Base sequence effects in radical cation migration in duplex DNA: support for the polaron-like hopping model

A series of anthraquinone-linked (AQ) duplex DNA oligomers were prepared and investigated. Irradiation of the AQ injects a radical cation into the DNA. The radical cation migrates through the DNA and reacts selectively at GG steps, which leads to strand cleavage after treatment with piperidine. The oligomers investigated in this work were selected to assess the effect on long-distance charge transport of placing a T base (or bases) in a strand of repeating purine bases. With notable exceptions, the amount of strand scission decreases with the distance between the AQ and the GG step. The results are consistent only with models for long-distance transport, such as thermally activated polaron-like hopping, that incorporate radical cation delocalization over two or more adjacent bases.     

Long-distance radical cation migration in duplex DNA: the effect of contiguous A.A and T.T mismatches on efficiency and mechanism

A series of DNA oligomers was prepared. Each oligomer contained an anthraquinone group (AQ, sensitizer) covalently linked at a 5'-end and two GG steps that surrounded a variable region. The variable region was composed of A.T base pairs or A.A or T.T mismatches. Irradiation of the AQ injected a radical cation (hole) into the DNA that migrated through the duplex, being trapped by reaction with H2O of O2 at the GG steps. The effect of substituting A.A or T.T mismatches for Watson-Crick base pairs was examined. For A.A mispairs, charge transfer through the mismatch region was as efficient as through normal DNA. For the T.T mismatches, radical cation transport was strongly distance-dependent. These findings suggest that A.A mismatches form a zipper-like motif, and charge transport proceeds by a hopping mechanism. In contrast, charge transport through the T.T mismatches (where there are no purines) may proceed by quantum mechanical tunneling.     

One-electron oxidation of DNA oligomers that lack guanine: reaction and strand cleavage at remote thymines by long-distance radical cation hopping

The anthraquinone (AQ) photosensitized one-electron oxidation of DNA introduces a radical cation (electron "hole") that migrates through the duplex by hopping. The radical cation normally is trapped irreversibly by reaction at guanine. We constructed AQ-linked DNA oligomers composed exclusively of A/T base pairs. Their irradiation led to reaction and strand cleavage primarily at thymines. Long-distance radical cation hopping to distant thymines was demonstrated by the distance dependence of the process and by experiments with DNA oligomers that contain a single remote GG step. The reaction of the radical cation at thymine was shown to involve its 5-methyl group by the replacement of selected thymines with uracils. These findings show that the reactivity of radical cations in DNA cannot be explained simply by exclusive reliance on the relative oxidation potential of the nucleobases. Instead, the site of reaction is determined in accord with the Curtin-Hammett principle for reactive species in rapid equilibrium.     

Role of the guanine N1 imino proton in the migration and reaction of radical cations in DNA oligomers

Oxidation of a guanine nucleobase to its radical cation in DNA oligomers causes an increase in the acidity of the N1 imino proton that may lead to its spontaneous transfer to N3 of the paired cytosine. This proton transfer is suspected of playing an important role in long-distance radical cation hopping in DNA and the decisive product-determining role in the reaction of the radical cation with H2O or O2. We prepared and investigated DNA oligomers in which certain deoxycytidines are replaced by 5-fluoro-2'-deoxycytidines (F5dC). The pKa of F5C was determined to be 1.7 units below that of dC, which causes proton transfer from the guanine radical cation to be thermodynamically unfavorable. Photoinitiated one-electron oxidation of the DNA by UV irradiation of a covalently attached anthraquinone derivative introduces a radical cation that hops throughout the oligomer and is trapped selectively at GG steps. The introduction of F5dC does not affect the efficiency of charge hopping, but it significantly reduces the amount of reaction at the GG sites, as revealed by subsequent reaction with formamidopyrimidine glycosylase. These findings suggest that transfer of the guanine radical cation N1 proton to cytosine does not play a significant role in long-range charge transfer, but this process does influence the reactions with H2O and/or O2.     

long-range charge transport in duplex DNA: anthraquinone sensitization results are independent of terminal ionic distribution

Irradiation of an anthraquinone (AQ) derivative linked to a 5'-terminus of duplex DNA results in the formation of a base radical cation ("hole") that can migrate through the DNA. Reaction of the radical cation occurs primarily at the 5'-G of GG sequences. This reaction results in the formation of strand breaks when the irradiated DNA is treated with piperidine. The strand breaks are detected by polyacrylamide gel electrophoresis of samples that are labeled at the 3'- or 5'-terminus with (32)P. In contrast to a previous report in which a linked rhodium metallointercalator is used as the sensitizer to oxidize the DNA (Williams, T. T.; Barton, J. K. J. Am. Chem. Soc. 2002, 124, 1840-1841), we find that the position of the label does not affect the relative reactivity of the GG steps when AQ is the sensitizer.     

Emergent functionality of nucleobase radical cations in duplex DNA: prediction of reactivity using qualitative potential energy landscapes

The one-electron oxidation of a series of DNA oligonucleotides was examined. Each oligomer contains a covalently linked anthraquinone (AQ) group. Irradiation of the AQ group with near-UV light results in a one-electron oxidation of the DNA that generates a radical cation (electron "hole"). The radical cation migrates through the DNA by a hopping mechanism and is trapped by reaction with water or molecular oxygen, which results in chemical reaction at particular nucleobases. This reaction is revealed as strand cleavage when the irradiated oligonucleotide is treated with piperidine. The specific oligomers examined reveal the existence of three categories of nucleobase sequences: charge shuttles, charge traps, and barriers to charge migration. The characterization of a sequence is not independent of the identity of other sequences in the oligonucleotide, and for this reason, the function of a particular sequence emerges from an analysis of the entire structure. Qualitative potential energy landscapes are introduced as a tool to assist in the rationalization and prediction of the reactions of nucleobases in oxidized DNA.     

Long-range oxidative damage to DNA: protection of guanines by a nonspecifically bound disulfide

One-electron oxidation of DNA generates a base radical cation ("hole") that migrates through the duplex and causes damage at guanines. Unrepaired damage may lead to mutations. It has been suggested that "sacrificial guanines" in intron regions of DNA might serve to protect genes from damage. We have investigated the ability of a noncovalently bound sacrificial reagent to protect DNA from damage. Irradiation of an anthraquinone (AQ)-linked DNA duplex injects a radical cation into the DNA that causes reactions at GG steps close to and farther from the AQ. Bis[2-(3-(aminopropyl)amino)ethyl]disulfide, an analogue of spermine, binds to duplex DNA. Irradiation of the AQ-linked DNA in the presence of this disulfide suppresses the reaction at both GG steps and protects the DNA from damage. It is suggested that evolutionary pressure for the preservation of genomic integrity would yield disulfide-containing compounds optimized to bind to DNA and neutralize base radical cations.     

Hole trapping at N6-cyclopropyldeoxyadenosine suggests a direct contribution of adenine bases to hole transport through DNA

Recent studies predict that adenine radical cation (A*+) contributes to the hole-trapping process through long A/T sequences and exists as a real chemical intermediate. However, the experimental evidence for the existence of A*+ has not been observed in the DNA-mediated hole transport reaction. To examine the direct contribution of A*+, we have developed a novel hole-trapping nucleobase N6-cyclopropyldeoxyadenosine (dCPA) which possesses a cyclopropyl group as a radical trapping device. One-electron oxidation of dCPA revealed that dCPA radical cation undergoes a rapid cyclopropane ring opening. With the use of the dCPA-containing DNA, we have demonstrated that the migrating hole was trapped at CPA incorporated into a long A/T bridge between two GG sites. The present results indicate that nucleobases possessing ionization potential higher than that of dG, such as dA, are able to participate directly in the multistep hopping mechanism.     

Near-UV induced interstrand cross-links in anthraquinone-DNA duplexes

Anthraquinone (AQ) has been extensively used as a photosensitizer to study charge transfer in DNA. Near-UV photolysis of AQ induces electron abstraction in oligonucleotides leading to AQ radical anions and base radical cations. In general, this reaction is followed by the transport of base radical cations to sites of low oxidation potential, that is, GG, and conversion of G radical cations to DNA breaks. Here, we show that AQ also produces interstrand cross-links in DNA duplexes. About half of the cross-links collapse to single strands in hot piperidine treatment. The structure of stable interstrand cross-links was deduced by MS, NMR, and sequence substitution. The cross-links consist of a covalent link between the methyl group of T on one strand with either C6 or C7 of AQ on the other strand. The formation of interstrand cross-links decreased in O2 compared to deoxygenated solutions. In the presence of O2, the yield of breaks at GG doublets was 10-fold greater than that of cross-links for end tethered AQ, while cross-links exceeded breaks for centrally located AQ. The formation of stable cross-links can be explained by initial charge transfer from T to excited AQ, deprotonation of T radical cations, and condensation of the latter species with AQ radicals. These studies reveal a novel pathway of damage in the photolysis of AQ-DNA duplexes.     

Oxidation of C-H bonds by [(bpy)2(py)RuIVO]2+ occurs by hydrogen atom abstraction

Anaerobic oxidations of 9,10-dihydroanthracene (DHA), xanthene, and fluorene by [(bpy)(2)(py)Ru(IV)O](2+) in acetonitrile solution give mixtures of products including oxygenated and non-oxygenated compounds. The products include those formed by organic radical dimerization, such as 9,9'-bixanthene, as well as by oxygen-atom transfer (e.g., xanthone). The kinetics of these reactions have been measured. The kinetic isotope effect for oxidation of DHA vs DHA-d(4) gives k(H)/k(D) > or = 35 +/- 1. The data indicate a mechanism of initial hydrogen-atom abstraction forming radicals that dimerize, disproportionate and are trapped by the oxidant. This mechanism also appears to apply to the oxidations of toluene, ethylbenzene, cumene, indene, and cyclohexene. The rate constants for H-atom abstraction from these substrates correlate well with the strength of the C-H bond that is cleaved. Rate constants for abstraction from DHA and toluene also correlate with those for oxygen radicals and other oxidants. The rate constant for H-atom transfer from toluene to [(bpy)(2)(py)Ru(IV)O](2+) appears to be close to that predicted by the Marcus cross relation, using a tentative rate constant for hydrogen atom self-exchange between [(bpy)(2)(py)Ru(III)OH](2+) and [(bpy)(2)(py)Ru(IV)O](2+).     

Intercalation of trioxatriangulenium ion in DNA: binding,electron transfer,x-ray crystallography,and electronic structure

Trioxatriangulenium ion (TOTA(+)) is a flat, somewhat hydrophobic compound that has a low-energy unoccupied molecular orbital. It binds to duplex DNA by intercalation with a preference for G-C base pairs. Irradiation of intercalated TOTA(+) causes charge (radical cation) injection that results in strand cleavage (after piperidine treatment) primarily at GG steps. The X-ray crystal structure of TOTA(+) intercalated in the hexameric duplex d[CGATCG](2) described here reveals that intercalation of TOTA(+) results in an unusually large extension of the helical rise of the DNA and that the orientation of TOTA(+) is sensitive to hydrogen-bonding interactions with backbone atoms of the DNA. Electronic structure calculations reveal no meaningful charge transfer from DNA to TOTA(+) because the lowest unoccupied molecular orbital of TOTA(+), (LUMO)(T), falls in the gap between the highest occupied molecular orbital, (HOMO)(D), and the (LUMO)(D) of the DNA bases. These calculations reveal the importance of backbone, water, and counterion interactions, which shift the energy levels of the bases and the intercalated TOTA(+) orbitals significantly. The calculations also show that the inserted TOTA(+) strongly polarizes the intercalation cavity where a sheet of excess electron density surrounds the TOTA(+).     

Turning on red and near-infrared phosphorescence in octahedral complexes with metalated quinones

We report the synthesis of π-bonded ruthenium, rhodium, and iridium o-benzoquinones [Cp*M(o-C(6)H(4)O(2))](n) [M = Ru (2), n = 1-; Rh (3), n = 0; Ir (4), n = 0] following a novel synthetic procedure. Compounds 2-4 were fully characterized by spectroscopic methods and used as chelating organometallic linkers, "OM-linkers", toward luminophore bricks such as Ru(bpy)(2)(2+), Rh(ppy)(2)(+), and Ir(ppy)(2)(+) (bpy = 2,2'-bipyridine; ppy = 2-phenylpyridine) for the design of a novel family of octahedral bimetallic complexes of the general formula [(L-L)(2)M(OM-linkers)][X](m) (X = counteranion; m = 0, 1, 2) whose luminescent properties depend on the choice of the OM-linker and the luminophore brick. Thus, dinuclear assemblies such as [(bpy)(2)Ru(2)][OTf] (5-OTf), [(bpy)(2)Ru(2)][Δ-TRISPHAT] (5-ΔT) {TRISPHAT = tris[tetrachlorobenzene-1,2-bis(olato)]phosphate}, [(bpy)(2)Ru(3)][OTf](2) (6-OTf), [(bpy)(2)Ru(4)][OTf](2) (7-OTf), [(bpy)(2)Ru(4)][Δ-TRISPHAT](2) (7-ΔT), [(ppy)(2)Rh(2)] (8), [(ppy)(2)Rh(3)][OTf] (9-OTf), [(ppy)(2)Rh(4)][OTf] (10-OTf), [(ppy)(2)Rh(4)][Δ-TRISPHAT] (10-ΔT), [(ppy)(2)Ir(2)] (11), [(ppy)(2)Ir(3)][OTf] (12-OTf), [(ppy)(2)Ir(4)][OTf] (13-OTf), and [(ppy)(2)Ir(4)][Δ-TRISPHAT] (13-ΔT) were prepared and fully characterized. The X-ray molecular structures of three of them, i.e., 5-OTf, 8, and 11, were determined. The structures displayed a main feature: for instance, the two oxygen centers of the OM-linker [Cp*Ru(o-C(6)H(4)O(2))](-) (2) chelate the octahedral chromophore metal center, whether it be ruthenium, rhodium, or iridium. Further, the carbocycle of the OM-linker 2 adopts a η(4)-quinone form but with some catecholate contribution due to metal coordination. All of these binuclear assemblies showed a wide absorption window that tailed into the near-IR (NIR) region, in particular in the case of the binuclear ruthenium complex 5-OTf with the anionic OM-linker 2. The latter feature is no doubt related to the effect of the OM-linker, which lights up the luminescence in these homo- and heterobinuclear compounds, while no effect has been observed on the UV-visible and emission properties because of the counteranion, whether it be triflate (OTf) or Δ-TRISPHAT. At low temperature, all of these compounds become luminescent; remarkably, the o-quinonoid linkers [Cp*M(o-C(6)H(4)O(2))](n) (2-4) turn on red and NIR phosphorescence in the binuclear octahedral species 5-7. This trend was even more observable when the ruthenium OM-linker 2 was employed. These assemblies hold promise as NIR luminescent materials, in contrast to those made from organic 1,2-dioxolene ligands that conversely are not emissive.     

Anthraquinone-sensitized photooxidation of 5-methylcytosine in DNA leading to piperidine-induced efficient strand cleavage

One-electron photooxidations of 5-methyl-2'-deoxycytidine (d(m)C) and 5-trideuteriomethyl-2'-deoxycytidine ([D(3)]d(m)C) by sensitization with anthraquinone (AQ) derivatives were investigated. Photoirradiation of an aerated aqueous solution containing d(m)C and anthraquinone 2-sulfonate (AQS) afforded 5-formyl-2'-deoxycytidine (d(f)C) and 5-hydroxymethyl-2'-deoxycytidine (d(hm)C) in good yield through an initial one-electron oxidation process. The deuterium isotope effect on the AQS-sensitized photooxidation of d(m)C suggests that the rate-determining step in the photosensitized oxidation of d(m)C involves internal transfer of the C5-hydrogen atom of a d(m)C-tetroxide intermediate to produce d(f)C and d(hm)C. In the case of a 5-methylcytosine ((m)C)-containing duplex DNA with an AQ chromophore that is incorporated into the backbone of the DNA strand so as to be immobilized at a specific position, (m)C underwent efficient direct one-electron oxidation by the photoexcited AQ, which resulted in an exclusive DNA strand cleavage at the target (m)C site upon hot piperidine treatment. In accordance with the suppression of the strand cleavage at 5-trideuterio-methylcytosine observed in a similar AQ photosensitization, it is suggested that deprotonation at the C5-methyl group of an intermediate (m)C radical cation may occur as a key elementary reaction in the photooxidative strand cleavage at the (m)C site. Incorporation of an AQ sensitizer into the interior of a strand of the duplex enhanced the one-electron photooxidation of (m)C, presumably because of an increased intersystem crossing efficiency that may lead to efficient piperidine-induced strand cleavage at an (m)C site in a DNA duplex.     

Reactions of platinum cluster ions with benzene

In this work, the cation and anion products of the reactions between platinum clusters produced by laser ablation and the benzene molecules seeded in argon have been studied using a high-resolution reflectron time-of-flight mass spectrometer (RTOFMS). The dominant cation products are [C(6n)H(6n - k)](+) and [Pt(m)(C(6)H(6))(n)](+) complexes, while the dominant anion products are dehydrogenated species, [C(6)H(5)PtH](-), [PtC(12)H(k)](-) and [Pt(m)C(6)H(4) . . . (C(6)H(6))(n)](-), etc. Some important intermediate structures ([PtC(6)H(6)](+), [Pt(C(6)H(6))(2)](+), [Pt(2)(C(6)H(6))(3)](+), [C(6)H(5)PtH](-), [Pt(2)C(6)H(4)](-), [Pt(3)C(6)H(4)](-) and [Pt(4)C(6)H(4)](-)) have been analyzed using density functional theory (DFT) calculations. Different reaction mechanisms are proposed for platinum cluster cations and anions with benzene, respectively.     

Increasing the speed limit for hole transport in DNA

Transport of positive charge or holes in DNA occurs via a thermally activated multi-step hopping mechanism. The fastest hopping rates reported to date are those for repeating poly(purine) sequences in which hopping occurs via a random walk mechanism with rate constants of k(hop) = 4.3 × 10(9) s(-1) for poly(dG) and 1.2 × 10(9) s(-1) for poly(dA). We report here the dynamics of charge separation in DNA conjugates possessing repeating 7-deazaadenine (dzA) sequences. These data provide an estimated value of k(hop) = 4.2 × 10(10) s(-1) for poly(dzA), an order of magnitude faster than for poly(dG).     

Relative and absolute kinetic studies of 2-butanol and related alcohols with tropospheric Cl atoms

A newly constructed chamber/Fourier transform infrared system was used to determine the relative rate coefficient, k(i), for the gas-phase reaction of Cl atoms with 2-butanol (k(1)), 2-methyl-2-butanol (k(2)), 3-methyl-2-butanol (k(3)), 2,3-dimethyl-2-butanol (k(4)) and 2-pentanol (k(5)). Experiments were performed at (298 +/- 2) K, in 740 Torr total pressure of synthetic air, and the measured rate coefficients were, in cm(3) molecule(-1) s(-1) units (+/-2sigma): k(1)=(1.32 +/- 0.14) x 10(-10), k(2)=(7.0 +/- 2.2) x 10(-11), k(3)=(1.17 +/- 0.14) x 10(-10), k(4)=(1.03 +/- 0.17) x 10(-10) and k(5)=(2.18 +/- 0.36) x 10(-10), respectively. Also, all the above rate coefficients (except for 2-pentanol) were investigated as a function of temperature (267-384 K) by pulsed laser photolysis-resonance fluorescence (PLP-RF). The obtained kinetic data were used to derive the Arrhenius expressions: k(1)(T)=(6.16 +/- 0.58) x 10(-11)exp[(174 +/- 58)/T], k(2)(T)=(2.48 +/- 0.17) x 10(-11)exp[(328 +/- 42)/T], k(3)(T)=(6.29 +/- 0.57) x 10(-11)exp[(192 +/- 56)/T], and k(4)(T)=(4.80 +/- 0.43) x 10(-11)exp[(221 +/- 56)/T](in units of cm(3) molecule(-1) s(-1) and +/-sigma). Results and mechanism are discussed and compared with the reported reactivity with OH radicals. Some atmospheric implications derived from this study are also reported.     

Isolation of (CO)1- and (CO2)1- radical complexes of rare earths via Ln(NR2)3/K reduction and [K2(18-crown-6)2]2+ oligomerization

Deep-blue solutions of Y(2+) formed from Y(NR(2))(3) (R = SiMe(3)) and excess potassium in the presence of 18-crown-6 at -45 °C under vacuum in diethyl ether react with CO at -78 °C to form colorless crystals of the (CO)(1-) radical complex, {[(R(2)N)(3)Y(μ-CO)(2)][K(2)(18-crown-6)(2)]}(n), 1. The polymeric structure contains trigonal bipyramidal [(R(2)N)(3)Y(μ-CO)(2)](2-) units with axial (CO)(1-) ligands linked by [K(2)(18-crown-6)(2)](2+) dications. Byproducts such as the ynediolate, [(R(2)N)(3)Y](2)(μ-OC≡CO){[K(18-crown-6)](2)(18-crown-6)}, 2, in which two (CO)(1-) anions are coupled to form (OC≡CO)(2-), and the insertion/rearrangement product, {(R(2)N)(2)Y[OC(═CH(2))Si(Me(2))NSiMe(3)]}[K(18-crown-6)], 3, are common in these reactions that give variable results depending on the specific reaction conditions. The CO reduction in the presence of THF forms a solvated variant of 2, the ynediolate [(R(2)N)(3)Y](2)(μ-OC≡CO)[K(18-crown-6)(THF)(2)](2), 2a. CO(2) reacts analogously with Y(2+) to form the (CO(2))(1-) radical complex, {[(R(2)N)(3)Y(μ-CO(2))(2)][K(2)(18-crown-6)(2)]}(n), 4, that has a structure similar to that of 1. Analogous (CO)(1-) and (OC≡CO)(2-) complexes of lutetium were isolated using Lu(NR(2))(3)/K/18-crown-6: {[(R(2)N)(3)Lu(μ-CO)(2)][K(2)(18-crown-6)(2)]}(n), 5, [(R(2)N)(3)Lu](2)(μ-OC≡CO){[K(18-crown-6)](2)(18-crown-6)}, 6, and [(R(2)N)(3)Lu](2)(μ-OC≡CO)[K(18-crown-6)(Et(2)O)(2)](2), 6a.     

Conjugated chromophore arrays with unusually large hole polaron delocalization lengths

We report variable temperature X-band EPR spectroscopic data for the cation radical states of meso-to-meso ethyne-bridged (porphinato)zinc(II) (PZnn) oligomers. These [PZn2-PZn7]+ species span an average 18-75 A length scale and display peak-to-peak EPR line widths (DeltaBp-p) that diminish with conjugation length. Analysis of these EPR data show that PZnn+ structures possess the largest hole polaron delocalization lengths yet measured; experiments carried out over a 4-298 K temperature domain demonstrate remarkably that the charge delocalization length remains invariant with temperature. These cation radical EPR data are well described by a stochastic, near barrierless, one-dimensional charge hopping model developed by Norris for N equivalent sites on a polymer chain, where the theoretical EPR line width is given by DeltaBp-p(N-mer) = (1/N1/2)DeltaBp-p(monomer); PZnn+ oligomers are the first such systems to verify a Norris-type hole delocalization mechanism over a substantial ( approximately 75 A) length scale. Given the time scale of the EPR measurement, these data show that either (i) Franck-Condon effects are incapable of driving charge localization in [PZn2-PZn7]+, resulting in cation radical wave functions which are globally delocalized over a spatial domain that is large with respect to established benchmarks for hole-doped conjugated materials, or (ii) polaron hopping rates in these oligomers exceed 107 s-1, even at 4 K. Finally, this study demonstrates that polymeric building blocks having low magnitude inner sphere reorganization energies enable the development of electronic materials having long polaron delocalization lengths.     

One-electron oxidation of DNA: the effect of replacement of cytosine with 5-methylcytosine on long-distance radical cation transport and reaction

One-electron oxidation of duplex DNA generates a radical cation that migrates through the nucleobases until it is trapped by an irreversible reaction with water or oxygen. The trapping site is often a GG step, because this site has a relatively low ionization potential and this causes the radical cation to pause there momentarily. Modifications to guanine that lower its ionization potential convert it to a better trap for the radical cation. One such modification is the formation of the Watson-Crick base pair with cytosine, which is reported to very significantly decrease its ionization potential. Methylation of cytosine to form 5-methylcytosine (5-MeC) is a naturally occurring reaction in genomic DNA that may be associated with regions of enhanced oxidative damage. The G.5-MeC base pair is reported to be more rapidly oxidized than normal G.C base pairs. We examined the oxidation of DNA oligomers that were substituted in part with 5-MeC. Irradiation of a covalently linked anthraquinone group injects a radical cation into the DNA and results in strand cleavage after piperidine treatment. For the sequences examined, substitution of 5-MeC for C has no measurable effect on the reactions. Cytosine methylation is not a general cause of enhanced oxidative damage in DNA.     

Long-distance radical cation transport in DNA: Horizontal charge hopping in a dimeric quadruplex

A series of DNA hairpins were synthesized and shown to associate to form quadruplexes formed by stacking five G-quartets in an antiparallel orientation. One of the hairpins in the quadruplex was linked covalently at the 5'-end to an anthraquinone (AQ) group and a 32P label was incorporated either at the 3'-terminus of the AQ-containing hairpin or on its partner hairpin in the quadruplex. Irradiation of the AQ group with UV light leads to the one-electron oxidation of the DNA and concomitant introduction of a radical cation into the DNA. Analysis by PAGE and autoradiography shows that the radical cation reacts at guanines both on the AQ-containing strand and with its partner hairpin in the quadruplex. This observation demonstrates that charge migration in DNA occurs vertically along a DNA chain and horizontally within a G-quartet.