Detection of Curvature-Radius-Dependent Interfacial pH/Polarity for Amphiphilic Self-Assemblies: Positive versus Negative Curvature

摘要

It is possible that a defined curvature at the membrane interface controls its pH/polarity to exhibit specific bioactivity. By utilizing an interface-interacting spiro-rhodamine pH probe and the Schiff base polarity probe, we have shown that the pH deviation from the bulk phase to the interface (Delta pH)/interfacial dielectric constant (kappa(i)) for amphiphilic self-assemblies can be regulated by the curvature geometry (positive/negative) and its radius. According to H-1 NMR and fluorescence anisotropy investigations, the probes selectively interact with an anionic interfacial Stern layer. The Delta pH/kappa(i) values for the Stern layer are estimated by UV-vis absorption and fluorescence studies. For the anionic sodium bis-2-ethylhexyl-sulfosuccinate (AOT) inverted micellar (IM) negative interface, the highly restricted water and proton penetration into the Stern layer owing to tight surfactant packing or a reduced water-exposed headgroup area may be responsible for the much lower Delta pH approximate to -0.45 and kappa(i) approximate to 28 in comparison to similar to-2.35 and similar to 44, respectively, for the anionic sodium dodecyl sulfate (SDS) micellar positive interface with a close similar Stern layer. With increasing AOT IM water-pool radius (1.7-9.5 nm) or [water]/[AOT] ratio (w(0)) (8.0-43.0), the Delta pH and kappa(i) increase maximally up to similar to-1.22 and similar to 45, respectively, due to a greater water-exposed headgroup area. However, the unchanged Delta pH approximate to -0.65 and kappa(i) approximate to 53.0 within radii similar to 3.5-8.0 nm for the positive interface of a mixed Triton X-100 (TX-100)/SDS (4:1) micelle justify its packing flexibility. Interestingly, the continuously increasing Delta pH trend for IM up to its largest possible water-pool radius of similar to 9.5 nm may rationalize the increase in Delta pH (similar to-1.4 to -1.6) with the change in the curvature radii (similar to 15 to 50 nm) for sodium 1,2-dimyristoyl-sn-g