Triplet State Interactions between Nucleic Acid Bases in Solution at Room Temperature: Intermolecular Energy and Electron Transfer

Triplet Excimer Formation in a DNA Duplex with Silver Ion-Mediated Base Pairs

The dynamics of excited electronic states in self-assembled structures formed between silver(I) ions and cytosine-containing DNA strands or monomeric cytosine derivatives were investigated by time-resolved infrared (TRIR) spectroscopy and quantum mechanical calculations. The steady-state and time-resolved spectra depend sensitively on the underlying structures, which change with pH and the nucleobase and silver ion concentrations. At pH ∼ 4 and low dC20 strand concentration, an intramolecularly folded i-motif is observed, in which protons, and not silver ions, mediate C-C base pairing. However, at the higher strand concentrations used in the TRIR measurements, dC20 strands associate pairwise to yield duplex structures containing C-Ag+-C base pairs with a high degree of propeller twisting. UV excitation of the silver ion-mediated duplex produces a long-lived excited state, which we assign to a triplet excimer state localized on a pair of stacked cytosines. The computational results indicate that the propeller-twisted motifs induced by metal-ion binding are responsible for the enhanced intersystem crossing that populates the triplet state and not a generic heavy atom effect. Although triplet excimer states have been discussed frequently as intermediates in the formation of cyclobutane pyrimidine dimers, we find neither computational nor experimental evidence for cytosine-cytosine photoproduct formation in the systems studied. These findings provide a rare demonstration of a long-lived triplet excited state that is formed in a significant yield in a DNA duplex, demonstrating that supramolecular structural changes induced by metal ion binding profoundly affect DNA photophysics.

Probing of DNA Photochemistry with C-Nucleosides of Xanthones and Triphenylene as Photosensitizers To Study the Formation of Cyclobutane Pyrimidine Dimers

The direct and sequence-dependent investigation of photochemical processes in DNA on the way to cyclobutane pyrimidine dimers (CPDs) as DNA damage requires the probing by photochemically different photosensitizers. The C-nucleosides of xanthone, thioxanthone, 3-methoxyxanthone, and triphenylene as photosensitizers were synthesized by Heck couplings and incorporated into ternary photoactive DNA architectures. This structural approach allows the site-selective excitation of the DNA by UV light. Together with a single defined site for T-T dimerization, not only the direct CPD formation but also the distance-dependent CPD formation in DNA as well as the possibility for energy transport processes could be investigated. Direct CPD formation was observed with xanthone, 3-methoxyxanthone, and triphenylene as sensitizers but not with thioxanthone. Only xanthone was able to induce CPDs remotely by a triplet energy transfer over up to six intervening A-T base pairs. Taken together, more precise information on the sequence dependence of the DNA triplet photochemistry was obtained.

Chemiexcited Neurotransmitters and Hormones Create DNA Photoproducts in the Dark

In DNA, electron excitation allows adjacent pyrimidine bases to dimerize by [2 + 2] cycloaddition, creating chemically stable but lethal and mutagenic cyclobutane pyrimidine dimers (CPDs). The usual cause is ultraviolet radiation. Alternatively, CPDs can be made in the dark (dCPDs) via chemically mediated electron excitation of the skin pigment melanin, after it is oxidized by peroxynitrite formed from the stress-induced radicals superoxide and nitric oxide. We now show that the dark process is not limited to the unusual structural molecule melanin: signaling biomolecules such as indolamine and catecholamine neurotransmitters and hormones can also be chemiexcited to energy levels high enough to form dCPDs. Oxidation of serotonin, dopamine, melatonin, and related biogenic amines by peroxynitrite created triplet-excited species, evidenced by chemiluminescence, energy transfer to a triplet-state reporter, or transfer to O2 resulting in singlet molecular oxygen. For a subset of these signaling molecules, triplet states created by peroxynitrite or peroxidase generated dCPDs at levels comparable to ultraviolet (UV). Neurotransmitter catabolism by monoamine oxidase also generated dCPDs. These results reveal a large class of signaling molecules as electronically excitable by biochemical reactions and thus potential players in deviant mammalian metabolism in the absence of light.

Direct Observation of Excitation Wavelength-Dependent Ultrafast Intersystem Crossing in Cytosine Nucleoside Solution

A triplet excited state can lead to different DNA photolesions, especially in cytosine and its nucleoside/nucleotide as they are hotspots for DNA mutations. However, the triplet state generation mechanism is in controversy, and experimental evidence of ultrafast intersystem crossing (ISC) has not been registered in these molecules. In this work, ultrafast ISC is directly observed in 2'-deoxycytidine (dCyd) solution by using femtosecond transient absorption spectroscopy. Surprisingly, we demonstrate that ISC in dCyd is sensitive to the excitation wavelength, and a spin-vibronic ISC mechanism is proposed. This finding is the last piece of the dCyd excited-state deactivation mechanism puzzle and sets the base for further investigation of triplet state-involved photophysics and photochemistry in dCyd-containing DNA.

Fundamental photophysics of isomorphic and expanded fluorescent nucleoside analogues

Fluorescent nucleoside analogues (FNAs) are structurally diverse mimics of the natural essentially non-fluorescent nucleosides which have found numerous applications in probing the structure and dynamics of nucleic acids as well as their interactions with various biomolecules. In order to minimize disturbance in the labelled nucleic acid sequences, the FNA chromophoric groups should resemble the natural nucleobases in size and hydrogen-bonding patterns. Isomorphic and expanded FNAs are the two groups that best meet the criteria of non-perturbing fluorescent labels for DNA and RNA. Significant progress has been made over the past decades in understanding the fundamental photophysics that governs the spectroscopic and environmentally sensitive properties of these FNAs. Herein, we review recent advances in the spectroscopic and computational studies of selected isomorphic and expanded FNAs. We also show how this information can be used as a rational basis to design new FNAs, select appropriate sequences for optimal spectroscopic response and interpret fluorescence data in FNA applications.

The spectral properties of DNA and RNA macromolecules at low temperatures: fundamental and applied aspects

This paper summarizes the results of studies of the spectral properties-optical absorption, fluorescence and phosphorescence-of DNA and RNA macromolecules and synthetic poly-, oligo- and mono-nucleotides, which have been carried out in our laboratory. The system of first excited singlet and triplet energy levels for DNA and RNA is evaluated using low-temperature (4.2 K-77 K) luminescent measurements. The traps of the singlet and triplet electronic excitations in these compounds are identified. An important self-protection mechanism against photo-damage of DNA and RNA by UV photons or penetrative radiation based on the capture of triplet electronic-energy excitations by the most photostable centers-in DNA, the complex formed by neighboring adenosine (A) and thymidine (T) links; in RNA, the adenosine links-is described. It is confirmed that despite similarities in the chemical and partly energy structures DNA is more stable than RNA. The spectral manifestation of the telomeres (the important functional system) in DNA macromolecules is examined. The results obtained on telomere fragments provide the possibility of finding the configuration peculiarities of the triplet excitations traps in DNA macromolecules. The resulting spreading length of the migrating singlet (l _s) and triplet (l _t) excitations for DNA and RNA macromolecules are evaluated.

Thymidine radical formation via one-electron transfer oxidation photoinduced by pterin: Mechanism and products characterization

UV-A radiation (320-400nm), recognized as a class I carcinogen, induces damage to the DNA molecule and its components through different mechanisms. Pterin derivatives are involved in various biological functions, including enzymatic processes, and it has been demonstrated that oxidized pterins may act as photosensitizers. In particular, they accumulate in the skin of patients suffering from vitiligo, a chronic depigmentation disorder. We have investigated the ability of pterin (Ptr), the parent compound of oxidized pterins, to photosensitize the degradation of the pyrimidine nucleotide thymidine 5'-monophosphate (dTMP) in aqueous solutions under UV-A irradiation. Although thymine is less reactive than purine nucleobases, our results showed that Ptr is able to photoinduce the degradation of dTMP and that the process is initiated by an electron transfer from the nucleotide to the triplet excited state of Ptr. In the presence of molecular oxygen, the photochemical process leads to the oxidation of dTMP, whereas Ptr is not consumed. In the absence of oxygen, both compounds are consumed to yield a product in which the pterin moiety is covalently linked to the thymine. This compound retains some of the spectroscopic properties of Ptr, such as absorbance in the UV-A region and fluorescence properties.

Remarkable effects of solvent and substitution on the photo-dynamics of cytosine: a femtosecond broadband time-resolved fluorescence and transient absorption study

Cytosine (Cyt) among all the nucleic acid bases features the most complex and least understood nonradiative deactivation, a process that is crucially important for its photostability. Herein, the excited state dynamics of Cyt and a series of its N1- and C5-derivatives, including the full set of Cyt nucleosides and nucleotides in DNA and RNA and the nucleosides of 5-methyl cytosine, 5-methylcytidine and 2'-deoxy-5-methylcytidine, have been investigated in water and in methanol employing femtosecond broadband time-resolved fluorescence coupled with fs transient absorption spectroscopy. The results reveal remarkable state-specific effects of the substitution and solvent in tuning distinctively the timescales and pathways of the nonradiative decays. For Cyt and the N1-derivatives, the nonradiative deactivations occur in a common two-state process through three channels, two from the light-absorbing ππ* state with respectively the sub-picosecond (∼0.2 ps) and the picosecond (∼1.5 ps) time constant, and the third is due to an optically dark nπ* state with the lifetime ranging from several to hundreds of picoseconds depending on solvents and substitutions. Compared to Cyt, the presence of the ribose or deoxyribose moiety at the N1 position of N1-derivatives facilitates the formation of the nπ* at the sub-picosecond timescale and at the same time increases its lifetime by ∼4-6 times in both water and methanol. In sharp contrast, the existence of the methyl group at the C5 position of the C5-derivatives eliminates completely the sub-picosecond ππ* channel and the channel due to the nπ*, but on the other hand slows down the decay of the ππ* state which after relaxation exhibits a single time constant of ∼4.1 to ∼7.6 ps depending on solvents. Varying the solvent from water to methanol accelerates only slightly the decay of the ππ* state in all the compounds; while for Cyt and its N1-derivatives, this change of solvent also retards strongly the nπ* channel, prolongs its lifetime from such as ∼7.7 ps in water to ∼52 ps in methanol for Cyt and from ∼30 ps in water to ∼186 ps in methanol for deoxycytidine. The spectral signatures we obtained for the ππ* and the nπ* states allow unambiguous evidence for clarifying uncertainties in the excited states of Cyt and the derivatives. The results provide a unifying experimental characterization at an improved level of detail about the photophysics of Cyt and its analogues under biologically relevant conditions and may help in understanding the photostability as well as photo-damages of the bases and related DNAs.

Singlet-Triplet States Interaction Regions in DNA/RNA Nucleobase Hypersurfaces

The present study provides new insight into the intrinsic mechanisms for the population of the triplet manifold in DNA nucleobases by determining, at the multiconfigurational CASSCF/CASPT2 level, the singlet-triplet states crossing regions and the main decay paths for their lowest singlet and triplet states after near-UV irradiation. The studied singlet-triplet interacting regions are accessible along the minimum energy path of the initially populated singlet bright (1)ππ* state. In particular, all five natural DNA/RNA nucleobases have, at the end of the main minimum energy path and near a conical intersection of the ground and (1)ππ* states, a low-energy, easily accessible, singlet-triplet crossing region directly connecting the lowest singlet and triplet ππ* excited states. Adenine, thymine, and uracil display additional higher-energy crossing regions related to the presence of low-lying singlet and a triplet nπ* state. These funnels are absent in guanine and cytosine, which have the bright (1)ππ* state lower in energy and less accessible nπ* states. Knowledge of the location and accessibility of these regions, in which the singlet-triplet interaction is related to large spin-orbit coupling elements, may help to understand experimental evidence such as the wavelength dependence measured for the triplet formation quantum yield in nucleobases and the prevalence of adenine and thymine components in the phosphorescence spectra of DNA.

Photosensitized Thymine Dimerization via Delocalized Triplet Excited States

A new mechanism of photosensitized formation of thymine (Thy) dimers is proposed, which involves generation of a delocalized triplet excited state as the key step. This is supported by chemical evidence obtained by combining one benzophenone and two Thy units with different degrees of freedom, whereby the photoreactivity is switched from a clean Paternò-Büchi reaction to a fully chemo-, regio-, and stereoselective [2+2] cycloaddition.

Reactivity pattern of bromonucleosides induced by 2-hydroxypropyl radicals: photochemical, radiation chemical, and computational studies

The bromonucleosides (BrdX's) 5-bromo-2'-deoxyuridine (BrdU), 5-bromo-2'-deoxycytidine (BrdC), 8-bromo-2'-deoxyadenosine (BrdA), and 5-bromo-2'-deoxyguanosine (BrdG) may substitute for ordinary nucleosides in DNA. As indicated by electron-stimulated desorption experiments, such a modified biopolymer is greater than 2-3-fold more sensitive to damage induced by excess electrons. The other major product of water radiolysis, the (•)OH radical, may form a number of other radicals in chemical reactions with the complex content of the cell. Thus, the well-proved BrdU-labeled DNA radiosensitivity may be, at least in part, related to secondary organic radicals. Therefore, in the current study, the propensity of BrdX's to damage induced by 2-hydroxypropyl radical (OHisop(•))-a prototype radical species-was investigated. The HPLC and LC-MS analyses revealed the formation of two major products from the brominated pyrimidine nucleosides, a native nucleoside and an adduct of BrdX and OHisop(•) , and only an adduct of BrdX from the bromopurine nucleosides. Quantum chemical calculations ascribed this evident difference between purines and pyrimidines to the electron transfer from OHisop(•) to BrdX that is especially favorable in pyrimidines.

Solar UV radiation-induced DNA Bipyrimidine photoproducts: formation and mechanistic insights

This review chapter presents a critical survey of the main available information on the UVB and UVA bipyrimidine photoproducts which constitute the predominant recipient classes of photo-induced DNA damage. Evidence is provided that UVB irradiation of isolated DNA in aqueous solutions and in cells gives rise to the predominant generation of cis-syn cyclobutane pyrimidine dimers (CPDs) and, to a lesser extent, of pyrimidine (6-4) pyrimidone photoproducts (6-4PPs), the importance of which is strongly primary sequence dependent. A notable change in the photoproduct distribution is observed when DNA either in the dry or in desiccated microorganisms is exposed to UVC or UVB photons with an overwhelming formation of 5-(α-thymidyl)-5,6-dihydrothymidine, also called spore photoproduct (dSP), at the expense of CPDs and 6-4PPs. UVA irradiation of isolated and cellular DNA gives rise predominantly to bipyrimidine photoproducts with the overwhelming formation of thymine-containing cyclobutane pyrimidine dimers at the exclusion of 6-4PPs. UVA photons have been shown to modulate the distribution of UVB dimeric pyrimidine photoproducts by triggering isomerization of the 6-4PPs into related Dewar valence isomers. Mechanistic aspects of the formation of bipyrimidine photoproducts are discussed in the light of recent photophysical and theoretical studies.

Early events of DNA photodamage

Ultraviolet (UV) radiation is a leading external hazard to the integrity of DNA. Exposure to UV radiation triggers a cascade of chemical reactions, and many molecular products (photolesions) have been isolated that are potentially dangerous for the cellular system. The early steps that take place after UV absorption by DNA have been studied by ultrafast spectroscopy. The review focuses on the evolution of excited electronic states, the formation of photolesions, and processes suppressing their formation. Emphasis is placed on lesions involving two thymine bases, such as the cyclobutane pyrimidine dimer, the (6-4) lesion, and its Dewar valence isomer.

Formation of cyclobutane thymine dimers by tiaprofenic acid and its photoproducts: approach to the photosensitizer triplet state energy limit value

Tiaprofenic acid family photosensitizes cyclobutane thymine dimer formation allowing to approach the lower limit threshold of a photosensitizer triplet state energy to excite the triplet state of thymine in DNA.

Contribution of cytosine-containing cyclobutane dimers to DNA damage produced by photosensitized triplet-triplet energy transfer

Mutagenic cyclobutane pyrimidine dimers (CPDs) can be induced in DNA through either direct excitation or photosensitized triplet-triplet energy transfer (TTET). In the latter pathway, thymines are expected to receive the excitation energy from the photosensitizer and react with adjacent pyrimidines. By using state-of-the art analytical tools, we provide herein additional information on the formation of cytosine-containing CPDs. We thus determined the yield of all possible CPDs upon TTET in a series of natural DNAs with various base compositions. We show that the distribution of CPDs cannot be explained only by excitation of individual thymines. We propose that the mechanism for TTET involves at least dinucleotides as the minimal targets. The observation of the formation of cytosine-cytosine CPDs also suggests that additional pathways are involved in this photosensitized reaction.

Benzophenone photosensitized DNA damage

Although the carcinogenic potential of ultraviolet radiation is well-known, UV light may interact with DNA by direct absorption or through photosensitization by endogenous or exogenous chromophores. These chromophores can extend the "active" fraction of the solar spectrum to the UVA region and beyond, which means that photosensitizers increase the probability of developing skin cancer upon exposure to sunlight. Therefore researchers would like to understand the mechanisms involved in photosensitized DNA damage both to anticipate possible photobiological risks and to design tailor-made photoprotection strategies. In this context, photosensitized DNA damage can occur through a variety of processes including electron transfer, hydrogen abstraction, triplet-triplet energy transfer, or generation of reactive oxygen species. In this Account, we have chosen benzophenone (BP) as a classical and paradigmatic chromophore to illustrate the different lesions that photosensitization may prompt in nucleosides, in oligonucleotides, or in DNA. Thus, we discuss in detail the accumulated mechanistic evidence of the BP-photosensitized reactions of DNA or its building blocks obtained by our group and others. We also include ketoprofen (KP), a BP-derivative that possesses a chiral center, to highlight the stereodifferentiation in the key photochemical events, revealed through the dynamics of the reactive triplet excited state ((3)KP*). Our results show that irradiation of the BP chromophore in the presence of DNA or its components leads to nucleobase oxidations, cyclobutane pyrimidine dimer formation, single strand breaks, DNA-protein cross-links, or abasic sites. We attribute the manifold photoreactivity of BP to its well established photophysical properties: (i) it absorbs UV light, up to 360 nm; (ii) its intersystem crossing quantum yield (ϕ(ISC)) is almost 1; (iii) the energy of its nπ* lowest triplet excited state (E(T)) is ca. 290 kJ mol(-1); (iv) it produces singlet oxygen ((1)O(2)) with a quantum yield (ϕ(Δ)) of ca. 0.3. For electron transfer and singlet oxygen reactions, we focused on guanine, the nucleobase with the lowest oxidation potential. Among the possible oxidative processes, electron transfer predominates. Conversely, triplet-triplet energy transfer occurs mainly from (3)BP* to thymine, the base with the lowest lying triplet state in DNA. This process results in the formation of cyclobutane pyrimidine dimers, but it also competes with the Paternò-Büchi reaction in nucleobases or nucleosides, giving rise to oxetanes as a result of crossed cycloadditions. Interestingly, we have found significant stereodifferentiation in the quenching of the KP triplet excited state by both 2'-deoxyguanosine and thymidine. Based on these results, this chromophore shows potential as a (chiral) probe for the investigation of electron and triplet energy transport in DNA.

Photoinduced damage to cellular DNA: direct and photosensitized reactions

The survey focuses on recent aspects of photochemical reactions to cellular DNA that are implicated through the predominant formation of mostly bipyrimidine photoproducts in deleterious effects of human exposure to sunlight. Recent developments in analytical methods have allowed accurate and quantitative measurements of the main DNA photoproducts in cells and human skin. Highly mutagenic CC and CT bipyrimidine photoproducts, including cyclobutane pyrimidine dimers and pyrimidine (6-4) pyrimidone photoproducts (6-4PPs) are generated in low yields with respect to TT and TC photoproducts. Another striking finding deals with the formation of Dewar valence isomers, the third class of bipyrimidine photoproducts that is accounted for by UVA-mediated isomerization of initially UVB generated 6-4PPs. Cyclobutadithymine (T<>T) has been unambiguously shown to be involved in the genotoxicity of UVA radiation. Thus, T<>T is formed in UVA-irradiated cellular DNA according to a direct excitation mechanism with a higher efficiency than oxidatively generated DNA damage that arises mostly through the Type II photosensitization mechanism. C<>C and C<>T are repaired at rates intermediate between those of T<>T and 6-4TT. Evidence has been also provided for the occurrence of photosensitized reactions mediated by exogenous agents that act either in an independent way or through photodynamic effects.

Alpha-methylene-gamma-butyrolactones: versatile skin bioactive natural products

Natural products containing an alpha-methylene-gamma-butyrolactone moiety, mainly of the sesquiterpene type, are widely observed in plants, which upon coming into contact with skin, will induce major skin toxicological side effects or phytodermatitis. Indeed two main dermatological pathologies have been associated with a skin exposure to molecules containing an alpha-methylene-gamma-butyrolactone moiety: allergic contact dermatitis (ACD) and chronic actinic dermatitis (CAD). ACD is an immunologically based disease resulting from modifications of epidermal proteins by sensitizers or haptens. Indeed, alpha-methylene-gamma-butyrolactones are highly electrophilic structures that can act as Michael acceptors towards nucleophilic residues of proteins. Cysteine and lysine are the most modified residues leading, in the case of enantiomerically pure lactones, to the formation of diastereomeric adducts. This chemical enantioselectivity induces an enantiospecificity of the allergic reaction, i.e., an individual sensitized to one enantiomer will not develop clinical symptoms when exposed to the other enantiomer and vice versa. Sesquiterpene lactones have been also associated with another pathology that involves UV irradiation and DNA modifications. Interestingly, it was found that alpha-methylene-gamma-butyrolactones, in addition to their electrophilic properties, were highly photoreactive molecules able to react with thymine/thymidine to form [2 + 2] photoadducts in very high yields. In all cases a syn regioselectivity was observed, probably associated with the polarization of the exomethylenic bond. This high photoreactivity of alpha-methylene-gamma-butyrolactones towards thymidine could be an explanation of the progressive evolution of allergic contact dermatitis towards chronic actinic dermatitis.

Molecular basis of DNA photodimerization: intrinsic production of cyclobutane cytosine dimers

Based on CASPT2 results, the present contribution establishes for the first time that cytosine photodimer formation (C< >C) is mediated along the triplet and singlet manifold by a singlet-triplet crossing, (T1/S0)X, and by a conical intersection, (S1/S0)CI, respectively. The former can be accessed in a barrierless way from a great variety of photochemical avenues and exhibits a covalent single bond between the ethene C6-C6' carbon atoms of each monomer. The efficiency of the stepwise triplet mechanism, however, would be modulated by the effectiveness of the intersystem crossing mechanism. The results provide the grounds for the understanding of the potential photogenotoxicity of endogenous and exogenous compounds via triplet-triplet sensitization, with a lower bound for cytosine oligonucleotides predicted to be 2.70 eV, and give support to the traditional view of the primary role of triplet excited states in the photochemistry of DNA, a well-known source of photoproducts in solution under triplet photosensitization conditions. The function played by singlet excimers (excited dimers) to explain both the red-shifted fluorescence and photoreaction is highlighted. A rationale on the pronounced wavelength dependence of the observed fluorescence is offered. Geometrical arrangements at the time of light irradiation close to, but energetically above, (S1/S0)CI are suggested as reactive orientations that become prone to produce C< >C directly, with no energy barrier. Because of the outstanding intrinsic ability of cytosine to form stable relaxed excimers, the system located near the bound relaxed excimer has to accumulate enough vibrational energy to surmount a small barrier of 0.2 eV to reach (S1/S0)CI, making the overall process to proceed at a slower relative rate as compared to other compounds such as thymine, which is not susceptible of forming so stable excimers.

Effect of the GC content of DNA on the distribution of UVB-induced bipyrimidine photoproducts

Solar UV radiation is a major mutagen that damages DNA through the formation of dimeric photoproducts between adjacent thymine and cytosine bases. A major effect of the GC content of the genome is thus anticipated, in particular in prokaryotes where this parameter significantly varies among species. We quantified the formation of UV-induced photolesions within both isolated and cellular DNA of bacteria of different GC content. First, we could unambiguously show the favored formation of cytosine-containing photoproducts with increasing GC content (from 28 to 72%) in isolated DNA. Thymine-thymine cyclobutane dimer was a minor lesion at high GC content. This trend was confirmed by an accurate and quantitative analysis of the photochemical data based on the exact dinucleotide frequencies of the studied genomes. The observation of the effect of the genome composition on the distribution of photoproducts was then confirmed in living cells, using two marine bacteria exhibiting different GC content. Because cytosine-containing photoproducts are highly mutagenic, it may be predicted that species with genomes exhibiting a high GC content are more susceptible to UV-induced mutagenesis.

The triplet energy of thymine in DNA

Norfloxacin (NFX) photosensitizes formation of thymine dimers (T<>T) in DNA, while its N(4') acetyl derivative (ANFX) does not. This is evident from the observation of single-strand breaks after enzymatic treatment with T4 endonuclease V and subsequent gel electrophoresis. The triplet energies of NFX and ANFX are estimated at 273 and 268 kJ/mol, respectively, on the basis of triplet-triplet energy transfer quenching by a set of biphenyl and naphthalene derivatives. Hence, the triplet energy of thymine in DNA (i.e., the value for a photosensitizer to produce T<>T) can be estimated at 270 kJ/mol.

Triplet excited fluoroquinolones as mediators for thymine cyclobutane dimer formation in DNA

A series of fluoroquinolones (FQs), including enoxacin (ENX), pefloxacin (PFX), norfloxacin (NFX), its N(4')-acetyl derivative (ANFX), ofloxacin (OFX), and rufloxacin (RFX) have been investigated to determine their potential as DNA photosensitizers via thymine cyclobutane dimer (T<>T) formation in DNA. At fluoroquinolone concentrations and light doses insufficient to produce direct single strand breaks, ENX, PFX, and NFX were able to produce T<>T dimers in DNA, revealed by enzymatic treatment with T4 endonuclease V. By contrast, ANFX, OFX, and RFX were inefficient in this assay. The absolute values of the triplet energies of ENX, PFX, NFX, ANFX, OFX, and RFX were estimated by means of laser flash photolysis, using flurbiprofen, 4-biphenylcarboxylic acid, and naproxen as energy acceptors. They were found to be 273, 269, 269, 265, 262, and 253 kJ/mol, respectively. Other triplet excited state properties of the FQs, including quantum yields and lifetimes, were also studied. All the results indicate that the threshold ET value required for a given compound to become a potential DNA photosensitizer via T<>T formation is in the range defined by the triplet energies of NFX and ANFX (265-269 kJ/mol). This provides the basis for an alert rule: any chemical (drugs, cosmetics, pesticides, etc.) with higher ET has to be considered with regard to its potential photogenotoxicity.