Association studies to transporting proteins of fac-Re^I(CO)₃(pterin)(H₂O) complex

A new synthetic route to acquire the water soluble complex fac-Re^I(CO)₃(pterin)(H₂O) was carried out in aqueous solution. The complex has been obtained with success via the fac-[Re^I(CO)₃(H₂O)₃]Cl precursor complex. Re^I(CO)₃(pterin)(H₂O) has been found to bind strongly with bovine and human serum albumins (BSA and HSA) with intrinsic-binding constants, K_b, of 6.5 × 10⁵ M^-1 and 5.6 × 10⁵ M^-1 at 310 K, respectively. The interactions of serum albumins with Re^I(CO)₃(pterin)(H₂O) were evaluated employing UV-vis fluorescence and absorption spectroscopy and circular dichroism. The results suggest that the serum albumins-Re^I(CO)₃(pterin)(H₂O) interactions occurred in the domain IIA-binding pocket without loss of helical stability of the proteins. The comparison of the fluorescence quenching of BSA and HSA due to the binding to the Re(I) complex suggested that local interaction around the Trp 214 residue had taken place. The analysis of the thermodynamic parameters ΔG⁰, ΔH⁰, and ΔS⁰ indicated that the hydrophobic interactions played a major role in both HSA-Re(I) and BSA-Re(I) association processes. All these experimental results suggest that these proteins can be considered as good carriers for transportation of Re^I(CO)₃(pterin)(H₂O) complex. This is of significant importance in relation to the use of this Re(I) complex in several biomedical fields, such as photodynamic therapy and radiopharmacy.