Department of Land, Air and Water Resources, University of California, Davis, California 95616, United States.
Langmuir. 2011 Mar 1;27(5):1778-87. doi: 10.1021/la103826p. Epub 2011 Jan 14.
Attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopy and quantum chemical calculations were used to elucidate the influence of solution chemistry (pH, amino acid concentration) on the binding mechanisms of glutamic and aspartic acid to rutile (α-TiO(2)). The amino acids, glutamate and aspartate, contain carboxyl and amine groups whose dissociation over a pH range results in changes of molecular charge and reactivity, including reactions with mineral surfaces. At pH 3, a decrease of IR bands corresponding to protonated carboxyl groups is observed upon reaction with TiO(2) and indicates involvement of distal carboxyl groups during sorption. In addition, decreased IR bands arising from carboxyl bonds at 1400 cm(-1), concomitant to shifts to higher wavenumbers for ν(as)(γ-COO(-)) and ν(as)(α-COO(-)) (particularly at low glutamate concentrations), are indicative of inner-sphere coordination of both carboxyl groups and therefore suggest a "lying down" surface species. IR spectra of aspartate reacted with rutile are similar to those of solution-phase samples, without peak shifts indicative of covalent bonding, and outer-sphere coordination is predicted. Quantum chemical calculations were carried out to assist in elucidating molecular mechanisms for glutamate binding to rutile and are in reasonable agreement with experimental data. The combined use of ATR-FTIR data and quantum calculations suggests three potential surface configurations, which include (1) bridging-bidentate where glutamate is "lying down" and binding occurs through inner-sphere coordination of both α- and γ-carboxyl groups; (2) chelating-monodentate in which glutamate binds through inner-sphere coordination with the γ-carboxyl group in a "standing up" configuration (with or without protonation of the α-carboxyl); and (3) another bridging-bidentate configuration where glutamate is binding to rutile via inner-sphere coordination of the α-carboxyl group and outer-sphere coordination with the γ-carboxyl ("lying down").
衰减全反射（ATR）傅里叶变换红外（FTIR）光谱和量子化学计算被用来阐明溶液化学（pH 值、氨基酸浓度）对谷氨酸和天冬氨酸与金红石（α-TiO2）结合机制的影响。氨基酸谷氨酸和天冬氨酸含有羧基和氨基，其在 pH 范围内的离解导致分子电荷和反应性的变化，包括与矿物表面的反应。在 pH 3 时，与 TiO2 反应时观察到对应于质子化羧基的 IR 带减少，表明在吸附过程中涉及远端羧基。此外，来自 1400 cm-1 处羧基键的 IR 带减少，同时 νas(γ-COO(-))和 νas(α-COO(-))的波数向高波数移动（特别是在低谷氨酸浓度下），表明两个羧基的内球配位，因此表明存在“躺下”表面物种。与金红石反应的天冬氨酸的 IR 光谱与溶液相样品的光谱相似，没有表明共价键合的峰位移，并且预测为外球配位。量子化学计算被用来协助阐明谷氨酸与金红石结合的分子机制，并且与实验数据吻合较好。ATR-FTIR 数据和量子计算的综合使用表明了三种潜在的表面构型，包括（1）桥联双齿，其中谷氨酸“躺下”，并且通过两个 α-和 γ-羧基的内球配位发生结合；（2）螯合单齿，其中谷氨酸通过内球配位以“站立”构型与 γ-羧基结合（或不带α-羧基的质子化）；和（3）另一个桥联双齿构型，其中谷氨酸通过内球配位与α-羧基结合，并且与γ-羧基通过外球配位（“躺下”）与金红石结合。
