Department of Chemical Engineering & Materials Science, University of California, Davis, California 95616, USA.
Langmuir. 2010 Jun 1;26(11):8614-24. doi: 10.1021/la9046885.
The phase behavior of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) (1/1 mol ratio)/cholesterol (0-60 mol %) supported lipid bilayers agreed with a DOPC/DSPC/cholesterol ternary phase diagram by Zhao et al. when a mica support was used (Zhao, J.; Wu, J.; Heberle, F. A.; Mills, T. T.; Klawitter, P.; Huang, G.; Costanza, G.; Feigenson, G. W. Biochim. Biophys. Acta, Biomembr. 2007, 1768, 2764-2776). However, when a silica xerogel support was used, the phase behavior deviated from the phase diagram. Specifically, miscibility and trend lines of DSPC-rich domain area fraction, domain shape, and domain size versus cholesterol, obtained by analysis of fluorescence and atomic force microscopy (AFM) images, were as expected for mica-supported lipid bilayers, but were substantially stretched to higher cholesterol concentrations for silica xerogel-supported lipid bilayers. In addition, this behavior was found in three other ternary lipid compositions substituting slightly shorter acyl chain lengths in comparison to DSPC or a saturated lipid versus unsaturated DOPC. Qualitative comparison of domain characteristics of DOPC/DSPC/cholesterol (0 and 15 mol %) bilayers supported by silica xerogel, mica, borosilicate glass, and quartz showed that the networked surface layer of high curvature (0.04 nm(-1)) silica beads was the dominant influence as opposed to the surface chemistry. Based upon the literature, we postulate two curvature-based mechanisms that explain our results. In the first mechanism, cholesterol was transferred from the higher curvature supported lipid bilayer to the lower curvature vesicles in the medium during the vesicle fusion and thermal cooling step, resulting in a lowered cholesterol concentration of the supported lipid bilayer. In the second mechanism, high curvature promoted sustained lipid demixing as the cholesterol concentration was increased, thus creating a new phase diagram in which coexisting phases persist to a higher cholesterol concentration. These results suggest that a surface layer of high curvature features can be used to observe and study curvature-induced intrabilayer transport or demixing over large areas and that curvature can play an important role in sorting and localization of biomembrane components.
当使用云母作为支撑时，1,2-二油酰基-sn-甘油-3-磷酸胆碱（DOPC）/1,2-二硬脂酰基-sn-甘油-3-磷酸胆碱（DSPC）（1/1 摩尔比）/胆固醇（0-60 摩尔%）支持的脂质双层的相行为与 Zhao 等人的 DOPC/DSPC/胆固醇三元相图一致（Zhao, J.; Wu, J.; Heberle, F. A.; Mills, T. T.; Klawitter, P.; Huang, G.; Costanza, G.; Feigenson, G. W. Biochim. Biophys. Acta, Biomembr. 2007, 1768, 2764-2776）。然而，当使用硅胶干凝胶作为支撑时，相行为偏离了相图。具体来说，通过荧光和原子力显微镜（AFM）图像分析获得的富含 DSPC 区域分数、域形状和域大小与胆固醇的混溶性和趋势线，与云母支撑的脂质双层一致，但对于硅胶干凝胶支撑的脂质双层，其被拉伸到更高的胆固醇浓度。此外，这种行为在另外三种替代 DSPC 或不饱和 DOPC 的短链酰基长度的三元脂质组成中也有发现。用硅胶干凝胶、云母、硼硅酸盐玻璃和石英支撑的 DOPC/DSPC/胆固醇（0 和 15 摩尔%）双层的域特征的定性比较表明，高曲率（0.04 nm(-1)) 硅胶珠的网络表面层是主要影响因素，而不是表面化学。根据文献，我们提出了两种基于曲率的机制来解释我们的结果。在第一种机制中，在囊泡融合和热冷却步骤中，胆固醇从较高曲率的支撑脂质双层转移到介质中的较低曲率的囊泡中，导致支撑脂质双层中的胆固醇浓度降低。在第二种机制中，高曲率促进了脂质持续的去混合，随着胆固醇浓度的增加，从而创建了一个新的相图，其中共存相持续到更高的胆固醇浓度。这些结果表明，高曲率特征的表面层可用于观察和研究大区域内的曲率诱导的层间运输或去混合，并且曲率可以在生物膜成分的分类和定位中发挥重要作用。
