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The interaction with proteins of metal-based drugs plays a crucial role in their transport, mechanism, and activity. For an active ML n complex, where L is the organic carrier, various binding modes (covalent and non-covalent, single or multiple) may occur and several metal moieties (M, ML, ML 2 , etc.) may interact with proteins. In this study, we have evaluated the interaction of [V IV O(malt) 2 ] (bis(maltolato)oxidovanadium(IV) or BMOV, where malt = maltolato, i.e., the common name for 3-hydroxy-2methyl-4H-pyran-4-onato) with the model protein hen egg white lysozyme (HEWL) by electrospray ionization mass spectrometry, electron paramagnetic resonance, and X-ray crystallography. The multiple binding of different V-containing isomers and enantiomers to different sites of HEWL is observed. The data indicate both noncovalent binding of cis-[VO(malt) 2 (H 2 O)] and [VO(malt)-(H 2 O) 3 ] + and covalent binding of [VO(H 2 O) 3−4 ] 2+ and cis-[VO(malt) 2 ] and other V-containing fragments to the side chains of Glu35, Asp48, Asn65, Asp87, and Asp119 and to the C-terminal carboxylate. Our results suggest that the multiple and variable interactions of potential V IV OL 2 drugs with proteins can help to better understand their solution chemistry and contribute to define the molecular basis of the mechanism of action of these intriguing molecules.

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Vanadium complexes (VCs) are promising agents for the treatment, among others, of diabetes and cancer. The development of vanadium-based drugs is mainly limited by a scarce knowledge of the active species in the target organs, which is often determined by the interaction of VCs with biological macromolecules like proteins. Here, we have studied the binding of [VIVO(empp)2] (where Hempp is 1-methyl-2-ethyl-3-hydroxy-4(1H)-pyridinone), an antidiabetic and anticancer VC, with the model protein hen egg white lysozyme (HEWL) by electrospray ionization-mass spectrometry (ESI-MS), electron paramagnetic resonance (EPR), and X-ray crystallography. ESI-MS and EPR techniques reveal that, in aqueous solution, both the species [VIVO(empp)2] and [VIVO(empp)(H2O)]+, derived from the first one upon the loss of a empp(−) ligand, interact with HEWL. Crystallographic data, collected under different experimental conditions, show covalent binding of [VIVO(empp)(H2O)]+ to the side chain of Asp48, and noncovalent binding of cis-[VIVO(empp)2(H2O)], [VIVO(empp)(H2O)]+, [VIVO(empp)(H2O)2]+, and of an unusual trinuclear oxidovanadium(V) complex, [VV 3O6(empp)3(H2O)], with accessible sites on the protein surface. The possibility of covalent and noncovalent binding with different strength and of interaction with various sites favor the formation of adducts with the multiple binding of vanadium moieties, allowing the transport in blood and cellular fluids of more than one metal-containing species with a possible amplification of the biological effects.

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