Evaluating Glutamate and Aspartate Binding Mechanisms to Rutile (α-TiO2) via ATR-FTIR Spectroscopy and Quantum Chemical Calculations

Attenuated total reflectance (AIR) 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 (alpha-TiO2). 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 TiO2 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 nu(as)(gamma-COO-) and nu(as)(alpha-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 alpha-and gamma-carboxyl groups; (2) chelating-monodentate in which glutamate binds through inner-sphere coordination with the gamma-carboxyl group in a standing up configuration (with or without protonation of the alpha-carboxyl); and (3) another bridging-bidentate configuration where glutamate is binding to rutile via inner-sphere coordination of the alpha-carboxyl group and outer-sphere coordination with the gamma-carboxyl (lying down).