Huang C, McIntosh T J
Department of Biochemistry, Health Sciences Center, University of Virginia, Charlottesville 22908, USA.
Biophys J. 1997 Jun;72(6):2702-9. doi: 10.1016/S0006-3495(97)78913-6.
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 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 beta[I]) 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).
我们使用高分辨率差示扫描量热法（DSC）研究了乙醇浓度[EtOH]对以下混合链磷脂酰胆碱（PC）主相变温度（T[m]）的影响：C(15):C(17)PC、C(17):C(15)PC和C(12):C(20)PC。这些脂质具有相同的分子量；然而，它们在sn-1和sn-2酰基链之间的表观酰基链长度差异ΔC明显不同。这三种混合链PC的ΔC值分别为0.5、3.5和6.5个碳-碳键长度。DSC结果表明，在T(m)对[EtOH]的图中，C(15):C(17)PC和C(17):C(15)PC双层膜的T(m)曲线为V形双相曲线，最低T(m)分别出现在乙醇浓度为50和73 mg/ml时。相比之下，C(12):C(20)PC双层膜的T(m)随[EtOH]增加呈近似线性下降。此外，还进行了X射线衍射实验，以评估这三种混合链PC在20℃的凝胶态双层膜中对高浓度乙醇的结构变化。X射线衍射数据表明，在没有乙醇的情况下，这三种层状脂质在水性介质中均以正常（Lβ'）凝胶相堆积。然而，在存在120 mg/ml乙醇的情况下，C(15):C(17)PC和C(17):C(15)PC层状结构以完全交错（Lβ[I]）凝胶相堆积。因此，通过乙醇诱导Lβ'→Lβ[I]等温相变可以解释这两种脂质在量热法中检测到的对[EtOH]的V形T(m)曲线。有趣的是，X射线衍射研究结果首次揭示，在高度不对称的C(12):C(20)PC凝胶态双层膜中发生了乙醇诱导的Lβ'→L(MI)（混合交错相）等温相变。因此，链不对称性被认为在T < T(m)时乙醇诱导的链交错中起重要作用。
