Pabst G, Amenitsch H, Kharakoz D P, Laggner P, Rappolt M
Institute of Biophysics and X-Ray Structure Research, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria.
Phys Rev E Stat Nonlin Soft Matter Phys. 2004 Aug;70(2 Pt 1):021908. doi: 10.1103/PhysRevE.70.021908. Epub 2004 Aug 19.
We have determined the structural properties and bending fluctuations of fully hydrated phosphatidylcholine multibilayers in the fluid (Lalpha) phase, as well as the structure of the ripple (Pbeta') phase near the main phase transition temperature (TM) by x-ray diffraction. The number of carbons, nHC, per acyl chain of the studied disaturated lipids varied from 14 to 22. All lipids exhibit a nonlinear increase of the lamellar repeat distance d in the Lalpha phase upon approaching TM, known as "anomalous swelling." The nonlinear increase reduces with chain length, but levels off at a constant value of about 0.5 A for lipids with more than 18 hydrocarbons per chain. A detailed analysis shows that anomalous swelling has two components. One is due to an expansion of the water layer, which decreases with chain length and finally vanishes for nHC >18. The second component is due to a bilayer thickness increase, which remains unchanged in its temperature dependence, including a nonlinear component of about 0.5 A in the vicinity of TM. Thus, anomalous swelling above 18 hydrocarbons per chain is due to the pretransitional effects on the membrane only. These results are supported by a bending fluctuation analysis revealing increased undulations close to TM only for the short chain lipids. We have further calculated the electron density maps in the ripple phase and find no coupling of the magnitude of the ripple amplitude to the chain length effects observed in the Lalpha phase. Hence, in agreement with an earlier report by Mason et al. [Phys. Rev. E 63, 030902 (2001)] there is no connection between the formation of the ripple phase and anomalous swelling.
我们通过X射线衍射确定了完全水合的磷脂酰胆碱多层膜在流体（Lα）相中的结构特性和弯曲波动，以及在主相变温度（TM）附近的波纹（Pβ'）相的结构。所研究的二饱和脂质每条酰基链的碳原子数nHC在14到22之间变化。所有脂质在接近TM时，Lα相中层状重复距离d呈现非线性增加，即所谓的“异常膨胀”。这种非线性增加随链长而减小，但对于每条链含有超过18个烃的脂质，在约0.5 Å的恒定值处趋于平稳。详细分析表明，异常膨胀有两个组成部分。一个是由于水层的膨胀，其随链长而减小，对于nHC > 18最终消失。第二个组成部分是由于双层厚度增加，其温度依赖性保持不变，包括在TM附近约0.5 Å的非线性部分。因此，每条链超过18个烃时的异常膨胀仅归因于对膜的预转变效应。这些结果得到弯曲波动分析的支持，该分析表明仅对于短链脂质，在接近TM时波动增加。我们进一步计算了波纹相中的电子密度图，发现波纹幅度的大小与在Lα相中观察到的链长效应没有耦合。因此，与Mason等人早期的报告[《物理评论E》63, 030902 (2001)]一致，波纹相的形成与异常膨胀之间没有联系。
