Garcia-Manyes Sergi, Domènech Oscar, Sanz Fausto, Montero M Teresa, Hernandez-Borrell Jordi
Departament de Química Física, Facultat de Química, Universitat de Barcelona, E-08028-Barcelona, Spain.
Biochim Biophys Acta. 2007 May;1768(5):1190-8. doi: 10.1016/j.bbamem.2007.02.004. Epub 2007 Feb 14.
Langmuir-Blodgett (LB) films of two heteroacid phospholipids of biological interest 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), as well as a mixed monolayer with chi(POPC)=0.4, were transferred onto mica in order to investigate by a combination of atomic force microscopy (AFM) and force spectroscopy (FS) their height, and particularly, their nanomechanical properties. AFM images of such monolayers extracted at 30 mN m(-1) revealed a smooth and defect-free topography except for the POPE monolayer. Since scratching such soft monolayers in contact mode was proved unsuccessful, their molecular height was measured by means of the width of the jump present in the respective force-extension curves. While for pure POPC a small jump occurs near zero force, for the mixed monolayer with chi(POPC)=0.4 the jump occurs at approximately 800 pN. Widths of approximately 2 nm could be established for POPC and chi(POPC)=0.4, but not for POPE monolayer at this extracting pressure. Such different mechanical stability allowed us to directly measure the threshold area/lipid range value needed to induce mechanical stability to the monolayers. AFM imaging and FS were next applied to get further structural and mechanical insight into the POPE phase transition (LC-LC') occurring at pressures >36.5 mN m(-1). This phase transition was intimately related to a sudden decrease in the area/molecule value, resulting in a jump in the force curve occurring at high force ( approximately 1.72 nN). FS reveals to be the unique experimental technique able to unveil structural and nanomechanical properties for such soft phospholipid monolayers. The biological implications of the nanomechanical properties of the systems under investigation are discussed considering that the annular phospholipids region of some transmembrane proteins is enriched in POPE.
具有生物学意义的两种杂酸磷脂，即1-棕榈酰-2-油酰基-sn-甘油-3-磷酸乙醇胺（POPE）和1-棕榈酰-2-油酰基-sn-甘油-3-磷酸胆碱（POPC）的朗缪尔-布洛杰特（LB）膜，以及χ（POPC）=0.4的混合单层膜，被转移到云母上，以便通过原子力显微镜（AFM）和力谱（FS）相结合的方法研究它们的高度，特别是它们的纳米力学性能。在30 mN m⁻¹下提取的此类单层膜的AFM图像显示，除了POPE单层膜外，表面形貌光滑且无缺陷。由于事实证明在接触模式下刮擦这种柔软的单层膜是不成功的，因此通过相应力-伸长曲线中出现的跳跃宽度来测量它们的分子高度。对于纯POPC，在接近零力处会出现一个小的跳跃，而对于χ（POPC）=0.4的混合单层膜，跳跃发生在大约800 pN处。在该提取压力下，POPC和χ（POPC）=0.4的跳跃宽度约为2 nm，但POPE单层膜的跳跃宽度无法确定。这种不同的机械稳定性使我们能够直接测量诱导单层膜机械稳定性所需的阈值面积/脂质范围值。接下来，应用AFM成像和FS来进一步深入了解在压力>36.5 mN m⁻¹时发生的POPE相变（LC-LC'）的结构和力学情况。这种相变与面积/分子值的突然减小密切相关，导致在高力（约1.72 nN）处力曲线出现跳跃。FS被证明是唯一能够揭示此类柔软磷脂单层膜的结构和纳米力学性能的实验技术。考虑到一些跨膜蛋白的环形磷脂区域富含POPE，讨论了所研究系统的纳米力学性能的生物学意义。
