Advances on the interaction of polypyridyl Cr(III) complexes with transporting proteins and its potential relevance in photodynamic therapy

The present study reports a detailed investigation into the interaction of Cr(phen)₂(dppz)]³⁺ and Cr(phen)₃]³⁺ with transferrin, the key protein for the transport of Fe³⁺ in blood plasma; its cycle holds promise as an attractive system for strategies of drug targeting to tumor tissues. This can allow us to understand further the role of both complexes as sensitizers in photodynamic therapy (PDT). Chromium(III) complexes, Cr(phen)₂(dppz)]³⁺ and Cr(phen)₃]³⁺, (phen = 1,10-phenanthroline and dppz = dipyridophenazine), where dppz is a planar bidentate ligand with an extended π system, have been found to bind strongly with apotransferrin (apoTf) with an intrinsic binding constant, K_b, of (1.8 ± 0.3) × 10⁵ M^− 1 and (1.1 ± 0.1) × 10⁵ M^− 1 at 299 K, for apoTf-Cr(phen)₂(dppz)]³⁺ and apoTf-Cr(phen)₃]³⁺, respectively. The interactions of apoTf with the different Cr(III) complexes were assessed employing UV-visible absorption, fluorescence and circular dichroism spectroscopy. The relative fluorescence intensity of the protein decreased when the increasing concentration of Cr(III) complex was added, suggesting that perturbation around the Trp and Tyr residues took place. The analysis of the thermodynamic parameters ΔG, ΔH, ΔS indicated that the presence of the Cr(III) complex stabilizes the protein with a strong entropic contribution. The binding distances and transfer efficiencies for apoTf-Cr(phen)₂(dppz)]³⁺ and apoTf-Cr(phen)₃]³⁺ binding reactions were calculated according to Föster theory of non-radiation energy transfer. All these experimental results suggest that Cr(phen)₂(dppz)]³⁺ and Cr(phen)₃]³⁺ bind strongly to apoTf indicating that this protein could act as a carrier of these complexes for further applications in PDT.