Nielsen M, Miao L, Ipsen J H, Zuckermann M J, Mouritsen O G
Centre for the Physics of Materials, Department of Physics, McGill University, 3600 University Street, Montreal, Canada H3A 2T8.
Phys Rev E Stat Phys Plasmas Fluids Relat Interdiscip Topics. 1999 May;59(5 Pt B):5790-803. doi: 10.1103/physreve.59.5790.
Lipid bilayers exhibit a phase behavior that involves two distinct, but coupled, order-disorder processes, one in terms of lipid-chain crystalline packing (translational degrees of freedom) and the other in terms of lipid-chain conformational ordering (internal degrees of freedom). Experiments and previous approximate theories have suggested that cholesterol incorporated into lipid bilayers has different microscopic effects on lipid-chain packing and conformations and that cholesterol thereby leads to decoupling of the two ordering processes, manifested by a special equilibrium phase, "liquid-ordered phase," where bilayers are liquid (with translational disorder) but lipid chains are conformationally ordered. We present in this paper a microscopic model that describes this decoupling phenomena and which yields a phase diagram consistent with experimental observations. The model is an off-lattice model based on a two-dimensional random triangulation algorithm and represents lipid and cholesterol molecules by hard-core particles with internal (spin-type) degrees of freedom that have nearest-neighbor interactions. The phase equilibria described by the model, specifically in terms of phase diagrams and structure factors characterizing different phases, are calculated by using several Monte Carlo simulation techniques, including histogram and thermodynamic reweighting techniques, finite-size scaling as well as non-Boltzmann sampling techniques (in order to overcome severe hysteresis effects associated with strongly first-order phase transitions). The results provide a consistent interpretation of the various phases of phospholipid-cholesterol binary mixtures based on the microscopic dual action of cholesterol on the lipid-chain degrees of freedom. In particular, a distinct small-scale structure of the liquid-ordered phase has been identified and characterized. The generic nature of the model proposed holds a promise for a unifying description for a whole series of different lipid-sterol mixtures.
脂质双层呈现出一种相行为，该行为涉及两个不同但相互关联的有序-无序过程，一个涉及脂链的晶体堆积（平移自由度），另一个涉及脂链的构象有序（内部自由度）。实验和先前的近似理论表明，掺入脂质双层中的胆固醇对脂链堆积和构象具有不同的微观影响，因此胆固醇会导致这两个有序过程解耦，表现为一种特殊的平衡相，即“液相有序相”，其中双层是液态的（具有平移无序），但脂链是构象有序的。我们在本文中提出了一个微观模型，该模型描述了这种解耦现象，并得出了与实验观察结果一致的相图。该模型是一个基于二维随机三角剖分算法的非晶格模型，用具有内部（自旋型）自由度且存在最近邻相互作用的硬核粒子来表示脂质和胆固醇分子。通过使用几种蒙特卡罗模拟技术，包括直方图和热力学重加权技术、有限尺寸标度以及非玻尔兹曼采样技术（以克服与强一级相变相关的严重滞后效应），计算了该模型所描述的相平衡，特别是根据相图和表征不同相的结构因子。结果基于胆固醇对脂链自由度的微观双重作用，对磷脂 - 胆固醇二元混合物的各个相提供了一致的解释。特别是，已识别并表征了液相有序相独特的小尺度结构。所提出模型的一般性为统一描述一系列不同的脂质 - 甾醇混合物带来了希望。
