Titanium(IV) and Vitamin C: Aqueous Complexes of a Bioactive Form of Ti(IV)

Ascorbic acid is among the biorelevant ligands that render Ti­(IV) stable in aqueous solution. A series of pH-dependent titanium­(IV) coordination complexes of l-ascorbic acid is described. Directed by spectropotentiometric methods, important aspects of the aqueous interactions in this system are investigated, including ligand binding mode, pH-dependent metal–ligand stoichiometry, and the importance of metal ion-promoted hydrolysis and the binding of hydroxide. Stability constants are determined for all metal ion–ligand–proton complexes by a process of model optimization and nonlinear least-squares fitting of the combined spectropotentiometric titration data to the log β_MLH values in the model. A speciation diagram is generated from the set of stability constants described in the model. In the range pH 3–10, the aqueous speciation is characterized by the sequential appearance of the following complexes as a function of added base: Ti­(asc)₂⁰ → Ti­(asc)₃^2‑ → Ti­(asc)₂(OH)₂^2‑ → Ti­(asc)­(OH)₄^2‑. These species dominate the speciation at pH < 3, pH 4–5, pH ∼ 8, and pH > 10, respectively, with minimum log stability constants (β values) of 25.70, 36.91, 16.43, and −6.91. Results from electrospray mass spectrometry, metal–ligand binding experiments, and kinetics measurements support the speciation, which is characterized by bidentate chelation of the ascorbate dianion to the titanium­(IV) ion via proton displacement, and a pH-dependent metal–ligand binding motif of ligand addition followed by metal ion-promoted hydrolysis, stepwise ligand dissociation, and the concomitant binding of hydroxide ion. Additionally, the kinetics of ligand exchange of titanium ascorbate with citrate are reported to understand better the possible fate of titanium ascorbate under biologically relevant conditions