pH-responsive aggregation states of chiral polymerizable amphiphiles from L-tyrosine and L-phenyl alanine in water.

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.