Li S, Lin H N, Wang G, Huang C
Department of Biochemistry, Health Sciences Center, University of Virginia, Charlottesville 22908, USA.
Biophys J. 1996 Jun;70(6):2784-94. doi: 10.1016/S0006-3495(96)79848-X.
The biphasic effect of ethanol on the main phase transition temperature (Tm) of identical-chain phosphatidyl-cholines (PCs) in excess H2O is now well known. This biphasic effect can be attributed to the transformation of the lipid bilayer, induced by high concentrations of ethanol, from the partially interdigitated L beta, phase to the fully interdigitated L beta I phase at T < Tm. The basic packing unit of the L beta I phase has been identified recently as a binary mixture of PC/ethanol at the molar ratio of 1:2. The ethanol effect on mixed-chain PCs, however, is not known. We have thus in this study investigated the alcohol effects on the Tm of mixed-chain PCs with different delta C values, where delta C is the effective acyl chain length difference between the sn-1 and sn-2 acyl chains. Initially, molecular mechanics (MM) simulations are employed to calculate the steric energies associated with a homologous series of mixed-chain PCs packed in the partially and the fully interdigitated L beta I motifs. Based on the energetics, the preference of each mixed-chain PC for packing between these two different motifs can be estimated. Guided by MM results, high-resolution differential scanning calorimetry is subsequently employed to determine the Tm values for aqueous lipid dispersions prepared individually from a series of mixed-chain PCs (delta C = 0.5-6.5 C-C bond lengths) in the presence of various concentrations of ethanol. Results indicate that aqueous dispersions prepared from mixed-chain PCs with a delta C value of less than 4 exhibit a biphasic profile in the plot of Tm versus ethanol concentration. In contrast, highly asymmetric PCs (delta C > 4) do not exhibit such biphasic behavior. In the presence of a longer chain n-alcohol, however, aqueous dispersions of highly asymmetric C(12):C(20)PC (delta C = 6.5) do show such biphasic behavior against ethanol. Our results suggest that the delta C region in a highly asymmetric PC packed in the L beta I phase is most likely the binding site for n-alcohol.
乙醇对过量水中相同链长磷脂酰胆碱（PCs）主相变温度（Tm）的双相效应现已广为人知。这种双相效应可归因于高浓度乙醇诱导的脂质双层在T < Tm时从部分交叉的Lβ相转变为完全交叉的LβI相。LβI相的基本堆积单元最近被确定为摩尔比为1:2的PC/乙醇二元混合物。然而，乙醇对混合链PCs的影响尚不清楚。因此，在本研究中，我们研究了乙醇对具有不同ΔC值的混合链PCs的Tm的影响，其中ΔC是sn-1和sn-2酰基链之间的有效酰基链长度差。首先，采用分子力学（MM）模拟来计算与以部分交叉和完全交叉的LβI基序堆积的一系列混合链PCs相关的空间能。基于能量学，可以估计每个混合链PC在这两种不同基序之间堆积的偏好。在MM结果的指导下，随后采用高分辨率差示扫描量热法来测定在各种乙醇浓度下由一系列混合链PCs（ΔC = 0.5 - 6.5个C-C键长度）单独制备的水性脂质分散体的Tm值。结果表明，由ΔC值小于4的混合链PCs制备的水性分散体在Tm与乙醇浓度的关系图中呈现双相特征。相比之下，高度不对称的PCs（ΔC > 4）不表现出这种双相行为。然而，在存在较长链的正醇时，高度不对称的C(12):C(20)PC（ΔC = 6.5）的水性分散体对乙醇确实表现出这种双相行为。我们的结果表明，在LβI相中堆积的高度不对称PC中的ΔC区域很可能是正醇的结合位点。
