Probing the ethanol-induced chain interdigitations in gel-state bilayers of mixed-chain phosphatidylcholines

Using high-resolution differential scanning calorimetry (DSC), we have studied the effects of ethanol concentrations, [EtOH], on the main phase transition temperatures (T-m) of the following mixed-chain phosphatidylcholines (PCs):C(15):C(17)PC, C(17):C(15)PC, and C(12):C(20)PC. These lipids have a common molecular weight; however, their apparent acyi chain-length differences between the sn-1 and sn-2 acyl chains, Delta C, are distinctively different. The Delta C values for these three mixed-chain PCs are, respectively, 0.5, 3.5, and 6.5 C-C bond lengths. DSC results show that the T-m profiles for C(15):C(17)PC and C(17):C(15)PC bilayers in the plot of T-m versus [EtOH] are V-shaped biphasic curves, with the minimum T-m occurring at 50 and 73 mg/ml of ethanol, respectively. In contrast, the C(12):C(20)PC bilayer exhibits a nearly linear decrease in T-m with increasing [EtOH]. In addition, x-ray diffraction experiments were also performed to assess the structural changes of these three mixed-chain PCs in the gel-state bilayers, at 20 degrees C, in response to high concentrations of ethanol. X-ray diffraction data indicate that, in the absence of ethanol, these three lamellar lipids are all packed in the normal (L-beta') gel phase in aqueous media. In the presence of 120 mg/ml of ethanol, however, the C(15):C(17)PC and C(17):C(15)PC lamellae are packed in the fully interdigitated (L beta') gel phase. The V-shaped T-m curves detected calorimetrically for these two lipids in response to [EtOH] can thus be explained by the ethanol-induced L-beta' --> L-beta I isothermal phase transition. Interestingly, the results of x-ray diffraction study reveal, for the first time, that an ethanol-induced L-beta' --> L-MI (mixed interdigitated phase) isothermal phase transition occurs in the gel-state bilayer of highly asymmetrical C(12):C(20)PC. Therefore, the chain asymmetry is recognized to play an important role in the ethanol-induced chain interdigitation at T < T-m.