Radhakrishnan A, McConnell H M
Department of Chemistry, Stanford University, Stanford, CA 94305, USA.
Proc Natl Acad Sci U S A. 2000 Feb 1;97(3):1073-8. doi: 10.1073/pnas.97.3.1073.
Monolayer mixtures of dihydrocholesterol and phospholipids at the air-water interface are used to model membranes containing cholesterol and phospholipids. Specific, stoichiometric interactions between cholesterol and some but not all phospholipids have been proposed to lead to the formation of condensed complexes. It is reported here that an externally applied electric field of the appropriate sign can destabilize these complexes, resulting in their dissociation. This is demonstrated through the application of an electric field gradient that leads to phase separations in otherwise homogeneous monolayers. This is observed only when the monolayer composition is close to the stoichiometry of the complex. The electric field effect is analyzed with the same mean field thermodynamic model as that used previously to account for pairs of upper miscibility critical points in these mixtures. The concentrations of dihydrocholesterol, phospholipid, and complex vary strongly and sometimes discontinuously in the monolayer membrane in the field gradient. The model is an approximation to a two-dimensional liquid in which molecules freely exchange between free and complexed form so that the chemical potentials are constant throughout the membrane. The calculations are illustrated for a complex of about 15 molecules, composed of 5 cholesterol molecules and 10 phospholipid molecules.
在气-水界面处，二氢胆固醇和磷脂的单层混合物被用于模拟含有胆固醇和磷脂的膜。有人提出，胆固醇与部分而非全部磷脂之间特定的化学计量相互作用会导致凝聚复合物的形成。据本文报道，施加适当符号的外部电场会使这些复合物不稳定，导致其解离。这通过施加电场梯度得以证明，该梯度会在原本均匀的单层中引发相分离。只有当单层组成接近复合物的化学计量时，才会观察到这种现象。电场效应采用与先前用于解释这些混合物中对上临界混溶点对相同的平均场热力学模型进行分析。在电场梯度作用下，单层膜中二氢胆固醇、磷脂和复合物的浓度变化强烈，有时甚至不连续。该模型是对二维液体的一种近似，其中分子在自由形式和复合形式之间自由交换，从而使整个膜中的化学势保持恒定。针对由5个胆固醇分子和10个磷脂分子组成的约15个分子的复合物进行了计算说明。
