Department of Chemistry , Jadavpur University , Kolkata 700032 , India.
Department of Chemistry , Maulana Azad College , Kolkata 700013 , India.
Langmuir. 2018 May 29;34(21):6271-6284. doi: 10.1021/acs.langmuir.7b03888. Epub 2018 Jan 10.
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.
有可能是膜界面的特定曲率控制其 pH/极性以表现出特定的生物活性。通过利用界面相互作用的螺环罗丹明 pH 探针和席夫碱极性探针，我们表明，亲水分子自组装的相对于体相的界面 pH 偏离（ΔpH）/界面介电常数（κ(i)）可以通过曲率几何形状（正/负）及其半径来调节。根据 1H NMR 和荧光各向异性研究，探针选择性地与阴离子界面斯特恩层相互作用。通过 UV-vis 吸收和荧光研究来估计斯特恩层的 ΔpH/κ(i)值。对于阴离子双(2-乙基己基)磺基琥珀酸钠（AOT）反胶束（IM）负界面，由于表面活性剂紧密堆积或暴露于水的头基面积减小，高度限制的水和质子渗透到斯特恩层中，可能导致低得多的 ΔpH ≈ -0.45 和 κ(i) ≈ 28，而对于具有相似斯特恩层的阴离子十二烷基硫酸钠（SDS）胶束正界面，分别为 ∼-2.35 和 ∼44。随着 AOT IM 水池半径（1.7-9.5nm）或[水]/[AOT]比（ w）（8.0-43.0）的增加，由于暴露于水的头基面积增加，ΔpH 和 κ(i) 最大增加至 ∼-1.22 和 ∼45。然而，对于混合 Triton X-100（TX-100）/SDS（4:1）胶束的正界面，半径在 3.5-8.0nm 范围内不变的 ΔpH ≈ -0.65 和 κ(i) ≈ 53.0 证明了其包装的灵活性。有趣的是，对于 IM，由于其最大可能的水池半径为 ∼9.5nm，ΔpH 呈现出连续增加的趋势，这可能可以解释对于具有不同曲率半径（∼15 至 50nm）的 1,2-二肉豆蔻酰基-sn-甘油-3-磷酸甘油（DMPG）/1,2-二肉豆蔻酰基-sn-甘油-3-磷酸胆碱（DMPC）（2:1）大单分子层囊泡（LUV）来说，ΔpH（∼-1.4 至-1.6）会增加。然而，类似于胶束正界面，对于具有相同脂质组成的巨大单分子层囊泡的荧光显微镜研究证实了正 LUV 界面处不变的 ΔpH。本研究提供了一种监测与生物相关膜的曲率半径依赖性界面 pH/极性的独特而简单的方法。
