School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore, Singapore.
Department of Biomedical Engineering, University of California, Davis, Davis, California 95616, United States.
J Am Chem Soc. 2020 Dec 30;142(52):21872-21882. doi: 10.1021/jacs.0c10674. Epub 2020 Dec 21.
Crystallization of membrane-embedded components within phospholipid bilayers represents a distinct class of phase transformation that occurs in structurally organized, molecularly crowded, and dimensionally constrained amphiphilic fluids. Using unstable supported lipid bilayers-transiently assembled via surface-mediated fusion and spreading of bicellar precursors containing supersaturating concentrations of cholesterol-we monitor here the morphological evolution and dynamics of cholesterol crystallization within the membrane media. We find that the three-dimensional (3D) crystallization of cholesterol from an unstable two-dimensional (2D) in-membrane state proceeds via well-defined sequence of intermediates, including filaments, rods, helices, and 2D rectangular plates, before transforming into three-dimensional quadrilateral crystals-characteristic triclinic habit of cholesterol monohydrate. Our observations thus demonstrate that these structurally distinct cholesterol polymorphs are related to one another, contrasting with the notion that they represent disparate crystal habits stabilized by differences in lipid environments. Moreover, these observations indicate that cholesterol crystallization within the membrane media follows nonclassical multistep crystallization governed by the heuristic "Ostwald's rule of stages", which predicts that the crystallization kinetics proceed down the free energy landscape in a multistage process where each successive phase transition incurs the smallest loss of free energy relative to its predecessor. Furthermore, we find that the well-known cholesterol extracting agent, β-cyclodextrin, acts by catalytically tipping the equilibrium in favor of crystal growth adding cholesterol from the membrane phase to the crystal in a layer-by-layer manner. Taken together, our results provide a new description of in-membrane cholesterol crystallization and may pave for a screening tool for identifying molecular candidates that target cholesterol crystals.
磷脂双层膜内膜嵌入成分的结晶是一种独特的相转变，发生在结构有序、分子拥挤和维度受限的两亲性流体中。使用不稳定的支撑磷脂双层膜-通过含有胆固醇过饱和浓度的双锥形前体的表面介导融合和扩展瞬时组装-我们在这里监测胆固醇在膜介质中的形态演变和结晶动力学。我们发现，胆固醇从不稳定的二维（2D）膜内状态向三维（3D）结晶的过程是通过一系列明确的中间体进行的，包括纤维、棒、螺旋和 2D 矩形板，然后转化为三维四边形晶体-胆固醇一水合物的典型斜方晶习惯。因此，我们的观察结果表明，这些结构上不同的胆固醇多晶型体是相互关联的，这与它们代表由脂质环境差异稳定的不同晶体习惯的观点形成对比。此外，这些观察结果表明，胆固醇在膜介质中的结晶遵循非经典的多步结晶，受启发式“Ostwald 阶段规则”控制，该规则预测结晶动力学沿着自由能景观以多步过程进行，其中每个连续的相变相对于其前体都会导致最小的自由能损失。此外，我们发现，众所周知的胆固醇提取剂β-环糊精通过催化作用使平衡有利于晶体生长，从而从膜相向晶体以逐层的方式添加胆固醇。总之，我们的结果提供了膜内胆固醇结晶的新描述，并可能为识别靶向胆固醇晶体的分子候选物的筛选工具铺平道路。
