Effect of secondary interactions, steric hindrance and electric charge on the interaction of VIVO species with proteins.

The interaction of two bis-chelated V^IVO²⁺ species, formed by pipemidic (Hpip) and 8-hydroxyquinoline-5-sulphonic acids (H₂hqs), with two small proteins, ubiquitin (Ub) and lysozyme (Lyz), was studied by EPR, ESI-MS, docking and DFT methods. The two complexes differ in electric charge (at neutral pH it is +2 and −2 for the species of Hpip and H₂hqs, respectively), coordination geometry (cis-[VO(Hpip)₂(H₂O)]²⁺ is in equilibrium with the square pyramidal [VO(Hpip)₂]²⁺, while cis-[VO(hqs)₂(H₂O)]²⁻ is the only species in solution), and size (pipemidic acid is more sterically crowded than 8-hydroxyquinoline-5-sulphonic acid). The ESI-MS spectra showed that the number of cis-VO(hqs)₂²⁻ moieties which interact with Ub is twice that of cis-VO(Hpip)₂²⁺ (adducts with formula n[VOL₂]–Ub, where n = 1–4 for H₂hqs and n = 1–2 for Hpip, were detected). The data can be rationalized in terms of the lower steric requirements of hqs²⁻ and its negative charge, which favour secondary interactions with the surface groups of the protein. The EPR spectra revealed the coordination of Asp/Glu-COO or His-N donors, while docking calculations allowed us to identify the Ub residues involved in the VO(Hpip)₂ (Asp39 and His68) and VO(hqs)₂ binding (Glu18, Asp39, Asp58 and His68). The steric hindrance of Hpip is so important that the interaction of cis-[VO(Hpip)₂(H₂O)]²⁺ with lysozyme is completely prevented, while with cis-[VO(hqs)₂(H₂O)]²⁻ the adducts n[VO(hqs)₂]–Lyz (n = 1–2) were detected. Finally, the general effect of secondary interactions such as hydrogen bonds and van der Waals contacts, steric hindrance and electric charge on the V binding and stability of the adducts formed is also discussed, suggesting that these factors should be taken into account when designing new pharmacologically active compounds. [ABSTRACT FROM AUTHOR]