Interaction of VO2+ ion with human serum transferrin and albumin.

The complexation of VO2+ ion with the high molecular mass components of the blood serum, human serum transferrin (hTf) and albumin (HSA), has been re-examined using EPR spectroscopy. In the case of transferrin, the results confirm those previously obtained, showing that VO2+ ion occupies three different binding sites, A, B1 and B2, distinguishable in the X-band anisotropic spectrum recorded in D2O. With albumin the results show that a dinuclear complex (VO)2dHSA is formed in equimolar aqueous solutions or with an excess of protein; in the presence of an excess of VO2+, the multinuclear complex (VO)xmHSA is the prevalent species, where x = 5-6 indicates the equivalents of metal ion coordinated by HSA. The structure of the dinuclear species is discussed and the donor atoms involved in the metal coordination are proposed on the basis of the measured EPR parameters. Two different binding modes of albumin can be distinguished varying the pH, with only one species being present at the physiological value. The results show that the previously named "strong" site is not the N-terminal copper binding site, and some hypothesis on the metal coordination is discussed, with the 51V Az values for the proposed donor sets obtained by DFT (density functional theory) calculations. Finally, preliminary results obtained in the ternary system VO2+/hTf/HSA are shown in order to determine the different binding strength of the two proteins. Due to the low VO2+ concentration used, the recording of the EPR spectra through the repeated acquisition of the weak signals is essential to obtain a good signal to noise ratio in these systems.