Spectroscopic characterization of the interactions of bovine serum albumin with medicinally important metal ions: platinum (IV), iridium (III) and iron (II).

Serum albumin protein plays a key role in the transportation and distribution of bioactive species including metal ions and metal-based drugs and, therefore, the nature of their binding could provide important insight for the development of new drugs. In the present investigation, binding interactions of bovine serum albumin (BSA) with three biologically important metal ions: Pt4+, Ir3+ and Fe2+ were screened using easy-to-use and cost-effective Fourier-Transform Infrared (FT-IR) and Ultraviolet-Visible (UV-Vis) spectroscopic techniques. Prior to the screening, the protein and metal ions were allowed to interact at physiological pH (7.4) and the spectral changes were monitored upon interaction. In FT-IR spectrum, the position of amide I band (C=O stretching) was shifted from 1652 cm-1 in case of free BSA to 1659, 1657 and 1656 cm-1 in BSA-Pt4+, BSA-Ir3+ and BSA-Fe2+ complexes, respectively. This spectral shifting was due to the binding of metal ions to N and O atoms of BSA peptide bonds. The interaction was further demonstrated by a remarkable reduction in spectral intensities of amide I and II bands. Secondary protein structure analysis revealed conformational changes characterized by a substantial decrease in α-helix (11.29-27.41%) accompanied by an increase in β-sheet and β-antiparallel contents. The absorption of BSA at a constant concentration at 280 nm was successively reduced as the concentration of Pt4+ and Ir3+ ions increased. On the other hand, the absorption of BSA-Fe2+ complex successively increased with the increase in the concentration of Fe2+ in the test solution. The binding constants for BSA-Pt4+, BSA-Ir3+ and BSA-Fe2+ complexes were calculated to be 1.55×104, 5.67×104 and 3.78×104 M-1, respectively. The results revealed that the three metal ions showed binding affinities with the BSA protein in the order: Ir3+>Fe2+>Pt4+.