New phosphotetradecavanadate hybrids: crystal structure, DFT analysis, stability and binding interactions with bio-macromolecules.

Two novel bicapped Keggin polyoxidovanadates with organic cations, (C ₆ H ₈ N) ₅ [H ₄ PV ₁₄ O ₄₂ ]·5H ₂ O (1) and (C ₆ H ₁₄ N ₄ ) ₂ (NH ₄ )[H ₄ PV ₁₄ O ₄₂ ]·11H ₂ O (2), (PV ₁₄ O ₄₂ ^6- = PV14, C ₆ H ₇ N = 3-picoline and C ₆ H ₁₂ N ₄ = methenamine) were synthesized. These compounds were isolated and characterized in the solid state and in solution by elemental analysis, powder X-ray diffraction, FTIR, UV-vis, ⁵¹ V, ³¹ P, ¹³ C and ¹ H NMR, and fluorescence spectroscopy. Further confirmation of the PV14 structures was obtained by single-crystal X-ray diffraction studies of 1 and 2. The Hirshfeld surface analysis was performed to confirm that within the intermolecular interactions occurring in the two crystals, the O⋯H/H⋯O, O⋯O and H⋯H interactions dominate. The protonation and one-electron reduction of the PV14 moiety were also analysed by means of DFT calculations; besides confirming the protonation sites and correctly predicting the p K _a values, the DFT results also indicate that molecular reduction is energetically more favourable in protonated PV14 anions. Upon the addition of PV14 anions to bovine serum albumin (BSA) up to a ratio of 1 : 1, the fluorescence decreased by 45% for both 1 and 2, indicating that the interaction of vanadium-containing species with this protein takes place; log( K _SV ) values of ca. 5.5 were obtained in both systems. Upon the addition of 1 or 2 to solutions of calf-thymus DNA (ctDNA), changes were observed in the UV-vis absorption and circular dichroism spectra. The significance of the changes observed is discussed considering the several V-containing species that form in the solution.