Glutamate surface speciation on amorphous titanium dioxide and hydrous ferric oxide

Hydrous ferric oxide (HFO) and titanium dioxide exhibit similar strong attachment of many adsorbates including biomolecules. Using surface complexation modeling, we have integrated published adsorption data for glutamate on HFO over a range of pH and surface coverage with published in situ ATR-FTIR studies of glutamate speciation on amorphous titanium dioxide. The results indicate that glutamate adsorbs on HFO as a deprotonated divalent anion at pH 3-5 and 0.2 micromol x m(-2) in the form of chelating-monodentate and bridging-bidentate species attached to the surface through three or four of the carboxylate oxygens, respectively. The amine group may interact weakly with the surface. However, at similar pH values and higher surface coverages, glutamate adsorbs mainly as a monovalent or divalent anion chelated to the surface by the gamma-carboxylate group. In this configuration the alpha-carboxylate and amine groups might be free to interact above the surface with the free ends of adjacent glutamates, suggesting a possible mechanism for chiral self-organization and peptide bond formation.