pH调控的两性多肽表面活性剂自组装

pH-tuned self-assembly of a zwitterionic peptide surfactant

综合运用圆二色光谱、透射电镜、原子力显微镜以及zeta电位测定等多种技术手段,研究了两性多肽表面活性剂分子I3CGK—OH的pH响应性自组装行为。I3CGK—OH分子含有多个pH敏感基团,分别是伯氨基、羧基和巯基,随pH变化可以带有不同数量的正电荷或负电荷。所研究的3个pH值2.5、7.0和12.0下I3CGK—OH分子所带净电荷分别为+0.5、0和-1.9。净电荷的不同会影响分子组装过程和分子构象变化。pH值7.0时,分子基本不带电荷,静电斥力非常弱,可以在较低浓度达到组装平衡;分子在组装体中紧密折叠堆积,形成直径较小的初级纤维;同时低静电斥力也导致纤维间强的侧向堆积作用,形成条带状的高级组装结构。而在pH值2.5和12.0,分子净电荷较多,静电斥力增大,分子的平衡组装浓度不同程度地增加;分子在组装体中采取较为伸展的构象,形成直径较大的初级纤维;初级纤维可以单独存在,两根或多根初级纤维也可以通过互相扭结形成高级结构。

The pH-responsive self-assembly of a zwitterionic peptide surfactant,I3CGK—OH,has been studied using a combination of techniques including circular dichroism,transmission electron microscopy,atomic force microscopy,and zeta potential measurements.I3CGK—OH has three pH-sensitive groups,namely,a primary amine group,a carboxyl group and a hydrosulfide group.The molecule can have either positive or negative charges with pH variation.The molecule is charged as ＋0.5,0 and –1.9 at pH 2.5,7.0 and 12.0 respectively,which affects both the self-assembly process and the molecular conformation during assembly.At pH 7.0,I3CGK—OH is nearly zero charged with weak inter-molecular charge repulsion;therefore,it can fulfill the self-assembly process at relatively lower concentrations.The molecules take a compact conformation during assembly and form primary fibers of smaller diameter.These primary fibers exhibit high propensity for lateral association and form sheet-like higher-order structures by parallel aligning.At pH 2.5 and 12.0,I3CGK—OH is relatively highly charged,and reasonably higher inter-molecular charge repulsion results in increased concentration to reach self-assembly equilibrium.The molecules take relatively extended conformations during assembly and form primary fibers of larger diameters.These fibers can exist separately or form higher-order fibrils through twisting of two or more primary fibers.