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

Abstract

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 (ΔpH)/interfacial dielectric constant (κ(i)) for amphiphilic self-assemblies can be regulated by the curvature geometry (positive/negative) and its radius. According to H NMR and fluorescence anisotropy investigations, the probes selectively interact with an anionic interfacial Stern layer. The ΔpH/κ(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 ΔpH ≈ -0.45 and κ(i) ≈ 28 in comparison to ∼-2.35 and ∼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) (8.0-43.0), the ΔpH and κ(i) increase maximally up to ∼-1.22 and ∼45, respectively, due to a greater water-exposed headgroup area. However, the unchanged ΔpH ≈ -0.65 and κ(i) ≈ 53.0 within radii ∼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 ΔpH trend for IM up to its largest possible water-pool radius of ∼9.5 nm may rationalize the increase in ΔpH (∼-1.4 to -1.6) with the change in the curvature radii (∼15 to 50 nm) for sodium 1,2-dimyristoyl- sn-glycero-3-phosphorylglycerol (DMPG)/1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC) (2:1) large unilamellar vesicles (LUV) owing to its negative interface. Whereas, similar to the micellar positive interface, the unchanged ΔpH at the positive LUV interface was confirmed by fluorescence microscopic studies with giant unilamellar vesicles of identical lipids composition. The present study offers a unique and simple method of monitoring the curvature-radius-dependent interfacial pH/polarity for biologically related membranes.

On the Coupling between Mechanical Properties and Electrostatics in Biological Membranes

“…Interesting, no variation in the local pH with curvature was detected in the external leaflet of the liposomes or in micelles (non-inverted). Therefore, the nature of interfacial curvature geometry and its magnitude contributes to the pH-deviation from the bulk phase to the interface [118]. The first experimental approach to study flexoelectricity in membranes was attained by AC currents registration on planar lipid bilayers subjected to a hydrostatic oscillating strain [119,120].…”

“…This explains the process of spontaneous vesiculation of ionizable membranes upon pH changes. Related to this, changes in the pH (and local dielectric constant) close to the polar headgroups have been reported to depend on the curvature for inverse micelles, and also on the internal leaflet (negative curvature) of anionic liposomes [ 118 ]. Interesting, no variation in the local pH with curvature was detected in the external leaflet of the liposomes or in micelles (non-inverted).…”

“…Interesting, no variation in the local pH with curvature was detected in the external leaflet of the liposomes or in micelles (non-inverted). Therefore, the nature of interfacial curvature geometry and its magnitude contributes to the pH-deviation from the bulk phase to the interface [ 118 ].…”

Amino-isocyanoacridines: Novel, Tunable Solvatochromic Fluorophores as Physiological pH Probes

“…It has been shown previously that the pH in the Stern layer of micelles formed from anionic surfactants such as sodium lauryl sulfate (SLS) can significantly differ from that of the bulk 28 . It is known that AO interacts with SLS micelles through both hydrophobic and electrostatic interactions.…”

“…Moreover, very recently Yeasmin et al . showed 28 by utilizing a spiro-rhodamine pH-probe, which selectively interacted with the anionic interfacial Stern layer, that it is possible that a defined curvature at the membrane interface controls its pH/polarity to exhibit specific bioactivity. We assume that an AO like smart dye would also be able to detect pH deviation in artificial and/or natural membranes.…”