Attachment of l-Glutamate to Rutile (α-TiO₂): A Potentiometric, Adsorption, and Surface Complexation Study

Abstract

Interactions between aqueous amino acids and mineral surfaces influence the bioavailability of amino acids in the environment, the viability of Ti implants in humans, and the role of mineral surfaces in the origin of life on Earth. We studied the adsorption of l-glutamate on the surface of rutile (alpha-TiO(2), pH(PPZC) = 5.4) in NaCl solutions using potentiometric titrations and batch adsorption experiments over a wide range of pH values, ligand-to-solid ratios, and ionic strengths. Between pH 3 and 5, glutamate adsorbs strongly, up to 1.4 micromol m(-2), and the adsorption decreases with increasing ionic strength. Potentiometric titration measurements of proton consumption for the combined glutamate-rutile-aqueous solution system show a strong dependence on glutamate concentration. An extended triple-layer surface complexation model of all the experimental results required at least two reaction stoichiometries for glutamate adsorption, indicating the possible existence of at least two surface glutamate complexes. A possible mode of glutamate attachment involves a bridging-bidentate species binding through both carboxyl groups, which can be thought of as "lying down" on the surface (as found previously for amorphous titanium dioxide and hydrous ferric oxide). Another involves a chelating species which binds only through the gamma-carboxyl group, that is, "standing up" at the surface. The calculated proportions of these two surface glutamate species vary strongly, particularly with pH and glutamate concentration. Overall, our results serve as a basis for a better quantitative understanding of how and under what conditions acidic amino acids bind to oxide mineral surfaces.

Mineral Surfaces, Geochemical Complexities, and the Origins of Life

“…The magnitude of this work is substantial and depends only on the stoichiometry of the surface reaction. When structures of adsorbed anions established in spectroscopic studies are used to calibrate an ETL model of bulk adsorption data, the model can independently predict proportions of innerto outer-sphere surface complexes as functions of pH, ionic strength, and surface coverageproportions that have been confirmed in experiments, for example in studies of aspartate and glutamate adsorption on titanium dioxide (e.g., Jonsson et al 2009). …”

“…Chemisorbed ions typically bond to one or two surface atoms, whereas larger molecules can adopt a variety of surface topologies with multiple attachments (Fig. 3), as discussed later in this article (Davis and Kent 1990;Zhang et al 2004;Sverjensky et al 2008;Jonsson et al 2009). …”

“…Details of molecular adsorption are dependent on several variables, most notably pH, the nature and concentrations of molecular solutes, and the identities and concentrations of electrolytes (Schindler 1990;Sverjensky 2005;Sverjensky and Fukushi 2006;Jonsson et al 2009). Additional complexities arise when organic molecules interact with crystal surface irregularities (Teng and Dove 1997;Teng et al 1998Teng et al , 2000Orme et al 2001;De Yoreo and Dove 2004;Elhadj et al 2006).…”

“…Proton titrations of oxide surfaces in electrolyte solutions, both with and without an organic adsorbate, provide powerful constraints on the possible reactions responsible for adsorption, particularly when used in combination with in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) studies (Holman and Casey 1996;Nordin et al 1997;Boily et al 2000aBoily et al , 2000bBoily et al , 2000cBoily et al , 2005Sheals et al 2002Sheals et al , 2003Lackovic et al 2003;Lindegren et al 2005). However, with few exceptions (Gisler 1981;Whitehead 2003;Vlasova and Golovkova 2004;Vlasova 2005;Jonsson et al 2009). Most adsorption studies of amino acids on oxide surfaces have been limited to systems without control of pH and ionic strength (e.g., Holm et al 1983;Matrajt and Blanot 2004).…”

Stochastic Prebiotic Chemistry within Realistic Geological Systems

“…More recently, a series of studies by Hazen and collaborators have addressed the issue of chemistry at geological interfaces, proposing that crystal surfaces of Hadean eon-relevant minerals provide an effective substrate upon which the concentration of prebiotic molecules could have been achieved [17][18][19][20][21][22][23][24] . These works have demonstrated the pivotal role played by mineralwater interfaces in both concentration and catalysis processes.…”