Soft Nano Laboratory, Physical Sciences Division, Institute of Advanced Study in Science and Technology, Vigyan Path, Paschim Boragaon, Garchuk, Guwahati, Assam 781035, India.
Soft Nano Laboratory, Physical Sciences Division, Institute of Advanced Study in Science and Technology, Vigyan Path, Paschim Boragaon, Garchuk, Guwahati, Assam 781035, India.
Colloids Surf B Biointerfaces. 2017 Nov 1;159:696-704. doi: 10.1016/j.colsurfb.2017.08.032. Epub 2017 Aug 24.
Successive compression-decompression cycles of the surface pressure (π) - specific molecular area (A) isotherms of protein (BSA) monolayers show that reversible hysteresis persists if the protein molecules contain effective positive or negative surface charges. However, for neutral condition, i.e., close to the isoelectric point of the protein, irreversibility in the hysteresis behaviour dominates. Out-of-plane structures obtained from the X-ray reflectivity analysis suggest that at lower surface pressure monomolecular layer of BSA is formed on the water surface. With increasing surface pressure, molecules start to lift-up from the water surface in such a way that semi-major axis makes an angle with the water surface. Depending on the surface pressure and surface charge of BSA, monomolecular or bimolecular layer of tilted BSA molecules is formed on the water surface, however, formation of bimolecular layer is observed when the pH is relatively closer to the BSA isoelectric point. After complete decompression, tilted monomolecular or bimolecular structures again transform into monomolecular layer as evidenced from the structural analysis of the films deposited at lower surface pressures in the second compression, however, structural hysteresis varies depending upon the subphase pH or protein surface charge. Structures obtained from the films deposited at first and second compressions at lower pressure implies that although structural dissimilarity is present but structural hysteresis is only present near the isoelectric point of BSA and becomes negligible below and above that pH. Competitive electrostatic and van der Waals interactions are responsible for such hysteresis behaviours and structural modifications.
连续的表面压力（π）-特定分子面积（A）等温线的压缩-解压循环表明，如果蛋白质分子含有有效的正或负表面电荷，那么可逆滞后仍然存在。然而，对于中性条件，即接近蛋白质的等电点，滞后行为的不可逆性占主导地位。从 X 射线反射分析中得到的面外结构表明，在较低的表面压力下，BSA 的单分子层在水面上形成。随着表面压力的增加，分子开始从水面抬起，使得长半轴与水面成一定角度。根据 BSA 的表面压力和表面电荷，BSA 的单分子层或倾斜的双分子层在水面上形成，然而，当 pH 值相对接近 BSA 等电点时，观察到双分子层的形成。在完全解压后，从结构分析中可以看出，倾斜的单分子层或双分子层再次转化为单分子层，这是在第二次压缩中较低表面压力下沉积的薄膜的结构分析所证明的，然而，结构滞后取决于亚相 pH 值或蛋白质表面电荷而变化。从第一次和第二次较低压力压缩中沉积的薄膜获得的结构表明，尽管存在结构差异，但结构滞后仅存在于 BSA 的等电点附近，在该 pH 值以下和以上变得可以忽略不计。竞争静电和范德华相互作用是导致这种滞后行为和结构修饰的原因。
