Department of Chemistry and Biochemistry, Queens College and the Graduate Center of the City University of New York, 65-30 Kissena Blvd, Flushing, NY 11367, USA.
J Am Soc Mass Spectrom. 2013 Jan;24(1):9-20. doi: 10.1007/s13361-012-0530-8. Epub 2012 Dec 18.
We report the formation and characterization of multiply negatively charged sodium bis(2-ethylhexyl) sulfosuccinate (NaAOT) aggregates in the gas phase, by electrospray ionization of methanol/water solution of NaAOT followed by detection using a guided-ion-beam tandem mass spectrometer. Singly and doubly charged aggregates dominate the mass spectra with the compositions of Na(n-z)AOT(n) (n = 1-18 and z = 1-2). Solvation by water was detected only for small aggregates Na(n-1)AOT(n)H(2)O of n = 3-9. Incorporation of glycine and tryptophan into Na(n-z)AOT(n) aggregates was achieved, aimed at identifying effects of guest molecule hydrophobicity on micellar solubilization. Only one glycine molecule could be incorporated into each Na(n-z)AOT(n) of n ≥ 7, and at most two glycine molecules could be hosted in that of n ≥ 13. In contrast to glycine, up to four tryptophan molecules could be accommodated within single aggregates of n ≥ 6. However, deprotonation of tryptophan significantly decrease its affinity towards aggregates. Collision-induced dissociation (CID) was carried out for mass-selected aggregate ions, including measurements of product ion mass spectra for both empty and amino acid-containing aggregates. CID results provide a probe for aggregate structures, surfactant-solute interactions, and incorporation sites of amino acids. The present data was compared with mass spectrometry results of positively charged Na(n+z)AOT(n) aggregates. Contrary to their positive analogues, which form reverse micelles, negatively charged aggregates may adopt a direct micelle-like structure with AOT polar heads exposed and amino acids being adsorbed near the micellar outer surface.
我们报告了在气相中形成和表征的多电荷的二（2-乙基己基）磺基琥珀酸钠（NaAOT）聚集体，方法是通过甲醇/水的 NaAOT 溶液的电喷雾电离，然后使用引导离子束串联质谱仪进行检测。单电荷和双电荷聚集体以Na(n-z)AOT(n)（n = 1-18 和 z = 1-2）的组成主导质谱。仅检测到小聚集体Na(n-1)AOT(n)H(2)O（n = 3-9）的水合作用。将甘氨酸和色氨酸掺入Na(n-z)AOT(n)聚集体中，旨在确定客体分子疏水性对胶束增溶的影响。只有一个甘氨酸分子可以掺入每个Na(n-z)AOT(n)中，其中 n ≥ 7，而在 n ≥ 13 中最多可以容纳两个甘氨酸分子。与甘氨酸相反，多达四个色氨酸分子可以容纳在单个 n ≥ 6 的聚集体中。然而，色氨酸的去质子化显著降低了其对聚集体的亲和力。对质量选择的聚集体离子进行了碰撞诱导解离（CID），包括对空聚集体和含氨基酸的聚集体的产物离子质谱进行了测量。CID 结果提供了聚集体结构、表面活性剂-溶质相互作用和氨基酸的掺入位点的探针。本数据与带正电荷的Na(n+z)AOT(n)聚集体的质谱结果进行了比较。与形成反胶束的正电荷类似物相反，负电荷聚集体可能采用类似于直接胶束的结构，其中 AOT 极性头暴露，氨基酸被吸附在胶束外表面附近。
