Reactions of aminopolycarboxylic acids with high-valent transition-metal ions: EDTA-assisted decomposition of carboxylato-bound chromium(V) complexes

Oxidative damage of DNA by chromium(V) complexes: relative importance of base versus sugar oxidation

Chromium(V)-mediated oxidative damage of deoxy-ribonucleic acids was investigated at neutral pH in aqueous solution by utilizing bis(2-ethyl-2-hydroxy-butanato)oxochromate(V) (I) and bis(hydroxyethyl)-amino-tris(hydroxymethyl)methane)oxochromate(V) (II). Single-stranded and double-stranded (ds) calf thymus and human placenta DNA, as well as two oligomers, 5'-GATCTAGTAGGAGGACAAATAGTGTTTG-3' and 5'-GATCCAAGCAAACACTATTTGTCCTCCTACTA-3', were reacted with the chromium(V) complexes. Most products were separated and characterized by chroma-tographic and spectroscopic methods. Polyacrylamide gel electrophoresis experiments reveal more damage at G sites in comparison to other bases. Three primary oxidation products, 5-methylene-2-furanone (5-MF), furfural and 8-oxo-2'-deoxyguanosine, were characterized. A minor product, which appears to be thymine propenal, was also observed. The dsDNA produces more furfural than furanone. The formation of these two products resulted from hydrogen ion or hydride transfer from C1' and C5' positions of the ribose to the oxo-chromium(V) center. Since no enhancements of these products (except propenal) were observed in the presence of oxygen, mechanisms pertaining to the participation of activated oxygen species may be ruled out. The oxidation of the G base is most likely associated with an oxygen atom transfer from the oxo-metallates to the double bond between C8 and N7 of the purine ring. The formation of the propenal may be associated with an oxygen-activated species, since a marginal enhancement of this product was observed in the presence of oxygen. The formation of furfural in higher abundance over 5-MF for dsDNA was attributed to the ease of hydrogen ion (or hydride transfer) from the C5' compared to C1' position of the ribose within a Cr(V)-DNA intermediate in which the metal center is bound to the phosphate diester moiety.

Mechanisms of DNA damage by chromium(V) carcinogens

Reactions of bis(2-ethyl-2-hydroxy-butanato)oxochromate(V) with pUC19 DNA, single-stranded calf thymus DNA (ss-ctDNA), a synthetic oligonucleotide, 5'-GATCTATGGACTTACTTCAAGGCCGGGTAATGCTA-3' (35mer), deoxyguanosine and guanine were carried out in Bis-Tris buffer at pH 7.0. The plasmid DNA was only nicked, whereas the single-stranded DNA suffered extensive damage due to oxidation of the ribose moiety. The primary oxidation product was characterized as 5-methylene-2-furanone. Although all four bases (A, C, G and T) were released during the oxidation process, the concentration of guanine exceeds the other three. Orthophosphate and 3'-phosphates were also detected in this reaction. Likewise, the synthetic oliogomer exhibits cleavage at all bases with a higher frequecncy at G sites. This increased cleavage at G sites was more apparent after treating the primary oxidation products with piperidine, which may indicate base oxidation as well. DNA oxidation is shown to proceed through a Cr(V)-DNA intermediate in which chromium(V) is coordinated through the phosphodiester moiety. Two alternative mechanisms for DNA oxidation by oxochromate(V) are proposed to account for formation of 5-methylene-2-furanone, based on hydrogen abstraction or hydride transfer from the C1' site of the ribose followed by hydration and two successive beta-eliminations. It appears that phosphate coordination is a prerequisite for DNA oxidation, since no reactions between chromium(V) and deoxyguanosine or guanine were observed. Two other additional pathways, hydrogen abstraction from C4' and guanine base oxidation, are also discussed.

Mechanisms of formation and decomposition of hypervalent chromium metabolites in the glutathione-chromium (VI) reaction

A long-lived chromium(IV) intermediate is generated during the reaction between Cr(VI) and glutathione in glycine below pH 3. The intermediate reacts with the tripeptide to produce Cr(III) and oxidized glutathione. A dynamic magnetic susceptibility measurement based on a nuclear magnetic resonance method yielded a 2.8 microB magnetic movement for the chromium(IV) species. The intermediate is formed by parallel third-order and second-order processes. The third-order process (k = 5.9 x 10(2) M-2 s-1) involves first-order participation by each of the oxidant, reductant, and hydrogen ions. A hydrogen ion independent pathway leads to a sluggish second-order process (k = 0.11 M-1 s-1) that is first order with respect to reduced glutathione [GSH] and [Cr(VI)]. Chromium(IV) species is reduced to Cr(III) by a second-order process (k = 0.13 M-1 s-1) that is first order in each of [Cr(IV)] and [GSH] and does not depend on [H+]. At pH 3.4, a chromium(V) species was detected as a minor intermediate as well. In the pH range 6.5-7.5, three dominant chromium(V) intermediates were detected. The existence of Cr(IV) in low pH offers an opportunity to examine the mechanism of DNA damage by this rare oxidation state.

Phosphonato complexes of platinum(II): kinetics of formation and phosphorus-31 NMR characterization studies

Reactions of cis-diamminedichloroplatinum(II) with phosphonoformic acid (PFA), phosphonoacetic acid (PAA), and methylenediphosphonic acid (MDP) yield various phosphonatoplatinum(II) chelates which were characterized by phosphorus-31 NMR spectroscopy. The P-31 resonances for the chelates appear at 6-12 ppm downfield as compared to the uncomplexed ligands. All complexes exhibit monoprotic acidic behavior in the pH range 2-10. The chemical shift-pH profiles yielded acidity constants, 1.0 x 10(-4), 1.5 x 10(-4), and 1.3 x 10(-6) M-1, for the PFA, PAA, and MDP chelates. In addition to the monomeric chelate, MDP formed a bridged diplatinum(II,II) complex when it reacted with cis-Pt (NH3)2(H2O)2(2)+. The P-31 resonance for this binuclear complex appears at 22 ppm downfield from the unreacted ligand. Rate data for the complexation reactions of the phosphonate ligands with the dichloroplatinum complex are consistent with a mechanism in which a monodentate complex is formed initially through rate-limiting aquation process of the platinum complex, followed by a rapid chelation. For the PFA and PAA complexes, initial binding sites are the carboxylato oxygens. Implications of the various binding modes of the phosphonates in relationship to their antiviral activities are discussed.