Cooperative and Competitive Adsorption of Amino Acids with Ca2+ on Rutile (α-TiO2)

CooperativeandCompetitiveAdsorptionofAminoAcidswithCa2+onRutile(α-TiO2)

NamheyLee,*, , ,§DimitriA.Sverjensky, , andRobertM.Hazen

DepartmentofEarthandPlanetarySciences,JohnsHopkinsUniversity,Baltimore,Maryland21218,UnitedStates

GeophysicalLaboratory,CarnegieInstitutionofWashington,5251BroadBranchRoadNW,Washington,DC20015,UnitedStatesWhileitisknownthatcationscanformbridgingcomplexesTheinteractionsbetweenmineralsurfacesandorganicwithorganicsbyformingtertiarycomplexes,17fewstudieshavemoleculesareubiquitous,rangingfromthefateoforganicbeendoneinvolvingcationsandaminoacids.18Studiesmatterduringweatheringandtransporttotheoceans,theinvolvingmetal humicacidinteractionsshowthatinthetransportofnutrientsinsoil,andthefateofcontaminants,topresenceofdivalentcations,theadsorptionoforganicsismedicalissuesincludingbiotechnology,pharmaceuticalsandenhancedathighpHvalues.19,20However,duetothecomplextheviabilityofmetalimplantsinthehumanbody.1 9RecentstructureofNOManditsmassivesize,itisnoteasytopinpointdramaticincreasesofinterestinnanoparticleshavealsobroughtwhatiscausingthisenhancedadsorption.Inthepresentstudy,ourattentionwebuilt

12tohoworganicmoleculesaltermineralgrowthat13,21onourpreviousstudiesofglutamateadsorptionon

surfaces.10 Furthermore,mineralsurfacesandorganicrutiletogetherwithoursurfacecomplexationmodelingmoleculeinteractionscanprovidevitalcluesfortheevolutionstudiesofcalciumonrutile,22toenableanexperimentalandofabioticallyformedorganicmoleculesinprebiotictimes.surfacecomplexationmodelingstudyoftheadsorptionofByde nition,allaminoacidshavebothpositiveandaminoacidsonrutileinthepresenceofcalcium.

negativelychargedfunctionalgroupsonthem( NH

-NH2

3+, COOH COO ).Asaresult,thenetchargeofthe2.MATERIALSANDMETHODS

moleculesvarieswithpH,leadingtoavarietyofinterestingAllsolutionsweremadefrommilli-Qwater(Milliporeadsorptionbehavior.Previousstudiesofaminoacidadsorptionresistance18.2Megaohm).L-glutamicacid(AcrosOrganics,havebeenrestrictedtosimplesystemstypicallycontainingone99%),L-lysinemonohydrochloride(AcrosOrganics,99%)andaminocalciumchloridestandardsolutionof0.5M(Flukaanalytical) 16acidandonemineralina1:1backgroundelectro-lyte.13However,near-surfacenaturalwatersaremorewereusedwithoutanyfurtherpuri cation.Thesolutionswerecomplex:theyaremostcommonlyCa2+-Na+-HCO3 -SiOsonicatedformorethan15minandvisuallycheckedpriortowaters.Asa rststeptobeginaddressingthiscomplexity,we2usetoensurecompletedissolution.ThepHwasadjustedbyinvestigatedthepotentialcompetitiveorcooperativeadsorp-addingprecisevolumesofstandardizedNaOHandHCl.tionofaminoacidsandCa2+ontherutilesurface.We

comparedhowtheadditionofCa2+changestheadsorption

characteristicsofglutamateandlysineonrutile.TheresultsReceived:April23,2014

illustratetheresponsesoftwoverydi erentaminoacidstotheRevised:July8,2014

Accepted:July10,2014

presenceofCa2+underarangeofenvironmentalconditions.