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Biomedical subjects

J K Barton

Publications and source records attributed to J K Barton.

At least 55 records · Page 3Linked to original sources
PubMed1999

Laser fluence for permanent damage of cutaneous blood vessels.

Treatment of vascular disorders may be improved by a more thorough understanding of laser-blood vessel interaction. In this study, the probability of permanent damage to a given type and size of blood vessel was determined as a function of fluence at the top (superficial edge) of the vessel lumen. A 532 nm wavelength, 10 ms pulse duration, 3 mm spot size laser was used to perform approximately 250 irradiations of subdermal blood vessels in the hamster dorsal skin flap preparation. The radiant exposure required for a 50% probability of permanent damage was calculated using a probit analysis of experimental results. Threshold radiant exposure increased with larger blood vessel diameters and was greater for arterioles than venules. Monte Carlo modeling of a typical blood vessel geometry revealed that fluence at the top of the blood vessel lumen was amplified by a factor of approximately 2.4 over tissue surface radiant exposure, due to light scattering in the tissue and internal reflection at the skin-air interfaces.

Animals↗
PubMed1998

Differential DNA recognition by the enantiomers of 1-Rh(MGP)2 phi: a combination of shape selection and direct readout.

The enantiomers of the symmetric metallointercalator complex 1-Rh(MGP)2phi5+ [MGP = 4-(guanidylmethyl)-1,10-phenanthroline; phi = phenanthrenequinone diimine] bound to DNA decamer duplexes containing their respective 6 bp recognition sequences have been investigated using 1H NMR. Shape selection due to the chirality of the metal center and hydrogen-bonding contacts of ancillary guanidinium groups to 3'-G N7 atoms define the recognition by complexes which bind by intercalation to duplex DNA. The titration of Lambda-Rh into the self-complementary decamer containing the recognition sequence (5'-GACATATGTC-3', L1) resulted in one symmetric bound conformation observed in the 1H NMR spectrum, indicating that the DNA duplex retains its symmetry in the presence of the metal complex. Upfield chemical shifts of duplex imino protons and the disruption of the NOE base-sugar contacts defined the central T5-A6 intercalation site. The downfield shift of the G8 imino proton supports the conclusion that the pendant guanidinium arms make simultaneous H-bonding contacts to the N7 atoms of 3'-G8 bases on either side of the site. A variable-temperature study of a partially titrated sample (2:3 Lambda-Rh/L1) showed the exchange rate (kobs) at 298 K to be 68 s-1 and the activation barrier to exchange (DeltaG of association) to be 2.7 kcal/mol, a value comparable to the stacking energy of one base step. The results presented coupled with biochemical data are therefore consistent with binding models in which Lambda-1-Rh(MGP)2phi5+ (Lambda-Rh) traps the recognition site 5'-CATATG-3' in an unwound state, permitting intercalation centrally and hydrogen bonding to guanines at the first and sixth base pair positions. The data suggest a different model of binding and recognition by Delta-Rh. The titration of Delta-Rh into a DNA decamer containing the 6 bp recognition site (D1, 5'-CGCATCTGAC-3'; D2, 5'-GTCAGATGCG-3') resulted in two, distinct conformers, in slow exchange on the NMR time scale. The rate of exchange between the two conformers (kobs) at 298 K is 37 s-1, most likely due to partial dissociation between binding modes. The slower rate relative to Lambda-Rh association reflects the relative rigidity of the D1 and/or D2 sequence in comparison to L1. NOE cross-peaks between the intercalating phi ligand and protons of T5-C6, as well as the upfield shifts observed for imino protons at this step, serve to define the central T5-C6 step as the single site of intercalation. The downfield shift of the 3'-G imino protons indicates the complex makes hydrogen bond contacts with these bases. The complex, which is too small to span a 6 bp B-form DNA sequence, nonetheless makes major groove contacts with 3'-G bases to either side of the site. Notably, both 3'-guanine bases are necessary to impart site specificity and slow dissociation kinetics with the 5'-CATCTG-3' site, as evidenced by the extremely exchange-broadened two-dimensional NOESY spectra of Delta-Rh bound to modified duplexes containing N7-deazaguanine at either G8 or G18; the loss of one major groove contact completely abolishes specificity for 5'-CATCTG-3'. DNA chemical shifts upon binding and intermolecular NOE contacts therefore support a model in which Delta-Rh intercalates in one of two canted binding conformations. Within this model, each intercalation mode allows one guanidinium-guanine hydrogen bond at a time, while bringing the other arm close to the phosphate backbone.

Binding Sites↗
PubMed1998

Long-range and short-range oxidative damage to DNA: photoinduced damage to guanines in ethidium-DNA assemblies.

Short-range and long-range photoreactions between ethidium and DNA have been characterized. While no DNA reaction is observed upon excitation into the visible absorption band of ethidium, higher-energy irradiation (313-340 nm) leads both to direct strand cleavage at the 5'-G of 5'-GG-3' doublets and to piperidine-sensitive lesions at guanine. This reactivity is not consistent with oxidation of guanine by either electron transfer or singlet oxygen as shown by comparison with reactions of a rhodium intercalator and methylene blue, respectively. By covalently tethering ethidium to one end of a DNA duplex, we demonstrate the presence of two distinct reactions, one short-range and the other long-range. The short-range reaction involves a covalent modification of guanine by ethidium, based upon HPLC analysis of the nucleoside products and studies with ethidium derivatives. The long-range reaction is entirely consistent with oxidation of guanine by DNA-mediated electron transfer. The yield of this electron-transfer reaction is not attenuated with distance; equal yields of guanine damage are observed at a proximal (17 A Et-GG separation) and distal (44 A Et-GG separation) site. These results are quite similar to those previously observed with a covalently tethered rhodium photooxidant and underscore the unique ability of the DNA base stack to facilitate long-range electron transfer so as to effect oxidative damage from a distance.

Alkylation↗
PubMed1998

Rh(phen)2phi3+ as a shape-selective probe of triple helices.

RNA pur*pur-pyr and pyr*pur-pyr (pur = purine, pyr = pyrimidine) triple helices consisting of a Watson-Crick base-paired 28mer hairpin duplex and a Hoogsteen base-paired purine or pyrimidine 12mer are targeted with photoactivated cleavage by the metal complex Rh(phen)2phi3+ (phen = phenanthroline, phi = 9, 10-phenanthrenequinone diimine). The metal complex interacts with these triple helices in a structure-specific manner. Different cleavage patterns are seen with the pyr*pur-pyr and pur*pur-pyr motifs. Cleavage is seen on both of the Watson-Crick strands of the former motif and primarily on the purine Watson-Crick strand of the latter motif. Little cleavage is seen on the Hoogsteen strand for either motif. Importantly, the metal complex shows no detectable cleavage on the A-form RNA duplex in the absence of the third Hoogsteen strand. The cleavage patterns are consistent with an intercalated model for the metal complex in the triple helix. Similar cleavage is seen on DNA triple helices, but over a background of duplex cleavage. Targeting of synthetic RNA triple helices, but not duplex regions, by Rh(phen)2phi3+ provides a basis for the chemical probing of triply bonded sites in folded RNA molecules.

Base Composition↗
PubMed1998

Oxidative charge transfer To repair thymine dimers and damage guanine bases in DNA assemblies containing tethered metallointercalators.

Potent oxidants which intercalate in DNA serve as tools to probe DNA-mediated electron-transfer reactions. A photoexcited rhodium intercalator, Rh(phi)2DMB3+ (phi = 9,10-phenanthrenequinone diimine and DMB = 4,4'-dimethyl-2,2'-bipyridine), tethered to DNA, promotes both oxidative damage to 5'-GG-3' doublets in DNA and the repair of thymine dimers from a remote site on the DNA duplex. DNA-mediated repair of a thymine dimer lesion by charge transfer from the tethered rhodium intercalator is quantitative, albeit with low photoefficiency, occurs in an intraduplex reaction over long range (36 A), and requires that the intervening bases be paired. When both oxidative reactions, repair and oxidative damage, are monitored on the same duplex, competition is evident; the presence of both a 5'-GG-3' site and the thymine dimer diminished the dimer repair efficiency by 20-40% and decreased damage at the 5'-GG-3' sites 2-fold compared to similar sequences lacking either the guanine doublet or thymine dimer, respectively. In addition to damage at the 5'-G of 5'-GG-3' sites, we also observe oxidation at the 3'-G of the 5'-GT<>TG-3' tetrad only in the presence of thymine dimer. Overall, the yield of repaired thymine strand was at least 10 times higher than the yield of oxidized guanine in the same sequences. While the 5-GG-3' may represent the thermodynamically favored site for oxidative reaction, repair of the thymine dimer appears to be kinetically more favorable. Dipyridophenanzine (dppz) complexes of ruthenium(III), less potent oxidants which intercalate in DNA, oxidize 5'-GG-3' doublets efficiently but cannot trigger the repair of the thymine dimer lesion. Oxidative damage to DNA from a distance, mediated by the DNA base pair stack, can, however, be utilized to probe the disruption in the base stack generated by the thymine dimer. The presence of the dimer does not diminish oxidation by a Ru(III) intercalator at a distal guanine doublet, suggesting that the disruption caused by the dimer does not block charge transfer through the DNA duplex. DNA-mediated electron-transfer reactions of metallointercalators therefore serve to illustrate important aspects of radical migration and its consequence with respect to reactions at a distance through the DNA base pair stack.

Binding Sites↗
PubMed1998

Imaging of the irradiation of skin with a clinical CO2 laser system: implications for laser skin resurfacing.

BACKGROUND AND OBJECTIVE: Several published reports describe the benefits of using the carbon dioxide laser for cutaneous resurfacing. The mechanisms on which skin resurfacing work are still not completely understood. This study was performed to obtain quantitative and qualitative information describing the thermal response of skin during high-energy, short-pulsed CO2 laser irradiation. STUDY DESIGN/MATERIALS AND METHODS: A Tissue Technologies TruPulse CO2 laser was used to irradiate an in vivo rat model. The laser parameters that were used were a 100-micros pulse width, a 1-Hz repetition rate, a 3 mm x 3 mm square spot size, and 2.4 J/cm2 and 3.9 J/cm2 radiant exposures. A 3-5 microm thermal camera was used to obtain temperature information during irradiation. Single spots were irradiated with one pulse, and the temperature-time history was obtained. In a different experiment, 15 pulses were applied to single spots, and both thermal and video images were obtained. RESULTS: Irradiation with one pulse at 2.4 J/cm2 and 3.9 J/cm2 led to peak temperatures >100 degrees C. The temperature relaxation time was approximately 25-150 ms. Multiple-pulse irradiation at 2.4 J/cm2 led to a slight rise in the peak temperature with each pulse. At 3.9 J/cm2, the peak temperature increased with successive pulses until pulse 10, after which the peak temperature oscillated between 300 and 400 degrees C. Video images showed concurrent burning events that occurred during pulses 10-15. CONCLUSION: Temperatures >100 degrees C were measured during CO2 laser irradiation of skin. Pulse stacking can lead to peak temperatures approaching 400 degrees C and to tissue charring with as few as three stacked pulses. It is crucial for the physician to manipulate the laser handpiece at parameters with which he or she can avoid pulse overlap.

Animals↗
PubMed1998

DNA-mediated electron transfer from a modified base to ethidium: pi-stacking as modulator of reactivity.

BACKGROUND: The DNA double helix is composed of an array of aromatic heterocyclic base pairs and, as a molecular pi-stack, represents a novel system for studying long-range electron transfer. Because many base damage and repair processes result from electron-transfer reactions, the ability of DNA to mediate charge transport holds important biological implications. Seemingly contradictory conclusions have been drawn about electron transfer in DNA from the many different studies that have been carried out. These studies must be reconciled so that this phenomenon can be understood both at a fundamental level and in the context of biological systems. RESULTS: The photoinduced oxidation of a modified base, 7-deazaguanine, has been examined as a function of distance, sequence, and base stacking in DNA duplexes covalently modified with ethidium. Over ethidium/deazaguanine separations of 6-27 A, the photooxidation reaction proceeded on a subnanosecond time scale, and the quenching yield exhibited a shallow distance dependence. The efficiency of the reaction was highly sensitive to small changes in base composition. Moreover, the overall distance-dependence of the reaction is sensitive to sequence, despite the constancy of photoexcited ethidium as acceptor. CONCLUSIONS: The remarkable efficiency of deazaguanine photooxidation by intercalated ethidium over long distances provides new evidence for fast electron-transfer pathways through DNA. By varying sequence as well as reactant separation, this work provides the first experimental demonstration of the importance of reactant stacking in the modulation of long-range DNA mediated electron transfer.

DNA↗
PubMed1997

Oxidative thymine dimer repair in the DNA helix.

The metallointercalator Rh(phi)2DMB3+ (phi, 9,10-phenanthrenequinone diimine; DMB, 4,4'-dimethyl-2,2'-bipyridine) catalyzed the repair of a thymine dimer incorporated site-specifically in a 16-base pair DNA duplex by means of visible light. This repair could be accomplished with rhodium noncovalently bound to the duplex and at long range (16 to 26 angstroms), with the rhodium intercalator tethered to either end of the duplex assembly. This long-range repair was mediated by the DNA helix. Repair efficiency did not decrease with increasing distance between intercalated rhodium and the thymine dimer, but it diminished with disruption of the intervening pi-stack.

2,2'-Dipyridyl↗
PubMed1997

Targeting the Tat-binding site of bovine immunodeficiency virus TAR RNA with a shape-selective rhodium complex.

The Tat-binding site of the bovine immunodeficiency virus TAR RNA hairpin has been targeted by Rh(phen)2phi3+ (phen = phenanthroline, phi = 9,10-phenanthrenequinone diimine), a photochemical probe of RNA tertiary structure. The primary site cleaved by the rhodium complex, upon photoactivation, is U24, a base which participates in the novel base triple (with bases A13 and U10) characteristic of this folded RNA. delta-Rh(phen)2phi3+ binds to this site with an affinity of 2 x 10(6) M-1. Upon mutation of U24 and A13 to A24 and U13, respectively, so that the RNA oligomer is unable to form the base triple, site-specific cleavage by the rhodium complex is abolished. Moreover, as determined through rhodium photocleavage, at a concentration of 20 microM, Rh(phen)2phi3+ inhibits specific binding of BIV-Tat peptide (2 microM) to its target site. Thus the rhodium complex, in matching its shape to the opened major groove of the properly folded RNA, specifically targets its site and is able to compete for its target with the BIV-Tat peptide.

Animals↗
PubMed1997

Long-range oxidation of guanine by Ru(III) in duplex DNA.

BACKGROUND: Theoretical and experimental studies have demonstrated that 5'-GG-3' sequences in DNA are 'hot spots' for oxidative damage, but few studies have definitively addressed whether oxidative damage to DNA may arise from a distance via long-range charge migration. Towards this end, we have prepared tethered ruthenium (Ru)-oligonucleotide duplexes and used a flash-quench strategy to demonstrate long-range charge transport through the DNA double helix. RESULTS: DNA assemblies containing a tethered Ru(II) intercalator have been synthesized. Ru(III), generated in situ in the presence of externally bound electron-transfer quenchers, promotes base damage selectively at the 5'-G of a 5'-GG-3' doublet located approximately 37 A from the binding site of the oxidant. In the absence of a guanine doublet, oxidative damage occurs equally at all guanine bases in the strand. Oxidative damage is also observed at long range for guanine in a G.A mismatch but not in a G.T mismatch. CONCLUSIONS: The present study expands the scope of long-range electron-transfer chemistry in terms of experiments, applications, and possible reactions within the cell. Here we demonstrate oxidative damage to DNA occurring with a high quantum yield over a distance of approximately 37ůsing a ground-state oxidant. These results point to the equilibration of the radical across the DNA duplex to the sites of lowest energy. In addition, this charge migration is sensitive to the intervening pi-stack formed by DNA base pairs and hence may be useful for the detection of mismatches.

DNA↗
PubMed1996

Oxidative DNA damage through long-range electron transfer.

The possibility has been considered for almost forty years that the DNA double helix, which contains a pi-stacked array of heterocyclic base pairs, could be a suitable medium for the migration of charge over long molecular distances. This notion of high charge mobility is a critical consideration with respect to DNA damage. We have previously found that the DNA double helix can serve as a molecular bridge for photo-induced electron transfer between metallointercalators, with fast rates (> or = 10(10)s-1) and with quenching over a long distance (>40 A). Here we use a metallointercalator to introduce a photoexcited hole into the DNA pi-stack at a specific site in order to evaluate oxidative damage to DNA from a distance. Oligomeric DNA duplexes were prepared with a rhodium intercalator covalently attached to one end and separated spatially from 5'-GG-3' doublet sites of oxidation. Rhodium-induced photo-oxidation occurs specifically at the 5'-G in the 5'-GG-3' doublets and is observed up to 37 A away from the site of rhodium intercalation. We find that the yield of oxidative damage depends sensitively upon oxidation potential and pi-stacking, but not on distance. These results demonstrate directly that oxidative damage to DNA may be promoted from a remote site as a result of hole migration through the DNA pi-stack.

Base Sequence↗
PubMed1996

Rates of DNA-mediated electron transfer between metallointercalators.

Ultrafast emission and absorption spectroscopies were used to measure the kinetics of DNA-mediated electron transfer reactions between metal complexes intercalated into DNA. In the presence of rhodium(III) acceptor, a substantial fraction of photoexcited donor exhibits fast oxidative quenching (>3 x 10(10) per second). Transient-absorption experiments indicate that, for a series of donors, the majority of back electron transfer is also very fast (approximately 10(10) per second). This rate is independent of the loading of acceptors on the helix, but is sensitive to sequence and pi stacking. The cooperative binding of donor and acceptor is considered unlikely on the basis of structural models and DNA photocleavage studies of binding. These data show that the DNA double helix differs significantly from proteins as a bridge for electron transfer.

Chemical Phenomena↗
PubMed1995

Sequence-selective DNA recognition and photocleavage: a comparison of enantiomers of Rh(en)2phi3+.

The recognition and photoinduced cleavage of DNA by the enantiomers of bis(ethylenediamine)-(9,10-phenanthrenequinone diimine)Rh(III) [Rh(en)2phi3+] have been characterized and the basis for enantioselective differences delineated. Rh(en)2phi3+ isomers bind strongly to DNA via intercalation and, upon photoactivation with near-UV light, produce direct strand cleavage. On the basis of product analysis, the photoinduced DNA cleavage appears to proceed by a mechanism consistent with that observed for the parent Rh(phen)2phi3+, involving direct abstraction of the 3'-hydrogen atom of the deoxyribose by the activated, intercalated phi. Quantitative photocleavage titrations indicate tight binding by both enantiomers to the DNA duplex. For delta-Rh(en)2phi3+, DNA site affinities range from 0.3 x 10(6) to 8.0 x 10(6) M-1, and a distinct preference for GC sites is evident. lambda-Rh(en)2phi3+ is found to be sequence neutral with an average site affinity of 2 x 10(6) M-1. The basis for sequence selectivity of the enantiomers has been examined through comparison of photocleavage patterns to those of several phi complexes of rhodium(III) containing or lacking axial amines; those complexes containing the axial amines are found to target GC sites. DNA photocleavage studies on oligonucleotides containing the modified bases O6-methylguanine, 7-deazaguanine, and deoxyuracil have been utilized to determine points of interaction on the DNA helix. These results establish binding by both complexes in the major groove of DNA. Differences in site recognition between enantiomers are attributed to the different hydrogen bonding and van der Waals contacts available in the major groove for the ancillary ethylenediamine ligands which differ in disposition in the two isomers.

Base Sequence↗
PubMed1995

Structural examination of enantioselective intercalation: 1H NMR of Rh(en)2phi3+ isomers bound to d(GTGCAC)2.

The enantioselective recognition of d(GTGCAC)2 by delta- and lambda-Rh(en)2phi3+ (en = ethylenediamine; phi = 9,10-phenanthrenequinone diimine) has been examined in a series of one-dimensional (1D) and two-dimensional (2D) 500 MHz 1H NMR experiments both to extend our understanding of the basis for the enantioselective DNA binding and to gain structural information concerning intercalation by the octahedral metal complexes. delta-Rh(en)2phi3+ forms a symmetric 1:1 complex with d(GTGCAC)2, and the metal complex is in slow exchange with the oligodeoxynucleotide bound form at 295 K. The strong upfield shifts of the phi ligand's aromatic protons (0.6-1.3 ppm) are consistent with full intercalation of the phi ligand into the DNA base stack. 2D-NOESY experiments reveal a loss in internucleotide connectivity between G3 and C4 bases, while new NOE cross peaks are observed between the phi ligand and the G3 deoxyribose sugar. In contrast to binding by delta-Rh(en)2phi3+, the 1:1 lambda-Rh(en)2phi(3+)-d(GTGCAC)2 complex shows much broader resonances, and both metal complex and DNA protons appear to be in the intermediate exchange regime. The loss of C2 symmetry in the 1:1 complex is consistent with binding by lambda-Rh(en)2phi3+ at the T2G3 step. Although the enantiomeric metal complexes display different sequence selectivities and exchange characteristics, lambda- and delta-Rh(en)2phi3+ interact with the oligonucleotide duplex in a fundamentally similar manner, through the full intercalation of the phi ligand. Upfield movements in chemical shifts of phi protons are nearly identical for the two enantiomers, and both lambda- and delta-Rh(en)2phi3+ stabilize the duplex to melting by 5-10 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Base Sequence↗
PubMed1995

Sequence-specific DNA binding by a rhodium complex: recognition based on sequence-dependent twistability.

The chemical construction of small molecules targeted to DNA depends upon the sequence-dependent structure of the double helix. Here we describe a new structural element to be considered in the sequence-specific recognition of DNA, sequence-dependent DNA twistability. The importance of sequence-dependent DNA twistability is demonstrated in the DNA recognition properties of a novel synthetic rhodium intercalator, lambda-1-Rh(MGP)2phi5+. This metallointercalator, containing pendant guanidinium groups, binds in the major groove of DNA at subnanomolar concentrations to the 6 base pair sequence 5'-CATATG-3' with enantiospecificity. An essential feature of this recognition is the sequence-specific unwinding of the DNA helix, which permits direct contacts between guanidinium functionalities on the metal complex and guanine residues. Through an assay developed to test for sequence-specific DNA unwinding, a 70 +/- 10 degrees unwinding of the sequence 5'-CATATG-3' is established with specific binding by the metal complex. This sequence-dependent twistability may be an essential feature of the recognition of sequences by DNA-binding proteins and may be exploited in future design.

Base Composition↗
PubMed1994

Sequence-specific recognition of DNA by phenanthrenequinone diimine complexes of rhodium(III): importance of steric and van der Waals interactions.

The importance of steric and van der Waals interactions in the sequence-specific recognition of DNA by [Rh(phi)]3+ complexes has been explored through the synthesis and application of a series of Rh(phi)3+ (phi: 9,10-phenanthrenequinone diimine) derivatives. [Rh(phi)]3+ complexes intercalate in the major groove of DNA via the phi ligand and promote strand scission in the presence of UV light. The complexes reported here are derivatives of the parent molecules [Rh(phi)2bpy]3+ and [Rh(bpy)2phi]3+ (bpy: 2,2'-bipyridyl). The [Rh(phi)]3+ complexes have comparable photoefficiencies; therefore, their different photocleavage patterns on 32P-end-labeled DNA fragments reflect their unique sequence-specific recognition characteristics. The shapes of the [Rh(phi)]3+ complexes are found to govern DNA recognition and reaction. Importantly and generally, the more sterically bulky complexes, containing methyl or phenyl groups on the ancillary ligands, cleave DNA at a subset of sequences recognized by their parent molecules. [Rh-(diphenylbpy)2phi]3+ specifically targets the site 5'-CTCTAGAG-3'. Furthermore, chiral discrimination in site selectivity is observed; the different isomers target different sites. delta- and lambda-[Rh(5,5'-dimethylbpy)2phi]3+ cleave specifically at sites that are defined by the consensus sequences 5'-C-T-N-G-3' and 5'-A-C/G-T-C/G-3', respectively. The sequence selectivities may be understood on the basis of both negative steric clashes and positive van der Waals interactions between methyl groups on the metal complex and thymine methyl groups in the DNA major groove.

Base Sequence↗
PubMed1994

Fast photoinduced electron transfer through DNA intercalation.

We report evidence for fast photoinduced electron transfer mediated by the DNA helix that requires metal complexes that are avid intercalators of DNA. Here the donor bis(phenanthroline)(dipyridophenazine)ruthenium(II) [Ru(phen)2dppz2+] and acceptor bis(9,10-phenanthrenequinone diimine)(phenanthroline)rhodium(III) [Rh(phi)2phen3+] intercalate into DNA with Kb > 10(6) M-1. Luminescence quenching experiments in the presence of two different lengths of DNA yield upward-curving Stern-Volmer plots and the loss of luminescence intensity far exceeds the change in emission lifetimes. In the presence of a nonintercalative electron acceptor, Ru(NH3)3+(6), Ru(phen)2dppz2+ luminescence is quenched much less efficiently compared to that found for the intercalative Rh(phi)2phen3+ quencher and follows linear Stern-Volmer kinetics; steady-state and time-resolved Stern-Volmer plots are comparable in scale. These experiments are consistent with a model involving fast long-range electron transfer between intercalators through the DNA helix.

DNA↗