THERMODYNAMIC ELUCIDATION OF ETHANOL-INDUCED INTERDIGITATION OF HYDROCARBON CHAINS IN PHOSPHATIDYLCHOLINE BILAYER VESICLES

The interdigitation of hydrocarbon chains in phosphatidylcholine bilayer vesicles was examined from the thermodynamic point of view. In 1,2-dipalmitoylphosphatidylcholine vesicles, the temperature (t(m)), the free energy (DELTA-G), the enthalpy (DELTA-H), and the entropy (DELTA-S) of the main lipid phase transition decrease in the presence of [EtOH] < 50 mg/mL (1.10 M), where no interdigitated phase is formed. Conversely, these parameters increase as [EtOH] increases from 50 to 103 mg/mL (1.10-2.24 M), where the interdigitated phase occurs. In contrast, for 1,2-dimyristoylphosphatidylcholine vesicles at [EtOH] < 112 mg/mL, where no interdigitated phase is formed, values of t(m), DELTA-G, DELTA-H and DELTA-S decrease in the presence of [EtOH] < 35 mg/mL (0.75 M), while DELTA-H and DELTA-S increase and t(m) and DELTA-G decrease at [EtOH] = 35-112 mg/mL (0.75-2.43 M). The obtained thermodynamic parameters were compared between these two lipid vesicle systems. The results were used to elucidate ethanol's modification of hydrophobic interactions in lipid bilayer vesicles in relation to the formation of the interdigitated phase. The dependence of fluorescence steady-state polarization (P) on [EtOH] in both lipid vesicles below and above t(m) was also examined to supplement thermodynamic observations.