Alteration of bleomycin cleavage specificity in a platinated DNA oligomer of defined structure

Synthesis, Molecular Structure, and Catalytic Potential of the Tetrairon Complex [Fe4(N3O2-L)4(μ-O)2]4+ (L = 1-Carboxymethyl-4,7-dimethyl-1,4,7-triazacyclononane)

The reaction of iron sulfate with 1-carboxymethyl-4,7-dimethyl-1,4,7-triazacyclononane (L) and hydrogen peroxide in aqueous ethanol gives a brown dinuclear complex considered to be [Fe2(N3O-L)2(mu-O)(mu-OOCCH3)] + (1), which converts upon standing in acetonitrile solution into the green tetranuclear complex [Fe4(N3O2-L)4(mu-O)2]4+ (2). A single-crystal X-ray structure analysis of [2][PF6]4.5MeCN reveals 2 to contain four iron(III) centers, each of which is coordinated to three nitrogen atoms of a triazacyclononane ligand and is bridged by one oxo and two carboxylato bridges, a structural feature known from the active center of methane monooxygenase. Accordingly, complex 2 was found to catalyze the oxidative functionalization of methane with hydrogen peroxide in aqueous solution to give methanol, methyl hydroperoxide, and formic acid; the total turnover numbers attain 24 catalytic cycles within 4 h. To gain more insight into the catalytic process, the catalytic potential of 2 was also studied for the oxidation of higher alkanes, cycloalkanes, and isopropanol in acetonitrile, as well as in aqueous solution. The bond selectivities of the oxidation of linear and branched alkanes suggest a ferroxy radical pathway.

Influence of extrinsic factors on electron transfer in a mixed-valence Fe(2+)/Fe(3+) complex: experimental results and theoretical considerations

The crystal structure of the mixed-valence complex (NEt(4))[Fe(2)(salmp)(2)].xMeCN(crystal) (x = 2,3) [1].xMeCN(crystal) was determined at temperatures between 153 and 293 K. The complex shows distinct Fe(2+) and Fe(3+) sites over this temperature interval. Variable temperature Mössbauer spectra confirm the valence-localized character of the complex. In contrast, spectroscopic investigation of powder samples generated from [1].xMeCN(crystal) indicate the presence of a valence-averaged component at temperatures above 150 K. To elucidate this apparent contradiction we have conducted a variable-temperature Mössbauer investigation of different forms of 1, including [1].xMeCN(crystal), [1].2DMF(crystal), [1].yMeCN(powder), and solution samples of 1 in acetonitrile. The low-temperature Mössbauer spectra of all forms are virtually identical and confirm the valence-localized nature of the S = (9)/(2) ground state. The high-temperature spectra reveal a subtle control of electron hopping by the environment of the complexes. Thus, [1].xMeCN(crystal) has valence-localized spectra at all explored temperatures, [1].2DMF(crystal) exhibits a complete collapse into a valence-averaged spectrum over a narrow temperature range, the powder exhibits partial valence averaging over a broad temperature interval, and the solution sample shows at 210 K the presence of a valence-averaged component in a minor proportion. The spectral transformations are characterized by a coexistence of valence-localized and valence-averaged spectral components. This phenomenon cannot be explained by intramolecular electron hopping between the valence-localized states Fe(A)(2+)Fe(B)(3+) and Fe(A)(3+)Fe(B)(2+) in a homogeneous ensemble of complexes, but requires relaxation processes involving at least three distinguishable states of the molecular anion. Hopping rates for [1].2DMF(crystal) and [1].yMeCN(powder) have been determined from spectral simulations, based on stochastic line shape theory. Analysis of the temperature dependences of the transfer rates reveals the existence of thermally activated processes between (quasi) degenerate excited states in both forms. The preexponential factors in the rate law for the hopping processes in the [1].yMeCN(powder) and [1].2DMF(crystal) differ dramatically and suggest an important influence of the asymmetry of the complex environment (crystal) on intramolecular electron hopping. The differences between the spectra for the crystalline sample [1].xMeCN and those for powders generated under vacuum from these crystals indicate that solvate depletion has a profound effect on the dynamic behavior. Finally, two interpretations for the three states involved in the relaxation processes in 1 are given and critically discussed (salmp = bis(salicyledeneamino)-2-methylphenolate(3-)).

Altered cleavage of DNA sequences by bleomycin and its deglycosylated derivative in the presence of actinomycin

The antitumour antibiotics bleomycin and actinomycin are commonly used therapeutically in combination. One causes metal ion- and oxygen-dependent oxidative damage to DNA, while the other acts at the level of DNA via intercalation of its phenoxazone chromophore and probable inhibition of topoisomerases. Both drugs bind and/or cleave DNA primarily at guanine-containing sequences, which could lead to mutual interference. Using three different restriction fragments we show that binding of actinomycin to DNA causes major alterations in the sequence specificity of bleomycin.Fe-mediated cleavage, including the appearance of new cleavage sites and the suppression of others. The subtle sequence-dependence of the interference is illustrated by the different effects of actinomycin on DNA cleavage by the deglycobleomycin.Fe complex. Actinomycin sharply decreases the extent of cleavage at GpC sites by both bleomycin and deglycobleomycin whereas cleavage at GpT sites is much less affected, while novel cleavage sites are generated at GpA, ApT and, to a lesser extent, TpT steps. A dramatic increase in bleomycin.Fe cutting at GpA is barely detectable with deglycobleomycin.Fe, confirming that the carbohydrate moiety of bleomycin is important for DNA recognition. The results contribute to a better understanding of how two individually well-characterized small molecules interact simultaneoulsy with specific sequences in DNA and as such assist clarification of the principles governing drug-DNA recognition.

Mechanistic Studies of the Formation and Decay of Diiron(III) Peroxo Complexes in the Reaction of Diiron(II) Precursors with Dioxygen

Mechanistic studies of the reactions of three analogous alkoxo-bridged diiron(II) complexes with O(2) have been carried out. The compounds, which differ primarily in the steric accessibility of dioxygen to the diiron(II) center, form metastable &mgr;-peroxo intermediates when studied at low temperature. At ambient temperatures, these intermediates decay to form (&mgr;-oxo)polyiron(III) products. The effect of ligand steric constraints on the O(2) reactivity was investigated. When access to the diiron center was unimpeded, the reaction was first-order with respect to both [Fe(II)(2)] and [O(2)] and the activation parameters for O(2) addition were similar to those for O(2) reacting with the dioxygen transport protein hemerythrin. When the binding site was occluded, however, reduced order with respect to [O(2)] was observed and a two-step mechanism was required to explain the kinetic results. Decay of all three peroxide intermediates involves a bimolecular event, implying the formation of tetranuclear species in the transition state.

Iron(II) bleomycin-mediated degradation of a DNA-RNA heteroduplex

The effect of iron(II) bleomycin on a DNA-RNA heteroduplex was investigated using a substrate formed by reverse transcription of Escherichia coli 5S ribosomal RNA. Both strands of the heteroduplex were cleaved by FeII.BLM A2 at comparable concentrations; complete digestion of both strands was observed using 5 microM FeII.BLM A2. The DNA strand of the heteroduplex was cleaved predominantly at 5'-G-pyr-3' sites; the sites of cleavage of the DNA strand were a subset of those observed for the corresponding DNA strand of a DNA duplex of identical sequence. The sites of cleavage of the RNA strand of the heteroduplex involved both purines and pyrimidines and were found to be different than the sites of cleavage of the 5S rRNA alone, demonstrating that cleavage of the former must actually have involved heteroduplex recognition by FeII.BLM A2. Both the DNA and RNA strands of the heteroduplex were cleaved by FeII.BLM A2 in the presence of physiological concentrations of Mg2+, consistent with the possibility that DNA-RNA heteroduplexes may be therapeutically relevant targets for bleomycin.

Molecular basis for potentiation of bleomycin-mediated degradation of DNA by polyamines. Experimental and molecular mechanical studies

The bleomycin-mediated degradation of DNA is stimulated (amplified) by certain DNA binding compounds, such as polyamines, that distort the double helix. Computer modelling studies suggest that putrescine (1), spermidine (2), and spermine (3) bind preferentially on the floor of the major groove of (dGdC)5.(dGdC)5. This interaction results in a bend of the oligomer helix toward the major groove and enlargement of the minor groove, both effects being in the order 1 less than 2 less than 3. These polyamine-induced distortions, as obtained from theoretical studies, parallel the experimental values of the amplification activities of 1-3 in the bleomycin-mediated degradation of poly(dGdC).poly(dGdC). The amplification mechanism of non-competitive binding of amplifier molecules in the major groove, and bleomycin in the minor groove, is proposed. It is suggested that the amplifier-induced conformational changes of the DNA helix increase affinity of the activated bleomycin complex toward the DNA minor groove and, consequently, result in an increased efficiency of the bleomycin-mediated degradation of the helix.