{"title":"Determination of divalent metal ion-regulated proton concentration and polarity at the interface of anionic phospholipid membranes†","authors":"Pratima Mandal, Snigdha Roy, Manisha Karmakar, Sushil Ranjan Bhatta, Chandi Charan Ghosh, Arunabha Thakur and Partha Pratim Parui","doi":"10.1039/D4SM00876F","DOIUrl":null,"url":null,"abstract":"<p >We studied the influence of trace quantities of divalent metal ions (M<small><sup>2+</sup></small>: Ca<small><sup>2+</sup></small>, Mg<small><sup>2+</sup></small>, and Zn<small><sup>2+</sup></small>) on proton concentration (−log[H<small><sup>+</sup></small>], 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). <small><sup>1</sup></small>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 (<em>κ</em>) was estimated using a porphyrin-based polarity-probe (GPP) that exhibits a <em>κ</em>-induced equilibrium between monomeric and oligomeric forms. M<small><sup>2+</sup></small> interaction decreased LUV interfacial <em>κ</em> from ∼67 to 61, regardless of lipid/M<small><sup>2+</sup></small> types. Fluorescence spectral and microscopic analysis revealed that low Ca<small><sup>2+</sup></small> and Mg<small><sup>2+</sup></small> amounts (M<small><sup>2+</sup></small>/lipid = 1 : 20 for unsaturated DOPG and POPG and ∼1 : 10 for saturated DMPG lipids), but not Zn<small><sup>2+</sup></small>, 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<small><sup>2+</sup></small>-mediated interfacial pH′ increase since there is little change in surface charges up to a low M<small><sup>2+</sup></small>/lipid ratio of <1/10. M<small><sup>2+</sup></small>-induced tight lipid headgroup packing and the resulting increased surface rigidity inhibit interfacial H<small><sup>+</sup></small>/H<small><sub>2</sub></small>O penetration, reducing interfacial acidity and polarity. Our findings revealed that in certain cases, essential M<small><sup>2+</sup></small> ion-induced bio-membrane reactivity can be attributed to the influence of interfacial pH′/polarity.</p>","PeriodicalId":103,"journal":{"name":"Soft Matter","volume":null,"pages":null},"PeriodicalIF":2.9000,"publicationDate":"2024-09-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Soft Matter","FirstCategoryId":"92","ListUrlMain":"https://pubs.rsc.org/en/content/articlelanding/2024/sm/d4sm00876f","RegionNum":3,"RegionCategory":"化学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q3","JCRName":"CHEMISTRY, PHYSICAL","Score":null,"Total":0}
引用次数: 0
Abstract
We studied the influence of trace quantities of divalent metal ions (M2+: Ca2+, Mg2+, and Zn2+) 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). 1H-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. M2+ interaction decreased LUV interfacial κ from ∼67 to 61, regardless of lipid/M2+ types. Fluorescence spectral and microscopic analysis revealed that low Ca2+ and Mg2+ amounts (M2+/lipid = 1 : 20 for unsaturated DOPG and POPG and ∼1 : 10 for saturated DMPG lipids), but not Zn2+, 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 M2+-mediated interfacial pH′ increase since there is little change in surface charges up to a low M2+/lipid ratio of <1/10. M2+-induced tight lipid headgroup packing and the resulting increased surface rigidity inhibit interfacial H+/H2O penetration, reducing interfacial acidity and polarity. Our findings revealed that in certain cases, essential M2+ ion-induced bio-membrane reactivity can be attributed to the influence of interfacial pH′/polarity.