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 (Tm]) 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 acyl 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 betaI]) 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 betaI] 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).