Determination of divalent metal ion-regulated proton concentration and polarity at the interface of anionic phospholipid membranes.

We studied the influence of trace quantities of divalent metal ions (M 2+ : Ca 2+ , Mg 2+ , and Zn 2+ ) on proton concentration (-log[H + ], designated as pH') and polarity at the interface of anionic PG-phospholipid membranes comprising saturated and unsaturated acrylic chains. A spiro-rhodamine-6G-gallic acid (RGG) pH-probe was synthesized to monitor the interfacial pH' of large unilamellar vesicles (LUVs) at a physiologically appropriate bulk pH (6.0-7.5). 1 H-NMR spectroscopy and fluorescence microscopy showed that RGG interacted with the LUV interface. The pH-dependent equilibrium between the spiro-closed and spiro-open forms of RGG at the interface from the bulk phase was compared using fluorescence spectra to obtain interfacial pH'. Interfacial dielectric constant ( κ ) was estimated using a porphyrin-based polarity-probe (GPP) that exhibits a κ -induced equilibrium between monomeric and oligomeric forms. M 2+ interaction decreased LUV interfacial κ from ∼67 to 61, regardless of lipid/M 2+ types. Fluorescence spectral and microscopic analysis revealed that low Ca 2+ and Mg 2+ amounts (M 2+ /lipid = 1 : 20 for unsaturated DOPG and POPG and ∼1 : 10 for saturated DMPG lipids), but not Zn 2+ , decreased LUV interfacial acidity from pH' ∼3.8 to 4.4 at bulk pH 7.0. Although membrane surface charges are normally responsible for pH' deviation from the bulk to the interface, they cannot explain M 2+ -mediated interfacial pH' increase since there is little change in surface charges up to a low M 2+ /lipid ratio of <1/10. M 2+ -induced tight lipid headgroup packing and the resulting increased surface rigidity inhibit interfacial H + /H 2 O penetration, reducing interfacial acidity and polarity. Our findings revealed that in certain cases, essential M 2+ ion-induced bio-membrane reactivity can be attributed to the influence of interfacial pH'/polarity.