Department of Earth & Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218, USA.
Langmuir. 2009 Oct 20;25(20):12127-35. doi: 10.1021/la901635t.
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.
水合氨基酸与矿物表面的相互作用影响环境中氨基酸的生物利用度、钛植入物在人体中的存活能力以及矿物表面在地球上生命起源中的作用。我们通过电位滴定和批式吸附实验，在广泛的 pH 值、配体-固体比和离子强度范围内研究了 L-谷氨酸在锐钛矿（α-TiO(2)，pH(PPZC) = 5.4）表面的吸附。在 pH 3 到 5 之间，谷氨酸强烈吸附，最高可达 1.4 微米摩尔/平方米，并且吸附随离子强度的增加而降低。对于组合的谷氨酸-锐钛矿-水溶液系统的质子消耗的电位滴定测量显示出对谷氨酸浓度的强烈依赖。扩展的三重层表面配合模型需要至少两种谷氨酸吸附的反应化学计量，以解释所有实验结果，这表明可能存在至少两种表面谷氨酸配合物。谷氨酸附着的一种可能模式涉及通过两个羧基桥联双齿物种的结合，可以被认为是“躺在”表面上（如先前在无定形二氧化钛和水合氧化铁上发现的）。另一种涉及仅通过γ-羧基结合的螯合物种，即在表面“站立”。这两种表面谷氨酸物种的计算比例变化很大，特别是随 pH 值和谷氨酸浓度而变化。总的来说，我们的结果为更好地定量理解酸性氨基酸与氧化物矿物表面结合的方式和条件提供了基础。
