Department of Daily Chemical Engineering, Beijing Technology and Business University, No. 11 Fucheng Road, Beijing 100048, People's Republic of China.
McIntire School of Commerce, University of Virginia, Charlottesville, Virginia 22903, United States.
Langmuir. 2022 May 24;38(20):6295-6304. doi: 10.1021/acs.langmuir.1c03415. Epub 2022 Apr 27.
Ion-specific effects widely exist in biological and chemical systems and cannot be explained by classical theories. The complexity of ion-specific effects in protein systems at the molecular level necessitates the use of mimetic models involving smaller molecules, such as amino acids, oligopeptides, and other organic molecules bearing amide bonds. Therefore, it is of theoretical value to determine the effect of additional salts on the aggregation transitions of acyl amino acid surfactants. Herein, the effects of specific tetraalkylammonium ions (TAA) on sodium lauroyl glycinate (SLG) aggregation were studied by dynamic light scattering (DLS) and transmission electron microscopy. Although previous studies have shown that the kosmotropic TAA ions tend to induce micellar growth or micelle-to-vesicle transitions of some anionic surfactants, TAA addition in the present study induced partial vesicle-to-micelle transitions in SLG solutions. The chemical trapping (CT) method was employed to estimate changes in the interfacial molarities of water, amide bonds, and carboxylate groups during such transitions. The vesicle-to-micelle transitions were accompanied by a marked rise in interfacial water molarity and a decline in interfacial amide bonds molarity, suggesting that the hydrated TAA entered the interfacial region and disrupted hydrogen bonding, thus preventing the SLG monomers from packing tightly. Molecular dynamic simulation was also performed to demonstrate the salt-induced cleavage of amide-amide bonds between SLG headgroups. Furthermore, both CT and DLS results show that the ability of tetraalkylammonium cations to induce such transitions increased with increasing size and hydrophobicity of the cation, which follows the Hofmeister series. The current study offers critical molecular-level evidence for understanding the specific effects of tetraalkylammonium ions on the aggregation transitions of an acyl amino acid surfactant.
离子特性效应广泛存在于生物和化学体系中，无法用经典理论解释。在分子水平上，蛋白质体系中离子特性效应的复杂性需要使用涉及较小分子的模拟模型，如氨基酸、寡肽和其他带有酰胺键的有机分子。因此，确定额外盐对酰基氨基酸表面活性剂聚集转变的影响具有理论价值。本文通过动态光散射（DLS）和透射电子显微镜研究了特定的四烷基铵离子（TAA）对十二酰基甘氨酸（SLG）聚集的影响。虽然先前的研究表明，亲溶盐 TAA 离子往往会诱导某些阴离子表面活性剂的胶束生长或胶束到囊泡的转变，但在本研究中，TAA 的加入诱导了 SLG 溶液中部分囊泡到胶束的转变。采用化学捕获（CT）方法来估计在这些转变过程中界面水、酰胺键和羧酸盐基团的摩尔浓度的变化。囊泡到胶束的转变伴随着界面水摩尔浓度的显著增加和界面酰胺键摩尔浓度的下降，表明水合的 TAA 进入了界面区域并破坏了氢键，从而阻止了 SLG 单体紧密堆积。还进行了分子动力学模拟，以证明盐诱导的 SLG 头基之间的酰胺-酰胺键的断裂。此外，CT 和 DLS 结果均表明，四烷基铵阳离子诱导这种转变的能力随着阳离子的尺寸和疏水性的增加而增加，这符合豪夫迈斯特序列。本研究为理解四烷基铵离子对酰基氨基酸表面活性剂聚集转变的特定影响提供了关键的分子水平证据。
