!donnan-equilibria-in-microbial-cell-walls-a-pH-homeostatic-mechanism-in-alkaliphiles

K.Tsujii/ColloidsandSurfacesB:Biointerfaces24(2002)247–2512482.Themodel2.1.StructuremodelingofmicrobialcellwallsTherearetwokindsofcellwallsinmicroorgan-isms.ThecellwallsofGram-positivebacteriaconsistmainlyofpeptidoglycanlayerandanionicpolymers(teichoicacid,teichuronicacidandte-ichuronopeptides)ofabrush-likestructure.InGram-negativebacteria,ontheotherhand,theanisotropicoutermembraneconsistingofphos-pholipids(innerleaflet),proteinsandlipo-polysaccharides(outerleaflet)isthesubstitutefortheanionicpolymerlayer.Fig.1showsasche-maticillustrationofthecellwallofGram-positivebacteria.Fromthephysico-chemicalpointofview,thecellwallstructureofGram-positivebacteriacanberegardedasahydrogelbearinganionicbrush-likepolymermoleculesonitssurface,sinceacross-linkedpolymerofpeptidoglycanisakindofhydrogel.Intheanionicpolymerlayer,anumberofanionsarefixedonthepolymerchainsofteichoicacidorteichuronicacidorte-ichuronopeptide,andthesamenumberofcationsispresentintheaqueousphaseofthelayer.Upontheaboveconditionsofthemicrobialcellwalls,Donnanequilibriumofelectrolytesshouldbeat-tainedbetweentheionicpolymerlayerandthebulkaqueousphase[4,5].2.2.DonnanequilibriainthecellwallsTheDonnanequilibriumtheoryprovidesusaquantitativedescriptionofthesaltdistributionbetweenthepolymerlayerofmicrobialcellwallsandthebulkaqueousphase.Letthesodiumconcentration(=theconcentrationoffixedan-ion)intheregionofanionicpolymerlayerbeC0,andtheinitialconcentrationofaddedNaClintheaqueousphasebeC1.SomepartoftheNaClinthebulkphasemoveintothepolymerlayer,andtheequilibrationisattained.TheconcentrationchangeduetothisNaClmovementiswrittenasx.Finalsodiumandchlorideconcentrationsin-sidethepolymerlayerareC0+xandx,respec-tively,andthebothconcentrationsofNa+andCl−inthebulkphaseareC1−x,ifthevolumesofbothphasesaresame.Theequilibriumcondi-tionofsodiumchlorideinbothphasescanbewrittenas:(C0+x)x=(C1−x)2(1)Theequationiseasilysolvedas:Fig.1.StructuralmodelforthecellwallofGram-positivebacteria.Manyanionicpolymerchainsaresituatedinabrush-likeshapeonthe