Double-stranded Calf Thymus DNA Research Articles

Interaction of the anticancer drug epirubicin with DNA

The interaction of epirubicin (EPR) with calf thymus double-stranded DNA (dsDNA) and calf thymus single-stranded DNA (ssDNA) was studied electrochemically by using differential pulse voltammetry (DPV) and cyclic voltammetry (CV) at carbon paste electrode (CPE). Experimental parameters, such as the concentration of EPR in CV, the concentration of DNA and the accumulation time of EPR, were studied by using DPV; in addition, the detection limit and the reproducibility were determined. It was observed that the signal at a bare electrode was higher than that at the dsDNA-modified CPE. The signals for EPR in CV were found to decrease in the order of bare CPE, ssDNA-modified and dsDNA-modified CPE. The interaction of EPR with smaller oligonucleotides was also evaluated for use as a hybridization indicator.

DNA binding properties of the marine sponge pigment fascaplysin

Association of fascaplysin with double-stranded calf thymus DNA was investigated by means of isothermal titration calorimetry, absorption spectroscopy, and circular dichroism. The UV spectroscopic data could be well interpreted in terms of a two-site model for the binding of fascaplysin to DNA revealing affinity constants of K1=2.5×106M−1 and K2=7.5×104M−1 (base pairs of DNA). Based on the typical change observed in the absorption and circular dichroism spectra, intercalation of fascaplysin is regarded as the major binding mode. The calorimetric titration curves showed an exothermic reaction which was exhausted at a 2:1 base pair/drug; ratio. This finding is in agreement with an intercalation model comprising nearest neighbor exclusion. In addition, significantly weaker non-intercalative DNA interactions can be observed at high drug concentration. By comparison of all these data with the binding behavior of known intercalating agents, it is concluded that fascaplysin intercalates into DNA.

Methylene blue as an electrochemical discriminator of single- and double-stranded oligonucleotides immobilised on gold substrates.

Self-assembly of thiol-terminated oligonucleotides on gold substrates provides a convenient and versatile route to DNA-functionalised surfaces. Here we show that the square-wave voltammetric peak position of methylene blue complexed to thiol-terminated single-stranded oligonucleotides immobilised on gold electrodes differs from that of methylene blue complexed to thiol-terminated double-stranded oligonucleotides immobilised on gold electrodes. The peak potential of methylene blue at the single-stranded oligonucleotide array was consistently found to occur at potentials ca. 10-15 mV more positive than that at double-stranded oligonucleotide arrays, the precise difference being dependent on the direction of the voltammetry. This voltammetric behaviour mirrors that found for methylene blue bound to freely diffusing single- and double-stranded calf thymus DNA and suggests that the immobilised oligonucleotides retain the methylene blue binding properties of their freely diffusing counterparts. Thus methylene blue provides a simple electrochemical indicator for the status of oligonucleotide-functionalised gold surfaces.

Interactions of benzyl viologen with surface-bound single- and double-stranded DNA.

The interactions of benzyl viologen (BV) with single- and double-stranded calf-thymus DNA immobilized onto gold electrodes have been studied by electrochemical methods. Benzyl viologen interacts electrostatically with both double-stranded (ds) and single-stranded (ss) DNA, and the strength of the interactions is dependent on ionic strength (mu). The dicationic form (BV2+) binds to dsDNA 9 times more strongly than the singly reduced form, BV*+, in a pH 7.4 Tris-HCl buffer solution at mu = 8.4 mM. BV2+ binds to ssDNA 5 times more strongly than the BV*+ form. From measurements at mu = 8.4 mM, a binding constant (K2+) of 2.0 (+/-0.2) x 10(4) M(-1) and a binding site size (s) of 1 base pair were obtained, respectively, for dsDNA. For ssDNA, at the same ionic strength, the values obtained for K and s were 3.6 (+/-0.4) x10(4) M(-1) and 2 nucleotides, respectively. The amount of BV bound, whether to dsDNA or ssDNA, decreased with increasing ionic strength. Whereas the binding rate of BV to both dsDNA and ssDNA immobilized onto gold electrodes is relatively low, once immobilized, it dissociates rapidly away from the electrode surface. The electron-transfer rate constant for BV is moderately fast at both dsDNA- and ssDNA-modified gold electrodes. The application of benzyl viologen as an electroactive indicator capable of differentiating between surface-immobilized single- and double-stranded DNA in denaturation/regeneration cycles has been explored.

Synthesis, characterization and DNA-binding properties of [Ru(phen) 2taptp] 2+ and [Ru(phen) 2dptatp] 2+

Two structurally related complexes [Ru(phen) 2taptp] 2+ ( 1) and [Ru(phen) 2dptatp] 2+ ( 2), where taptp is 4,5,9,18-tetraazaphenanthreno[9,10-b]triphenylene and dptatp is 2,3-diphenyl-1,4,8,9-tetraazatriphenylene, were synthesized and characterized by elemental analyses and 1H NMR. The spectroscopic and electrochemical properties of complexes 1 and 2 were also examined. Both complexes can luminesce in organic solvent but are quenched in water to different extents. The interaction of the complexes with calf thymus DNA was studied by absorption and emission spectra, as well as viscosity and lifetime measurements. All the data indicate that both complexes bind with double-stranded calf thymus DNA via intercalative mode. Unlike its analog [Ru(bpy) 2taptp] 2+, [Ru(phen) 2taptp] 2+ shows negligible luminescence in water buffer but bright luminescence on binding to DNA, enabling it to be a sensitive probe for DNA conformation.

Acridine Orange Sensitization and DNA Inhibition Effects on the Cyclic Voltammetry of Naphthoquinones Using Bare and DNA Modified Gold Electrodes

An ion pair mechanism is used to characterize the inhibition by DNA, and sensitization by acridine orange (AO) of the peak currents in the cyclic voltammetry (CV) of naphthoquinones with differing ionic substituents. Gold electrodes modified with adsorbed synthetic ′5-thiol-(GC)10 double strand DNA oligomers are used to study the redox activity of ferricyanide, 1,2-naphthoquinone-4-sulfonic acid sodium salt (NQS), 4-amino-1,2-naphthoquinone (ANQ), or 1,2-naphthoquinone (NQ) in the presence of AO. Parallel studies are performed using bare gold electrodes with double strand calf thymus DNA (CT-DNA) in the bulk solution. AO is known to bind intercalatively to DNA and is seen to sensitize the redox activity of ferricyanide or NQS with DNA either adsorbed on the electrode or present in the electrolyte. AO otherwise does not sensitize the peak currents in the CV of either ANQ or NQ regardless of the presence of DNA, nor does the DNA inhibit their CV, as it does the AO sensitized CV of the anions in the presence of DNA. The electroanalysis is done in pH 7 phosphate buffer with the solute and calf thymus double strand DNA base pair concentrations in the 10–3 molar range, and AO concentrations in the 10–4 molar range. The anion selective electrolytic sensitization combined with the intercalative property of AO may be applied to either the design of DNA based biosensors for the detection of anions, or DNA sensitive biosensors using AO as a marker.

Flow-injection chemiluminescence determination of DNA based on a photochemical reaction

The determination of DNA using the coupling between a photochemical reaction and a chemiluminescent reaction in a flow injection (FI) system is described. The method is based on intercalating 9,10-anthraquinone-2,6-disulphonic acid (AQ) (disodium salt) within the helix of double-stranded calf thymus (CT) DNA. The anthraquinone molecules that are thus intercalated cannot sensitize the photooxidation of ethanol to yield hydrogen peroxide, which is monitored through the chemiluminescent reaction with luminol catalysed by hematin. There is a linear relationship between the decrease in the chemiluminescent response and the DNA concentration at a constant concentration of anthraquinone. Under the optimal conditions, the calibration graph is linear in the range 0.04–5.5 μg ml −1 with a throughput of 45 samples per hour and relative standard deviations between 0.17% and 1.3%. The 3 σ limit of detection is 17 ng ml −1. The method was used for the determination of DNA extracted from rabbit tissues and leaves of the orange tree and the flame tree.

Interaction between Ellagic Acid and Calf Thymus DNA Studied with Flow Linear Dichroism UV–VIS Spectroscopy

The interaction between ellagic acid and DNA has been characterized with respect to the geometry of the ellagic acid–DNA complex, and the active form of ellagic acid has been identified. Optical spectroscopic methods have been employed to examine the interaction between double-stranded calf thymus DNA and ellagic acid in low-ionic-strength aqueous solutions at pH values of 5.5, 7.0, and 8.8. Based on normal absorption titration and flow linear dichroism experiments, it is confirmed that the neutral form of ellagic acid present at pH 5.5 binds to double-stranded DNA. It is found that the plane of the ellagic acid chromophore is positioned at an angle relative to the DNA helix axis, which is in accordance with intercalation of ellagic acid in DNA. It is concluded that at higher values of pH no or a very limited amount of ellagic acid binds to DNA. These results prove that the direct interaction between ellagic acid and DNA must be taken into account when evaluating the mechanism underlying the observed biological effects of this plant phenol.

Electrochemical Study of Electron Transport through Thin DNA Films

The electron/hole conduction of disordered bulk double-stranded (ds) calf thymus DNA and of one-dimensionally aligned 12-base pair single- and double-stranded oligonucleotide monolayers on gold was probed by testing for the occurrence of Faradaic processes. The disordered ds-DNA film was probed by doping it with soybean peroxidase, an easy to “wire” thermostable polycationic enzyme and measuring the current density of electroreduction of H2O2 to water at SCE potential. Although the current density in films with hydrophilic electron-conducting polymeric “wires” is ∼0.5 mAcm-2, when ds-DNA was used to “wire” soybean peroxidase, the current density was only 0.1 μA cm-2, similar to that in the absence of an electron-conducting enzyme-“wiring” polymer. We conclude that the diffusivity of electrons in unaligned and unstretched calf thymus DNA is less than 10-11 cm2 s-1. Nevertheless, the occurrence of a Faradaic reaction was observed in the Au−S−(CH2)2−ds-oligo-NH−PQQ/Au−S−CH2−CH2−OH monolayer on gold, in which...

Natural and synthetic double-stranded DNA binding studies of macrocyclic tetraamine zinc(II) complexes appended with polyaromatic groups.

The characteristic binding mode of zinc(II) complexes of macrocyclic tetraamines (1,4,7,10-tetraazacyclododecane, cyclen) appended with one or two arylmethyl group(s) [(4-quinolyl)methyl-, 1,7-bis(4-quinolyl)methyl-, (1-naphthyl)methyl-, 1,7-bis(1-naphthyl)methyl-, and (9-acridinyl)methyl-cyclen] to double-stranded calf thymus DNA and synthetic DNAs [poly(dA)-poly(dT), poly(dA-dT)2, poly(dI).poly(dC), poly(dI-dC)2, poly(dG) poly(dC), and poly(dG-dC)2] has been examined by spectrophotometric methods, Tm measurement, and inhibition of these DNA-directed transcriptions in vitro. Various hypochromic and bathochromic effects on the pendant aromatic absorption spectra of the complexes were observed in titration with the native and synthetic DNA. The binding constants Kapp (=[bound cyclen derivatives]/[unbound cyclen derivatives][DNA phosphates] M(-1)), at 25 degrees C in 10 mM EPPS (pH 8.0) containing 0.1 M Na+, were determined and compared with those of the corresponding Zn2+ -free ligands. The results showed that the Zn2+ -cyclen complexes interact with the DNA more strongly than the corresponding diprotonated ligands, leading to a stronger stacking of the pendant aromatic rings. The binding of Zn2+ -(9-acridinyl)methyl-cyclen to calf thymus DNA was competed by an AT-selective, minor groove binder, distamycin, but not by a major groove binder, methyl green. In an unusual interaction of excess Zn2+ -(9-acridinyl)methyl-cyclen with poly(dA).poly(dT), the Zn2+ -cyclen moiety went into the minor groove to make coordination bonds with the deprotonated imides of the thymines, resulting in disruption of the poly(dA).poly(dT) duplex. Thymine-containing DNA-directed transcription with Escherichia coli RNA polymerase in vitro was inhibited by the Zn2+ -(9-acridinyl)methyl-cyclen. The 50% inhibition concentrations of the transcription (IC50) were 22-45 microM with poly(dA).poly(dT) or poly(dA-dT)2 as templates, while with poly(dG-dC)2 as a template the IC50 value was 110 microM.

Synthesis, characterization and the effect of ligand planarity of [Ru(bpy) 2L] 2+ on DNA binding affinity

Two structurally related ligands (L) 4,5,9,18-tetraazaphenanthreno[9,10- b] triphenylene (taptp) and 2,3-diphenyl-1,4,8,9-tetraazatriphenylene (dptatp), and their related complexes of [Ru(bpy) 2L] 2+ have been synthesized and characterized by elemental analyses, 1H NMR and mass spectra. Their electrochemical properties were also examined. Both complexes emit intense luminescence in organic solvent but are quenched in water to different extents. The interactions of the complexes with calf thymus DNA have been investigated by viscosity, absorption, emission and circular dichroism spectra. The intrinsic binding constants of [Ru(bpy) 2(taptp)] 2+ and [Ru(bpy) 2(dptatp)] 2+ are 1.7×10 5 and 3.8×10 4 M −1, respectively. All data indicate that both complexes bind enantioselectively to double-stranded calf thymus DNA via the intercalative mode, with stronger affinity for the fully planar ligand complex of [Ru(bpy) 2(taptp)] 2+.

In vitro reactions of butadiene monoxide with single- and double-stranded DNA: characterization and quantitation of several purine and pyrimidine adducts.

We have previously shown that butadiene monoxide (BM), the primary metabolite of 1,3-butadiene, reacted with nucleosides to form alkylation products that exhibited different rates of formation and different stabilities under in vitro physiological conditions. In the present study, BM was reacted with single-stranded (ss) and double-stranded (ds) calf thymus DNA and the alkylation products were characterized after enzymatic hydrolysis of the DNA. The primary products were regioisomeric N-7-guanine adducts. N-3-(2-hydroxy-3-buten-1-yl)adenine and N-3-(1-hydroxy-3-buten-2-yl)adenine, which were depurinated from the DNA more rapidly than the N-7-guanine adducts, were also formed. In addition, N6-(2-hydroxy-3-buten-1-yl)deoxyadenosine and N6-(1-hydroxy-3-buten-2-yl)deoxyadenosine were detected and evidence was obtained that these adducts were formed by Dimroth rearrangement of the corresponding N-1-deoxyadenosine adducts, not while in the DNA, but following the release of the N-1-alkylated nucleosides by enzymatic hydrolysis. N-3-(2-hydroxy-3-buten-1-yl)deoxyuridine adducts, which were apparently formed subsequent to deamination reactions of the corresponding deoxycytidine adducts, were also detected and were stable in the DNA. Adduct formation was linearly dependent upon BM concentration (10-1000 mM), with adduct ratios being similar at the various BM concentrations. At a high BM concentration (750 mM), the adducts were formed in a linear fashion for up to 8 h in both ssDNA and dsDNA. However, the rates of formation of the N-3-deoxyuridine and N6-deoxyadenosine adducts increased 10- to 20-fold in ssDNA versus dsDNA, whereas the N-7-guanine adducts increased only slightly, presumably due to differences in hydrogen bonding in ssDNA versus dsDNA. These results may contribute to a better understanding of the molecular mechanisms of mutagenesis and carcinogenesis of both BM and its parent compound, 1,3-butadiene.

The dinuclear ruthenium(II) complex [{Ru(Phen)2}2(HAT)]4+ (HAT = 1,4,5,8,9,12-hexaazatriphenylene), a new photoreagent for nucleobases and photoprobe for denatured DNA

The photophysical and photochemical properties of a dinuclear ruthenium(II) complex, [{Ru(Phen)2}2(HAT)]4+ (Phen = 1,10-phenanthroline, HAT = 1,4,5,8,9,12-hexaazatriphenylene), have been examined in the presence of mononucleotides and different polynucleotides. Characteristic new features, not observed with monometallic ruthenium(II) complexes, appear with this dimeric compound. First it forms strong ion pairs with the mononucleotides, adenosine- and guanosine-5′-monophosphate, detected from the absorption and emission characteristics under steady state and time-resolved conditions. Secondly, under steady state illumination, very weak luminescence enhancements are induced by the addition of double stranded calf thymus DNA (CT-DNA) whereas important increases of emission occur by the addition of denatured CT-DNA. The dinuclear species may thus be regarded as an excellent photoprobe for denatured DNA. On the other hand, the same photoreactivity with the nucleobases as that of the monometallic TAP (1,4,5,8-tetraazaphenanthrene) and HAT complexes has been retained by the dinuclear species, i.e. (i) a photoelectron transfer from guanosine monophosphate to the excited complex correlated with the formation of a photoproduct and (ii) a photoelectron transfer with DNA; in that case, however, this process is not systematically correlated with the formation of photoproduct in contrast to the monometallic species.

Interactions between aminocalixarenes and nucleotides or nucleic acids †

Four calixarenes with (trimethylammonium)methyl groups at the phenyl rings in the upper rim were prepared. Association constants K with mononucleotides were determined in D2O by NMR shift titration, partially also by fluorescence competition titration using ANS as dye. The complexation free energies ΔG obtained with the derivatives of the calix[4]cone (AC4c) and the calix[4]-1,3-alternate (AC4a) conformation were similar, but increased from AMP (18 ± 1 kJ mol–1) to ADP (20 ± 1 kJ mol–1), to ATP (22 ± 1 kJ mol–1). With the calix[6] derivative (AC6) the corresponding values were 22, 24, 27 kJ mol–1, with the calix[8] host (AC8) 24, 26, 28 kJ mol–1, respectively. The large contribution of salt bridging to the complexation was obvious from the ΔG difference between adenosine and e.g. AMP (with the calix[4]cone derivative 5.6 and 17.7 kJ mol–1, respectively). Affinity differences between different nucleobases increased moderately with the size of the macrocyclic host, e.g. ΔΔG between AMP and TMP was 1 kJ mol–1 with calix[4]cone, 2 kJ mol–1 with calix[6], and 3 kJ mol–1 with calix[8] compounds. The results are in line with computer simulated complex structures in which the nucleobase or sugar parts are only partially inserted into the calix cavity. This agrees with the observed complexation induced NMR shifts (CIS), which are small but increase with the ring size of the host. Noticeably the CIS values are substantially larger for much weaker bound nucleosides.Affinities of the four aminocalixarenes with double-stranded calf thymus (CT) DNA, with polydA*polydT and with polydG*polydC were characterized by ΔTm of the double-strand denaturation temperature and by fluorimetric assays using ethidium bromide (C50 values). The calix[4]cone derivative AC4c shows, due to the four positive charges converging at one side, the strongest effects. They surpass spermine although this also bears four protonated ammonium groups, indicating additional binding contributions from the phenyl moieties. The larger, more flexible calix[6]- and calix[8]-derivatives AC6 and AC8 show only small affinity increases in spite of their 6 or 8 positive charges. Preliminary molecular modeling studies indicate that based on the distances between the ammonium centers only partial contact of all centers to the groove phosphates can materialize. The ligands AC4c, AC4a and AC6 exhibit a remarkable preference for DNA in comparison to RNA mimics.

An unusual DNA adduct derived from the powerfully mutagenic environmental contaminant 3-nitrobenzanthrone.

The covalent binding of an N-hydroxy metabolite of the powerfully mutagenic 3-nitrobenzanthrone (NBA) to 2'-deoxyguanosine (dG) and calf thymus DNA has been investigated in vitro. The major adduct obtained from the reaction of the N-acetoxy-N-acetyl derivative (N-Aco-N-Ac-ABA) of 3-aminobenzanthrone (ABA) and dG was identified as N-acetyl-3-amino-2-(2'-deoxyguanosin-8-yl)benzanthrone (dG-N-Ac-ABA) by 1H NMR and mass spectroscopies as well as by the reaction of N-Aco-N-Ac-ABA with the double-stranded calf thymus DNA. The coupling with the dG moiety occurred exclusively at C-2 of benzanthrone (BA), suggesting a significant contribution of a resonance-stabilized arenium ion intermediate derived from BA to the production of this new type of adduct. The preferred conformation of the adduct has been shown to be syn by 1H and 13C NMR.

Interactions of antitumor drug daunomycin with DNA in solution and at the surface

The interaction of the antitumor drug daunomycin with double-stranded (ds) calf thymus DNA was studied in solution and at the electrode surface by means of cyclic voltammetry and particularly by constant-current chronopotentiometric stripping analysis (CPSA) with the carbon paste electrodes (CPE). As a result of intercalation of this drug between the base pairs in dsDNA, the CPSA daunomycin peak δ decreased and a new more positive shoulder δ b appeared. This shoulder was attributed to the oxidation of the drug intercalated in DNA. Under the same conditions almost no changes in the DNA peak G ox (due to oxidation of guanine residues) were observed. It was shown that daunomycin adheres strongly to the bare CPE (resisting washing) so that a daunomycin-modified electrode can be easily prepared. Daunomycin immobilized at CPE interacted with DNA on immersion of the modified electrode into the dsDNA solution, showing a decrease of peak δ and a well-separated peak δ b instead of the shoulder (which resulted from the interaction of DNA with daunomycin in solution). When the DNA-modified CPE was immersed into a daunomycin solution the peak G ox increased in dependence on daunomycin concentration or on the time of interaction of daunomycin with dsDNA at the electrode surface. Such changes in peak G ox were observed only at submicromolar concentrations of daunomycin. At higher daunomycin concentrations or at longer interaction time intervals a daunomycin peak appeared, which was substantially smaller and more positive than the peak of free daunomycin. The increase of the DNA peak G ox was attributed to interaction of daunomycin from the side of the DNA double helix not contacting the electrode surface. Such binding may induce changes in the DNA structure including bending of the DNA molecule which may result in the increase of peak G ox. Our results thus suggest that the interaction of daunomycin with DNA anchored at the surface may significantly differ from that with DNA in solution. The prospects of using of electroanalytical methods in studies of DNA–drug interactions are discussed.

Migration and Trapping of Photoinjected Excess Electrons in Double-Stranded B-Form DNA But Not in Single-Stranded DNA

Photoexcitation of a pyrene derivative covalently bound to double-stranded calf thymus DNA with 355 nm Nd:YAG laser pulses (fwhm = 24 ps, 50 mJ/cm2/pulse) results in the efficient two-photon-induced ionization of the pyrenyl residues. By use of nanosecond transient absorption techniques, it is shown that the excess electrons injected into the DNA can reduce methylviologen cations (MV2+) that are noncovalently bound to the DNA but not MV2+ in the outer aqueous solution phase. In double-stranded DNA, this reduction of MV2+ to MV•+ occurs via two kinetic phases, a rapid one that is complete within ≤7 ns and a slower one (200−300 ns) due to the diffusive reduction of MV2+ by hydrated electrons. The appearance of the first, rapid reduction phase of MV2+ depends on the secondary structure of the DNA, since it is observed only in the double-stranded form but not in the denatured, single-stranded form. This rapid reduction phase is entirely eliminated upon the addition of magnesium ions, which displace the positively charged MV2+ cations from the double-stranded DNA molecules. By variation of the concentration of MV2+ cations at a constant distribution of covalently bound pyrenyl residues (60 base pairs per pyrenyl residue), a mean distance of migration of excess electrons in double-stranded DNA of ca. 40 Å is estimated.

Intercalation mechanisms with ds-DNA: binding modes and energy contributions with benzene, naphthalene, quinoline and indole derivatives including some antimalarials †

Intercalation mechanisms into double-stranded calf thymus DNA have been probed with 38 different ligands, largely based on naphthalene or quinoline systems. It is shown how NMR shift and line width changes of the ligand signals can be used to unequivocally differentiate intercalation from groove binding modes, and to obtain by curve fitting association constants K. These show acceptable agreement if derived from several independent NMR signals. Less reliable information is obtained from selected UV titrations, from DNA melting differences, from calorimetric measurements and from affinity comparison to polyamines, the latter being based on a fluorescence assay with ethidium bromide. In contrast to literature expectations the naphthalene-shaped ligands show similar affinities irrespective of the presence of nitrogen atoms, or even of charges within the aromatic system. Quinolinium and naphthalene derivatives only intercalate if they bear a positively charged side chain, and then with similar binding constants. Comparison of all systems as well as salt effects demonstrate that the binding can be quantified with additive contributions from salt bridges of the ammonium centres in the side chains, and from the stacking effects of the aromatic parts. There is no evidence for non-classical intercalation by partial insertion between the nucleobases, nor for any intercalation of phenyl units.

Deoxyribonucleic acid binding and photocleavage studies of rhenium(I) dipyridophenazine complexes

The intercalative binding interaction of [Re(dppz)(CO) 3 (py)][O 3 SCF 3 ] (dppz = dipyrido[3,2-a:2′ ,3′-c]phenazine, py = pyridine) and [Re(dppn)(CO) 3 (py)][O 3 SCF 3 ] (dppn = benzo[i]dipyrido[3,2-a :2′,3′-c]phenazine) with double-stranded calf thymus DNA, and synthetic oligonucleotides poly(dA)·poly(dT) and poly(dC)·poly(dG) has been studied with spectroscopic methods. The complexes have been found to promote cleavage of plasmid pBR322 DNA from the supercoiled form I to the open circular form II upon irradiation. The crystal structure of [Re(dppz)(CO) 3 (py)][O 3 SCF 3 ] has also been established.

Single-Strand Breaks in Double-Stranded DNA Irradiated in Anoxic Solution: Contribution of tert-Butanol Radicals

Yields of single-strand breaks induced by 60Co gamma or pulse irradiation in double-stranded calf thymus DNA have been measured in N2O-saturated aqueous solution as a function of the concentration of tert-butanol. The yields were found to be dependent on dose rate. The experimental data were analyzed using a theoretical model based on non-homogeneous scavenging kinetics. It is concluded from this analysis that after 60Co gamma irradiation in the absence of oxygen, aside from breaks caused by hydroxyl radicals, additional breaks occur which are initiated by hydrogen atoms and secondary radicals of tert-butanol. The efficiency of hydrogen atoms in causing single-strand breaks in double-stranded calf thymus DNA was determined to be 2.3%, while the rate constant for the reaction of tert-butanol radicals with DNA and their efficiency in causing single-strand breaks was determined to be 4.1 x 10(3) dm3 mol-1 s-1 and 2%, respectively.