Giri Rajendra P, Chakrabarti Abhijit, Mukhopadhyay Mrinmay K
Saha Institute of Nuclear Physics, HBNI , 1/AF, Bidhannagar, Kolkata 700064, India.
J Phys Chem B. 2017 Apr 27;121(16):4081-4090. doi: 10.1021/acs.jpcb.6b12587. Epub 2017 Apr 17.
Lateral and out-of-plane organization of cholesterol and its effect on regulating the physicochemical properties of zwitterionic phospholipid model membranes have been investigated by a pressure-area isotherm study from the Langmuir monolayer, atomic force microscopy (AFM), and X-ray reflectivity (XRR) measurements from supported binary monolayer films. The systematic isotherm studies on the Langmuir monolayer of phospholipids and the subsequent extraction of excess Gibbs free energy (ΔG) revealed the mechanism of cholesterol interaction and the molecular cooperativeness for different arrangements in the phospholipid model membranes. We have found a critical cholesterol molar concentration (χ) up to which the lipid-cholesterol miscibility gradually increases and then further increase in the concentration leads to an inhomogeneous structure formation similar to raft structures. The thickening in the lipid acyl chain and the subsequent lowering of the lipid head group thickness up to χ are also evident from the XRR study. Beyond χ, large-sized domains are observed in the AFM images from the deposited monolayer. χ has also been observed to depend on the phase of the monolayer, in particular, ∼25 molar % in the gel phase and ∼40 molar % in the fluid phase, wherein a regular distribution has been found with the highest separation between the cholesterol molecules. The extracted isothermal compressibility coefficient (C) and ΔG from the monolayer isotherms indicate that the molecular arrangement at χ are the most stable configurations of the monolayer. Our study provides direct evidence into cholesterol-induced evolution in phase behavior and the consequent model on the structure at different phases in the phospholipid Langmuir monolayers.
通过对Langmuir单层膜进行压力-面积等温线研究、原子力显微镜（AFM）以及对支撑的二元单层膜进行X射线反射率（XRR）测量，研究了胆固醇的横向和面外组织及其对两性离子磷脂模型膜物理化学性质的调节作用。对磷脂Langmuir单层膜进行的系统等温线研究以及随后对过量吉布斯自由能（ΔG）的提取，揭示了胆固醇相互作用的机制以及磷脂模型膜中不同排列的分子协同性。我们发现了一个临界胆固醇摩尔浓度（χ），在该浓度之前，脂质-胆固醇的混溶性逐渐增加，而浓度的进一步增加会导致形成类似于筏状结构的不均匀结构。从XRR研究中也可以明显看出，脂质酰基链增厚以及脂质头基厚度在达到χ之前随之降低。超过χ后，在沉积单层膜的AFM图像中观察到大型区域。还观察到χ取决于单层膜的相，特别是在凝胶相中约为25摩尔％，在流体相中约为40摩尔％，其中发现胆固醇分子之间的间距最大且分布规则。从单层等温线中提取的等温压缩系数（C）和ΔG表明，χ处的分子排列是单层膜最稳定的构型。我们的研究为胆固醇诱导的相行为演变以及磷脂Langmuir单层膜不同相结构的相应模型提供了直接证据。
