Kubica Krystian, Langner Marek, Gabrielska Janina
Agricultural University, Department of Physics and Biophysics, Norwida 25, 50-375 Wrocław, Poland.
Cell Mol Biol Lett. 2003;8(4):943-54.
The degree of dependence of a lipid bilayer's surface properties on its conformational state is still an unresolved question. Surface properties are functions of molecular organization in the complex interfacial region. In the past, they were frequently measured using fluorescence spectroscopy. Since a fluorescent probe provides information on its local environment, there is a need to estimate the effect caused by the probe itself. In this paper, we address this question by calculating how lipid head-group orientation effects the fluorescence intensity of Fluorescein-PE (a probe that is sensitive to surface potential). In the theoretical model assumed the lipid bilayer state and the interactions between the charged fluorescent probe and the surrounding lipid molecules was evaluated. The results of this theoretical analysis were compared with experimentally obtained data. A lipid bilayer formed from DPPC was chosen as the experimental system, since it exhibits all the major conformational states within a narrow temperature range of 30 degrees C-45 degrees C. Fluorescein-PE fluorescence intensity depends on local pH, which in turn is sensitive to local electrostatic potential in the probe's vicinity. This local electrostatic potential is generated by lipid head-group dipole orientation. We have shown that the effect of the probe on lipid bilayer properties is limited when the lipid bilayer is in the gel phase, whereas it is more pronounced when the membrane is liquid-crystalline. This implies that Fluorescein-PE is a good reporter of local electrostatic fields when the lipid bilayer is in the gel phase, and is a poor reporter when the membrane is in the liquid-crystalline state.
脂质双层的表面性质对其构象状态的依赖程度仍是一个未解决的问题。表面性质是复杂界面区域中分子组织的函数。过去，它们经常使用荧光光谱法进行测量。由于荧光探针可提供有关其局部环境的信息，因此有必要估计探针本身所造成的影响。在本文中，我们通过计算脂质头部基团的取向如何影响荧光素 - 磷脂酰乙醇胺（一种对表面电位敏感的探针）的荧光强度来解决这个问题。在理论模型中，对脂质双层状态以及带电荧光探针与周围脂质分子之间的相互作用进行了评估。将该理论分析的结果与实验获得的数据进行了比较。选择由二棕榈酰磷脂酰胆碱（DPPC）形成的脂质双层作为实验系统，因为它在30℃至45℃的狭窄温度范围内呈现出所有主要的构象状态。荧光素 - 磷脂酰乙醇胺的荧光强度取决于局部pH值，而局部pH值又对探针附近的局部静电势敏感。这种局部静电势是由脂质头部基团的偶极取向产生的。我们已经表明，当脂质双层处于凝胶相时，探针对脂质双层性质的影响是有限的，而当膜处于液晶相时，这种影响更为明显。这意味着当脂质双层处于凝胶相时，荧光素 - 磷脂酰乙醇胺是局部静电场的良好报告分子，而当膜处于液晶状态时，它是较差的报告分子。
