Interaction of polypyridyl Cr(III) complexes with bovine serum albumin

We report a detailed investigation of the interaction of Cr(NN)₃³⁺ with bovine serum albumin (BSA), an important protein for the transport of drugs in blood plasma which allows us to understand further the role of Cr(NN)₃³⁺ as a sensitizer in photodynamic therapy (PDT). Chromium(III) complexes, Cr(5Cl-phen)₃³⁺, Cr(5Me-phen)₃³⁺ and Cr(5Ph-phen)₃³⁺ (where Cl = chlorine, Me = methyl and Ph = phenyl are substituents in position 5 of the phen = 1,10-phenanthroline bidentate ligand), were used for the present study. The interactions of BSA with Cr(NN)₃³⁺ were assessed employing fluorescence spectroscopy and UV-Vis absorption spectroscopy; in addition electrochemical experiments carried out at a liquid/liquid interface gave insight into the relative hydrophobicities of the complexes. We found that chromium complexes bind strongly with bovine serum albumins (BSA) with intrinsic binding constants, K_b, of (3.33 ± 0.08) × 10⁵ M^-1, (5.92 ± 0.08) × 10⁵ M^-1 and (1.64 ± 0.05) × 10⁵ M^-1 at 300.3 K. Analysis of the thermodynamic parameters ΔG, ΔH, and ΔS indicated that hydrophobic interactions played a major role in all the BSA-Cr(NN)₃³⁺ association processes. The binding distances and transfer efficiencies for BSA binding reactions were calculated according to the Förster theory of non-radiation energy transfer giving distance (r) of 2.63 nm, 2.94 nm and 3.00 nm for 5Clphen, 5Mephen and 5Ph phenanthroline complexes, respectively. All these experimental results indicate that Cr(NN)₃³⁺ binds to serum albumins, by which these proteins could act as carriers of this complex for further applications in PDT.