Rothbauer Gabriel A, Rutter Elisabeth A, Reuter-Seng Chelsea, Vera Simon, Billiot Eugene J, Fang Yayin, Billiot Fereshteh H, Morris Kevin F
Department of Chemistry, Carthage College, 2001 Alford Park Drive, Kenosha, WI 53140, USA.
Department of Physical and Environmental Sciences, Texas A&M University-Corpus Christi, 6300 Ocean Drive, Corpus Christi, TX 78412, USA.
J Surfactants Deterg. 2018 Jan;21(1):139-153. doi: 10.1002/jsde.12015. Epub 2018 Feb 14.
Micelle formation by the anionic amino acid-based surfactant undecyl l-phenylalaninate (und-Phe) was investigated as a function of pH in solutions containing either Na, l-arginine, l-lysine, or l-ornithine counterions. In each mixture, the surfactant's critical micelle concentration (CMC) was the lowest at low pH and increased as solutions became more basic. Below pH 9, surfactant solutions containing l-arginine and l-lysine had lower CMC than the corresponding solutions with Na counterions. Nuclear magnetic resonance (NMR) diffusometry and dynamic light scattering studies revealed that und-Phe micelles with Na counterions had hydrodynamic radii of approximately 15 Å throughout the investigated pH range. Furthermore, l-arginine, l-lysine, and l-ornithine were found to bind most strongly to the micelles below pH 9 when the counterions were cationic. Above pH 9, the counterions became zwitterionic and dissociated from the micelle surface. In und-Phe/l-arginine solution, counterion dissociation was accompanied by a decrease in the hydrodynamic radius of the micelle. However, in experiments with l-lysine and l-ornithine, micelle radii remained the same at low pH when counterions were bound and at high pH when they were not. This result suggested that l-arginine is attached perpendicular to the micelle surface through its guanidinium functional group with the remainder of the molecule extending into solution. Contrastingly, l-lysine and l-ornithine likely bind parallel to the micelle surface with their two amine functional groups interacting with different surfactant monomers. This model was consistent with the results from two-dimensional ROESY (rotating frame Overhauser enhancement spectroscopy) NMR experiments. Two-dimensional NMR also showed that in und-Phe micelles, the aromatic rings on the phenylalanine headgroups were rotated toward the hydrocarbon core of micelle.
研究了基于阴离子氨基酸的表面活性剂十一烷基-L-苯丙氨酸酯（und-Phe）在含有Na⁺、L-精氨酸、L-赖氨酸或L-鸟氨酸抗衡离子的溶液中形成胶束的情况，作为pH的函数。在每种混合物中，表面活性剂的临界胶束浓度（CMC）在低pH时最低，并随着溶液碱性增强而增加。在pH 9以下，含有L-精氨酸和L-赖氨酸的表面活性剂溶液的CMC低于相应的含Na⁺抗衡离子的溶液。核磁共振（NMR）扩散测定和动态光散射研究表明，在整个研究的pH范围内，含Na⁺抗衡离子的und-Phe胶束的流体动力学半径约为15 Å。此外，当抗衡离子为阳离子时，发现L-精氨酸、L-赖氨酸和L-鸟氨酸在pH 9以下与胶束结合最强。在pH 9以上，抗衡离子变成两性离子并从胶束表面解离。在und-Phe/L-精氨酸溶液中，抗衡离子解离伴随着胶束流体动力学半径的减小。然而，在L-赖氨酸和L-鸟氨酸的实验中，当抗衡离子结合时，低pH下胶束半径保持不变，当它们不结合时，高pH下胶束半径也保持不变。这一结果表明，L-精氨酸通过其胍基官能团垂直附着于胶束表面，分子的其余部分延伸到溶液中。相反，L-赖氨酸和L-鸟氨酸可能与胶束表面平行结合，其两个胺官能团与不同的表面活性剂单体相互作用。该模型与二维ROESY（旋转框架Overhauser增强光谱）NMR实验结果一致。二维NMR还表明，在und-Phe胶束中，苯丙氨酸头基上的芳香环向胶束的烃核旋转。
