Aqueous Coordination Chemistry of Vanadocene Dichloride, V(η5-C5H5)2Cl2, with Nucleotides and Phosphoesters. Mechanistic Implications for a New Class of Antitumor Agents

Article abstract of DOI:10.1021/ja00283a022

This paper reports an investigation of the mode of interaction of the organometallic antitumor agent Cp2VCl2 (Cp = η⁵-C5H5) with nucleotides and phosphoesters, in aqueous solution near physiological pH, employing high-field 1H and 31P FT NMR and EPR.Paramagnetic (d¹) aqueous Cp2VCl2 is found to selectively interact with the phosphate functionalities of nucleotides and to significantly shorten the 31P nuclear relaxation times.A quantitative analysis of the paramagnetic contributions to the longitudinal (T₁) and transverse (T₂) relaxation rates of the 31P nucleus of 2'-deoxyadenosine-5'-monophosphate reveals that the average internuclear vanadium-phosphorus distance in the solution complex is 6.2 (2) or 5.5 (1) Angstroem, depending on whether each vanadium ion interacts with one or two phosphate moieties, respectively.The temperature dependence of the 31P relaxation rates yields kinetic parameters characterizing the labile outer-sphere complexation of aqueous Cp2VCl2 to the phosphate groups.At 25 deg C, the mean lifetime of the metal-nucleotide complex is estimated to be 0.49 (8) ms.Activation parameters for the ligand dissociation at 25 deg C are the following: ΔG^<*> = 19.5 (2.6) kcal/mol, ΔH^<*> = 13.8 (1.0) kcal/mol, and ΔS^<*> = -19.1 (4.3) e.u.Nucleotide-nucleotide Watson-Crick base-pairing is not disrupted by Cp2VCl2 in aqueous solution, as shown by 1H NMR.An X-ray crystallographic study was also carried out on the model compound, Cp2V(OH2)2*2O2P(OPh)2 (1).The crystal structure of 1 serves to define the coordination of the Cp2V²⁺ unit to a diesterified phosphoric acid, which possesses metrical parameters similar to those of polynucleotides.The complex crystallizes in the monoclinic space group P2₁ (No. 4) with four molecules in a unit cell of dimensions a = 10.571 (2) Angstroem, b = 12.108 (3) Angstroem, c = 25.277 (6) Angstroem, and β = 98.50 (2) deg at 163 (2) K.Least-squares refinement led to a value for the conventional R index (on F) of 0.031 for 6197 unique reflections having 2θ_MoKα<*>55 deg and I > 3?(I).The molecular structure consists of pseudotetrahedral V(η⁵-C5H5)2(OH2)2²⁺ cations connected to diphenyl phosphate anions via strong hydrogen bonds.Average metrical parameters for the V(η⁵-C5H5)2(OH2)2²⁺ cation are as follows: V-C distance, 2.297 (5) Angstroem; V-O(water) distance, 2.050 (8) Angstroem; ring centroid-V-ring centroid angle, 133.0 (4) deg.Average metrical parameters for the PO2(OPh)2^- anion are P-OPh distance, 1.601 (6) Angstroem; P-O (non-ester), 1.480 (2) Angstroem; and C-O distance, 1.392 (3) Angstroem, and these are typical for a phosphodiester anion.The vanadium-phosphorus nonbonded contacts are in the range 5.07-6.44 Angstroem, in good agree ment with the nucleotide NMR results in solution.Implications of these results for the observed biological activity of Cp2VCl2 are briefly discussed.