Krivanek Roland, Okoro Linus, Winter Roland
University of Dortmund, Department of Chemistry, Physical Chemistry I-Biophysical Chemistry, Dortmund, Germany.
Biophys J. 2008 May 1;94(9):3538-48. doi: 10.1529/biophysj.107.122549. Epub 2008 Jan 16.
Although sterol-phospholipid interactions have been of interest for many years now, a complete thermodynamic profile of these systems is still missing. To contribute to a better understanding of the thermodynamic functions of these systems, we determined isothermal compressibility coefficient data for dipalmitoylphosphocholine (DPPC) and DPPC-containing cholesterol and ergosterol vesicles by means of molecular acoustics (ultrasound velocimetry and densimetry) and differential scanning and pressure perturbation calorimetric techniques. A particular focus was on the influence of the differential structural properties of the two sterols on the thermodynamic properties of lipid bilayers, and on the nature of the critical point region of phospholipid-sterol systems by determining thermodynamic fluctuation parameters. Contrary to significant changes in conformational and dynamical properties of the DPPC-sterol membranes, no marked differences were found in the various thermodynamic properties studied, including the adiabatic (beta(S)(lipid)) and isothermal (beta(T)(lipid)) compressibility, as well as the volume fluctuations. Differences in beta(T)(lipid) and beta(S)(lipid) become dramatic in the gel-fluid transition region only, due to a significant degree of slow relaxational processes in the microsecond time range in the transition region. Our data show no evidence for the existence of a typical critical point phenomenon in the concentration and temperature range where a critical point in the DPPC-sterol phase diagram is expected to appear. Hence, on a macroscopic level, it seems more appropriate to describe the sterol-phospholipid binary mixtures in the liquid-ordered/liquid-disordered coexistence region as a phase region consisting essentially of small nanodomains only. Such small-domain dimensions, with a series of particular properties such as increased line energy, spontaneous curvature, and limited lifetime, seem also to be typical of raftlike domains in cell membranes.
尽管多年来人们一直对甾醇 - 磷脂相互作用感兴趣，但这些体系完整的热力学概况仍未可知。为了有助于更好地理解这些体系的热力学功能，我们通过分子声学（超声测速法和密度测定法）以及差示扫描量热法和压力扰动量热技术，测定了二棕榈酰磷脂酰胆碱（DPPC）以及含胆固醇和麦角固醇的DPPC囊泡的等温压缩系数数据。特别关注的是两种甾醇不同的结构特性对脂质双层热力学性质的影响，以及通过测定热力学涨落参数来研究磷脂 - 甾醇体系临界点区域的性质。与DPPC - 甾醇膜的构象和动力学性质的显著变化相反，在所研究的各种热力学性质中未发现明显差异，包括绝热压缩系数（β(S)(脂质)）和等温压缩系数（β(T)(脂质)）以及体积涨落。仅在凝胶 - 流体转变区域，β(T)(脂质)和β(S)(脂质)的差异才变得显著，这是由于转变区域中微秒时间范围内存在显著程度的缓慢弛豫过程。我们的数据表明，在预期会出现DPPC - 甾醇相图临界点的浓度和温度范围内，没有证据表明存在典型的临界点现象。因此，在宏观层面上，将液体有序/液体无序共存区域中的甾醇 - 磷脂二元混合物描述为基本上仅由小纳米域组成的相区域似乎更为合适。这种具有一系列特殊性质（如增加的线能、自发曲率和有限寿命）的小域尺寸，似乎也是细胞膜中筏状域的典型特征。
