Department of Physics and Center for Supramolecular Studies, California State University (CSU), Northridge, California 91330-8268, United States.
Langmuir. 2013 May 14;29(19):5734-41. doi: 10.1021/la4008922. Epub 2013 Apr 29.
Sodium salts of maleamic acid derivatives of lauryl ester of tyrosine (MTNa) and phenyl alanine (MPNa) in water exhibited strong pH-responsive behaviors of viscosity and specific conductivity that originate from the concentration and pH dependence of their aggregation states. The aggregates were characterized by a novel spin-probe-partitioning electron paramagnetic resonance (SPPEPR) method and dynamic light scattering (DLS). Results of high-precision fitting of the second-harmonic EPR spectra of the small spin probe di-tert-butyl nitroxide (DTBN) in these aggregates together with viscosity, conductivity, and DLS showed that, at pH ~ 7.54, MTNa formed micelles and MPNa vesicles and MTNa exhibited a pH-induced micelle to vesicle transition as pH was lowered toward 6. MTNa, at pH ~ 7.54, formed small micelles at low concentrations that transformed to long worm-like micelles for concentrations ≥ 0.05 M, accompanied by a 30-fold increase in solution viscosity. The hydrodynamic radii from DLS confirmed the presence of small micellar aggregates of radius ~ 2 nm in MTNa at pH ~ 7.54 at the lower concentrations, with coexisting micelles (2 nm) and vesicles (50 nm) at pH near 6.5, vesicles (radii ~ 70 nm) at pH near 6, and large vesicles (85 nm) in MPNa at pH ~ 7.60. Both MTNa and MPNa precipitated upon reduction of pH below 6 and below 7, respectively. The rate of transfer of DTBN between the aqueous phase and the aggregate was calculated from the high-field Lorentzian linewidths of the electron paramagnetic resonance (EPR) spectra. The activation energy for the transfer determined from the temperature dependence of the rate of transfer is 12.7 kJ/mol for MTNa vesicles (pH ~ 6) and 20.6 ± 1.3 kJ/mol for MPNa (pH ~ 7.60). The pH-induced transformations were reversible.
在水中，酪氨酸月桂酯（MTNa）和苯丙氨酸（MPNa）的马来酸酰胺盐衍生物的钠盐表现出强烈的 pH 响应的粘度和比导率行为，这些行为源于其聚集态的浓度和 pH 依赖性。这些聚集态通过一种新的自旋探针分区电子顺磁共振（SPPEPR）方法和动态光散射（DLS）来表征。在这些聚集物中，小自旋探针二叔丁基硝酮（DTBN）的二次谐波 EPR 光谱的高精度拟合结果，以及粘度、电导率和 DLS 结果表明，在 pH7.54 时，MTNa 形成胶束，MPNa 形成囊泡，并且当 pH 降低至 6 时，MTNa 表现出 pH 诱导的胶束到囊泡的转变。在 pH7.54 时，MTNa 在低浓度下形成小胶束，当浓度≥0.05 M 时，这些胶束转化成长的蠕虫状胶束，溶液粘度增加 30 倍。来自 DLS 的水动力半径证实了在 pH7.54 时 MTNa 存在半径约为 2nm 的小胶束聚集物，在 pH6.5 附近存在共存的胶束（2nm）和囊泡（50nm），在 pH6 时存在囊泡（半径70nm），在 pH7.60 时在 MPNa 中存在大囊泡（85nm）。当 pH 降低到 6 以下和 7 以下时，MTNa 和 MPNa 都会沉淀。从电子顺磁共振（EPR）光谱的高场洛伦兹线宽计算出 DTBN 在水相和聚集相之间转移的速率。从转移速率的温度依赖性确定的转移的活化能对于 MTNa 囊泡（pH6）为 12.7kJ/mol，对于 MPNa（pH~7.60）为 20.6±1.3kJ/mol。pH 诱导的转变是可逆的。
