Feng S S, MacDonald R C
Department of Biochemistry, Molecular Biology, and Cell Biology, Northwestern University, Evanston, Illinois 60208, USA.
Biophys J. 1995 Aug;69(2):460-9. doi: 10.1016/S0006-3495(95)79919-2.
An equation of state for lipid monolayers at the air-water interface is presented, which takes into account the effects of the conformation and the number and position of double bonds of the hydrocarbon chains. The total Hamiltonian of the monolayer is assumed to consist of three terms. Two of them are calculated exactly within the limitations of the formulation. These are the two-dimensional entropy of mixing of the lipid and water molecules at the surface and the conformational entropy of the chains using a model available from the literature. These two terms give rise to positive surface pressure. The third term, which includes all energies that are not amenable to calculation, was obtained as the difference between the sum of the two calculated terms and experimental data and is found to represent an approximately area-independent tension. The effects of chain unsaturation on the equation of state were modeled in the present theory in two ways; the chain bend caused by cis double bonds increases the minimal molecular area, and the double bond linkage on a chain decreases the degrees of freedom of the chain. Calculations revealed that the former is highly significant whereas the latter is negligible. The deduced equation of state reproduces experimental data with appropriate values for three parameters, which represent the collapse area, the overlap of adjacent chains, and the combined effects of the intra- and intermolecular interactions other than the surface mixing entropy and the chain conformational energy.
提出了一种用于描述气-水界面脂质单分子层的状态方程，该方程考虑了烃链构象、双键数量及位置的影响。假设单分子层的总哈密顿量由三项组成。其中两项在公式的限制范围内进行了精确计算。这两项分别是表面脂质分子与水分子的二维混合熵，以及使用文献中的模型计算得到的链的构象熵。这两项会产生正的表面压力。第三项包含所有难以计算的能量，它是通过已计算的两项之和与实验数据的差值得到的，并且发现它代表了一种近似与面积无关的张力。在本理论中，通过两种方式对链不饱和度对状态方程的影响进行了建模；顺式双键导致的链弯曲增加了最小分子面积，而链上的双键连接减少了链的自由度。计算表明，前者影响非常显著，而后者可忽略不计。推导得到的状态方程通过三个参数的适当取值再现了实验数据，这三个参数分别代表塌陷面积、相邻链的重叠，以及除表面混合熵和链构象能之外的分子内和分子间相互作用的综合影响。
