Interaction of n-Propanol and Isopropanol with Dipalmitoylphosphatidylcholine Bilayer
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Differential scanning calorimetry (DSC) was employed to study the effects of n-propanol and isopropanol on the phase transitions of dipalmitoylphosphatidylcholine (DPPC), a 16-carbon acyl chain phospholipid. Various concentrations of both alcohols were used in the investigation. The biphasic effect, observed above 0.39M n-propanol and 0.52M isopropanol, was identified by the large hysteresis between the heating and cooling scans on the gel-to-liquid crystalline main transition temperatures. At low alcohol concentrations, the main transition temperatures decreased with increasing alcohol concentrations. After the critical concentration, the main transition temperatures started to increase due to the formation of the more stable interdigitated gel phase. The induction of this particular gel phase is identified not only by the biphasic effect, but also the disappearance of the pretransitions. This critical concentration was found to be higher for isopropanol than for n-propanol, suggesting that the branching in isopropanol makes it less effective in inducing the formation of the interdigitated phase in DPPC. On some heating and cooling scans, an extra peak was also observed around and beyond the critical concentration, suggesting a mixture of phases as the interdigitated phase forms.