibility of phosphatidylcholine liposomes containing bacteriorhodopsin
The effects of bacteriorhodopsin (BR) interaction with large unilamellar vesicles (LUV) of dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) were examined at various BR/lipid ratios, using differential scanning calorimetry (DSC) and ultrasound velocimetry (USV). On DSC, a shift of T c towards higher temperatures and considerable dynamics of structural transition of membranes were found for DMPC proteoliposomes. The changes of thermodynamical parameters suggest long-distance interactions between regions of altered bilayer structure which arise around each BR molecule. The changes of ultrasound absorbance and velocity were in coincidence with DSC results. The concentration increment of ultrasound velocity that characterizes volume compressibility of vesicles, increased with the increasing BR concentration in DPPC LUV in gel state (T < T c ). Saturation of concentration increment was observed at BR/LUV ratio 1/0.5 mol/mol. No significant changes of compressibility were observed at liquid-crystal state (T > T c ). This means that, in the gel state of lipid bilayer, one BR molecule is enough to change the physical properties of one LUV.
Key wordsBacteriorhodopsin phosphatidylcholine liposomes phase transitions calorimetry ultrasound velocimetry
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