Photoinduced C-F Bond Cleavage in Some Fluorinated 7-Amino-4-quinolone-3-carboxylic Acids

Photosensitized DNA damage: the case of fluoroquinolones

This review focuses on DNA damage photosensitized by the fluoroquinolone (FQ) antibacterial drugs. The in vivo evidence for photocarcinogenesis mediated by FQs is presented in the introduction. The different methods employed for detection of DNA-photodamage mediated by FQs are then summarized, including gel electrophoresis (with whole cells, with isolated DNA and with oligonucleotides) and chromatographic analysis (especially HPLC with electrochemical and MS/MS detection). The chemical mechanisms involved in the formation of the reported lesions are discussed on the basis of product studies and transient spectroscopic evidence. In general, the literature coverage is limited to the last decade, although some earlier citations are also included.

An exploratory and mechanistic study of the defluorination of an (aminofluorophenyl)oxazolidinone: S(N)1(Ar*) vs. S(R(+)N)1(Ar*) mechanism

The morpholinofluorophenyloxazolidinone 1 (the antibacterial drug linezolid) is found to undergo reductive defluorination upon irradiation in water (Phi 0.33), in some of the products accompanied by the simultaneous oxidative degradation of the morpholine side chain. In the presence of chloride, iodide and pyrrole, the fluorine is substituted by these groups (with pyrrole, in position 2). The defluorination is less efficient in methanol and mainly leads to reduction (Phi 0.053). These data can be accommodated through two different mechanisms, viz. either C-F bond heterolysis to give a phenyl cation [S(N)1(Ar*)], or ionization to give a radical cation [S(R(+)N)1(Ar*)]. Steady-state and time resolved data have been gathered for clarifying this issue. It is found that, indeed, ionization of 1 is efficient and proceeds from the singlet, but leads to no irreversible change. On the contrary, triplet (3)1 (lifetime 0.5 micros in MeOH, <0.1 micros in water) fragments and gives the corresponding triplet phenyl cation. The last intermediate explains well the observed hydrogen abstraction both inter- (from the solvent, when this is reducing) and intramolecularly (from the morpholine group), as well as addition to a charged anion or to a neutral pi nucleophile such as pyrrole. The rationalization is supported by the study of some related molecules. Thus, the only photochemical reaction from the non fluorinated analogue of linezolid (that ionizes just as 1) is an inefficient degradation of the morpholine chain (Phi 0.001), while a simple model such as N-(2-fluorophenyl)morpholine undergoes photosolvolysis in water and is not trapped by pyrrole.

Characterizing Ionic Liquids as Reaction Media through a Chemical Probe

The triplet N,N-dimethylaminophenyl cation, a highly reactive but chemospecific electrophile, has been used as a probe for characterizing the properties of reaction media for a series of imidazolium ILs. With the N-hexyl-N-methyl imidazolium derivatives (not with the N-butyl analogues), hydrogen transfer leading to the aniline was the main process. Trapping by iodide occurred with an inverse dependence on viscosity. Trapping by pi nucleophiles exhibited a more complex behavior. This was explained by the effect of both the bulk viscosity and the structure of the IL cation on both steps of the reaction, namely, initial electrophilic attack and ensuing cation elimination or nucleophile addition. However, with an excellent nucleophile, such as thiophene, or when the latter step was intramolecular, as with 4-pentenol, the difference was obliterated and trapping became uniform. Incorporation of the probe into the IL cation (through insertion into the C--H bond alpha to the imidazolium ring) was demonstrated, while no addition to the anion tested (including bis(trifluoromethanesulfonimide)) took place.

Product and Pulse Radiolysis Studies on Radical-Ion Splitting of N(1)−C(5‘)-Linked Dimer Hydrates of 5-Substituted Uracils by One-Electron Reduction in Anoxic Aqueous Solution

Steady-state gamma-radiolysis, pulse radiolysis, and cyclic voltammetry have been performed to identify the mechanism by which N(1)-C(5')-linked homodimer hydrates [1-(6'-hydroxy-5',6'-dihydrothymin-5'-yl)thymine (2a) and [1-(5'-fluoro-6'-hydroxy-5',6'-dihydrouracil-5'-yl)-5-fluorouracil (2b)], N(1)-C(6')-linked dimer hydrate [1-(5'-hydroxy-5',6'-dihydrothymin-6'-yl)thymine (3a)], and N(1)-C(5')-linked heterodimer hydrate [1-(6'-hydroxy-5',6'-dihydrothymin-5'-yl)-5-fluorouracil (2ba)] undergo radiolytic reductive splitting to regenerate the parent monomers in anoxic aqueous solution. Radiolytic reductions of the thymine homodimer hydrates 2a and 3a by hydrated electrons (e(aq)-) regenerated the parent thymine (1a) almost quantitatively, while the 5-fluorouracil homodimer hydrates cis-2b and trans-2b afforded 1-(uracil-5'-yl)-5-fluorouracil efficiently along with a small amount of the parent 5-fluorouracil (1b). In contrast to 2b, the heterodimer hydrate analogue 2ba with noneliminating 5'-methyl substituent releases 5-fluorouracil 1b almost quantitatively in the radiolytic reduction. The pulse radiolysis studies suggested that the electron adducts are produced primarily at the thymine and 5-fluorouracil structural unit in the dimer hydrates 2a,b, respectively, in which the resulting dimer hydrate radical anion of 2b (2b*-) was more stable than that of 2a (2a*-). As characterized by pulse radiolysis and cyclic voltammetry, the 5-fluorouracil homodimer hydrate 2b bearing F-substituent at C(5') undergoes one-electron reduction to eliminate exclusively fluoride ion along with the formation of dimer hydrate C(5') radical (2b(-F)*) with oxidizing property. The formation of a possible dimer hydrate radical intermediate 2b(-F)* was also supported by the effect of amines as the reducing additives on the yields of 1b and 4b in the radiolytic reduction of 2b.

Generation of Detectable Singlet Aryl Cations by Photodehalogenation of Fluoroquinolones

Nanosecond laser flash photolysis (lambdaexc = 355 nm) of neutral aqueous solutions of lomefloxacin (LFX, a 8-fluorinated 7-amino-4-quinolone-3-carboxylic acid derivative) produces a detectable transient species, which shows an absorption maximum at 490 nm and can be assigned to an aryl cation. This intermediate has a lifetime of ca. 200 ns in net water, reacts with Br- and Cl- with rate constants of 3.6 x 10(9) M(-1) s(-1) and 4.1 x 10(8) M(-1) s(-1), respectively, and shows a lack of reactivity toward molecular oxygen. From the photolysis of BAY y3118 (BAY, a 8-chlorinated analogue), an aryl cation is also generated, showing absorption maximum at 480 nm (lifetime of ca. 1 micros in net water) and a reaction rate constant of 9 x 10(9) M(-1) s(-1) with Br(-). The existence of these highly reactive species arising from direct photolysis of LFX and BAY can justify the photogenotoxic properties associated with these antibacterial drugs likely due to direct reaction of their cations with DNA.

Role of Excited State Intramolecular Charge Transfer in the Photophysical Properties of Norfloxacin and Its Derivatives

The photophysical properties of 1-ethyl-6-fluoro-7-(1-piperazinyl)-1,4-dihydro-4-oxoquinoline-3-carboxylic acid (norfloxacin, NFX) and some of its derivatives have been studied to evaluate the role of the free carboxylic acid and the nonprotonated piperazinyl group in the behavior of the 1,4-dihydro-4-oxoquinoline ring. Steady state and time-resolved fluorescence measurements at different pHs provide clear evidence in favor of singlet excited-state deactivation of NFX and its N(4')-methyl derivative pefloxacin (PFX) via intramolecular electron transfer from the N(4') atom of the piperazinyl ring to the fluoroquinolone (FQ) main system. This is a very efficient, energy-wasting pathway, which becomes dramatically enhanced in basic media. Acetylation at N(4') (as in ANFX) decreases the availability of the lone pair, making observable its fluorescence and the transient absorption spectrum of its triplet excited state even at high pH. It also reveals that the geometry of FQs changes from an almost sp3 hybridization of the N(1') of the piperazinyl substituent in the ground state to nearly sp2 in the singlet excited state (rehybridization accompanied by intramolecular charge transfer, RICT); accordingly, the singlet energy of ANFX is significantly lower than that of NFX and PFX. The fluorescence measurements using acetonitrile as a polar nonprotic organic solvent further support deactivation of the singlet excited state of nonacetylated NFX derivatives via intramolecular electron transfer from the N(4') atom.

Selective Entrapment of the Cationic Form of Norfloxacin within Anionic Sodium Dodecyl Sulfate Micelles at Physiological pH and its Effect on the Drug Photodecomposition†

The binding of the photosensitizing fluoroquinolone (FQ) antibiotic norfloxacin (NX) to sodium dodecyl sulfate (SDS) micelles and the photoreactivity of the NX/SDS complex under physiological pH conditions are investigated by means of absorption and emission spectroscopy, steady-state and laser flash photolysis. It is shown that the photolabile zwitterionic form of NX, which is dominant at physiological pH, is not the most abundant species in the presence of SDS micelles. This medium exhibits a high preference for the cationic form of the drug, which is selectively and successfully entrapped within the micellar cage (K(ass) = 6 x 10(4) M(-1) +/- 3000), becoming the largely dominant species at neutral pH. The effect of this trapping is drastically reflected on both efficiency and nature of the drug photodecomposition. It is observed that the photostability of NX incorporated in the micellar pseudophase increases of more than one order of magnitude if compared to that of the "free" drug. Furthermore, the radical photodecomposition mechanism occurring in phosphate buffered solution is suppressed by the micellar medium and the low photodegradation observed seems to take place preferentially through an ionic pathway. Hopefully, the results presented herein may contribute to a better understanding of the bio-distribution of NX in biological systems and provide helpful and stimulating information in order to get the control of FQ photoreactivity under physiological pH conditions.

Rufloxacin induced photosensitization in bio-models of increasing complexity

Rufloxacin belongs to the class of fluoroquinolones that act mainly as specific inhibitors of bacterial Topoisomerase II. These drugs are widely known to be involved in various diseases ranging from cutaneous reactions to aging. The type II photosensitizing activity of Rufloxacin has been already demonstrated on calf thymus DNA and free nucleosides. The aim of this study is to examine in control untreated and UVA irradiated human fibroblasts the modifications on DNA status induced by Rufloxacin added in the culture medium. This allows to investigate the photosensitizing activity of Rufloxacin in a more complex cell model. Fibroblasts, either in the presence or in the absence of Rufloxacin, were exposed to UVA irradiation for different times. An experimental protocol was followed in order to evaluate the amount of single-strand breaks (SSB) and double-strand breaks (DSB) DNA fragmentation by comet assay, and plasmid photocleavage. The presence of oxidized bases was also evaluated using the 8-OH-dGuo test. The comet assay test was also employed to assess cellular repair capacity. The intracellular drug concentration was verified by HPLC-MS. The results confirming the role of Rufloxacin as photosensitizer were: (i) a time-dependent increase in DNA fragmentation when fibroblasts were irradiated in the presence of Rufloxacin; (ii) the efficiency of the cellular repair machinery to be exhaustive after 2 h (whereas no correlation between irradiation time and DNA damage repair was observed with a higher level of DNA fragmentation after shorter irradiation times); (iii) the increased number of cells exhibiting high DNA fragmentation, seen as comets with long tails, was not accompanied by a similar large extent of oxidised DNA base formation, as measured by 8-OH-dGuo analysis; (iv) the double helix SSB, formed in plasmid photosensitization, agreed with the comet assay results, pointing out a good correlation among the cell system and the simpler models used.

Photonucleophilic Aromatic Substitution of 6-Fluoroquinolones in Basic Media: Triplet Quenching by Hydroxide Anion

Photoreaction of 1-ethyl-6-fluoro-7-(1-piperazinyl)-1,4-dihydro-4-oxoquinoline-3-carboxylic acid (norfloxacin, NFX) and other 6-fluoroquinolones in aqueous solution gives rise to the corresponding 6-hydroxy derivatives. Although two mechanisms have been proposed for this photonucleophilic aromatic substitution, direct evidence for any of them is still missing. Obtaining such evidence requires work in basic media, where intramolecular electron transfer from the piperazine ring to the quinolone system is the almost exclusive singlet deactivation pathway. To overcome this problem, the 4'-N-acetyl derivative of norfloxacin (ANFX) has been employed in the present paper due to the lower availability of the N lone pair. The photochemical and photophysical properties of ANFX have been studied in aqueous solutions at pH between 7.4 and 13. As expected, fluorescence of ANFX is not significantly quenched in basic media. Furthermore, the excited triplet state (lambda(max) = 620 nm) reacts with hydroxide anions with a rate constant of (0.3 +/- 0.1) x 10(6) M(-)(1) s(-)(1). This supports a direct attack by hydroxide anions to the excited triplet state with subsequent release of fluoride as the operating mechanism. The fact that the reaction is inhibited by the presence of naproxen (a water-soluble naphthalene derivative) as triplet quencher clearly confirms the mechanistic assignment.

Simultaneous determination of fluoroquinolone, sulfonamide, and trimethoprim antibiotics in wastewater using tandem solid phase extraction and liquid chromatography–electrospray mass spectrometry

A robust and sensitive method for the detection of fluoroquinolones, sulfonamides and trimethoprim has been developed. Wastewater samples were acidified and extracted through an anion-exchange cartridge in tandem with a hydrophilic-lipophilic balance (HLB) cartridge, a procedure that reduced interferences from wastewater organic matter. The extracted antibiotics were analyzed using liquid chromatography electrospray mass spectrometry and selected ion monitoring. Quantification of antibiotics was assessed both by internal standard and standard addition methods. Average recoveries for a range of wastewater matrices were 37 to 129% for a 1 microg/L spiking concentration. The method detection limits (MDLs) of antibiotics in deionized water, final and secondary effluent ranged from 2 to 7 ng/L, from 20 to 50 ng/L, and from 30 to 90 ng/L, respectively. Assessment of matrix interference shows that signal suppression and MDL increases with higher amounts of organic matter in the sample. Analyses of samples from two municipal wastewater treatment plants indicate that ciprofloxacin, ofloxacin, sulfamethoxazole and trimethoprim are present in the secondary effluents at median concentrations of 100-160, 205-305, 395-575, and 40-705 ng/L, respectively.

Characterisation of the lowest singlet and triplet excited states of S-flurbiprofen

The photophysical properties of S-flurbiprofen [S-2-fluoro-alpha-methyl-4-biphenylacetic acid], a nonsteroidal anti-inflammatory drug, have been examined using steady-state and time-resolved spectroscopic techniques. The energy of its first singlet excited state is 99 kcal mol(-1). The fluorescence quantum yields and lifetimes (at 300 nm) have been determined in acetonitrile, methanol, hexane and PBS; they are in the range 0.15<phi(F)< 0.33 and 0.7<tau(F)<2.0 ns. The intersystem crossing quantum yields are between 0.45 and 0.71; the lambda(max) of the T-T absorption is 360 nm, and the triplets live from 15 to 106 micros. Steady state photolysis in aqueous medium leads to S-2-hydroxy-alpha-methyl-4-biphenylacetic acid via photonucleophilic aromatic substitution, in addition to the photodecarboxylation products observed in organic solvents.

Homolytic vs Heterolytic Paths in the Photochemistry of Haloanilines

The photochemistry of 4-haloanilines and 4-halo-N,N-dimethylanilines has been studied in apolar, polar aprotic, and protic solvents. Photophysical and flash photolysis experiments show that the reaction proceeds in any case from the triplet state. It is rather unreactive in apolar media, the highest value being Phi = 0.05 for the iodoanilines in cyclohexane. Changing the solvent has little effect for iodoanilines and for the poorly reacting bromo analogue, while it leads to a variation of over 2 orders of magnitude in the quantum yield for the chloro and fluoro derivatives. The triplets have been characterized at the UB3LYP/6-31G(d) level of theory, evidencing a deformation and an elongation (except for C-F) of the C-X bond. Homolytic fragmentation is in every case endothermic, but calculations in acetonitrile solution show that heterolytic cleavage of C-Cl and C-Br is exothermic. Experimentally, the occurrence of heterolytic fragmentation has been monitored through selective trapping of the resulting phenyl cation by allyltrimethylsilane. Heterolytic dechlorination occurs efficiently in polar media (e.g., Phi = 0.77 in MeCN), while debromination remains ineffective due to the short lifetime of the triplet. Heterolytic defluorination is efficient only in protic solvents (Phi = 0.48 in MeOH), in accord with calculations showing that in the presence of an ancillary molecule of water fragmentation is exothermic due to the formation of the strong H-F bond. The energy profile for both homo- and heterolytic dissociation paths has been mapped along the reaction coordinates in the gas phase and in acetonitrile. The conditions determining the efficiency and mode of dehalogenation have been defined. This is significant for devising synthetic methods via photogenerated phenyl cations and for rationalizing the photodegradation of halogenated aromatic pollutants and the phototoxic effect of some fluorinated drugs.

Photochemical properties of ofloxacin involved in oxidative DNA damage: a comparison with rufloxacin

Photodegradation of ofloxacin (OFX) under aerobic conditions gives rise to N-demethylation, mainly involving coupling of radical cation OFX(*)(+ )()with superoxide radical anion. Although H(2)O(2) is produced as a byproduct, oxidative damage to DNA to give 8-OH-dGuo is associated with a type II mechanism. When the photosensitizing potentials of OFX and rufloxacin (RFX) are compared under the same conditions, the latter is shown to produce a much higher degree of DNA oxidation despite the close structural similarity. This is explained by a decrease of the triplet energy when sulfur instead of oxygen is attached to position 8 of the fluoroquinolone ring system. As a consequence, phosphate anions are able to quench OFX triplet but not RFX triplet; this reveals that the reaction medium has a strong influence on the photochemistry of OFX and hence on its photobiological properties.

Photoinduced N-demethylation of rufloxacin and its methyl ester under aerobic conditions

Irradiation of rufloxacin (RF) under aerobic conditions gives rise to N-demethylation of the piperazinyl ring, which is enhanced in aerated D2O. Two primary processes seem to be involved in RF N-demethylation: photoionization from 1RF and singlet oxygen generation from 3RF. Both processes may lead to the same key intermediates, namely, RF*+ and superoxide radical anion; coupling of these intermediates explains N-demethylation of RF via an iminium cation. Formation of the hydrated electron by a monophotonic process (with a quantum yield of 0.09) is detected along with 3RF (with a intersystem-crossing quantum yield phiISC = 0.36) by laser flash photolysis. Studies performed on RF methyl ester give qualitatively similar results.

Physicochemical properties of quinolone antibiotics in various environments

The progress and photosensitivity of quinolone antibiotics are briefly described. By the photolysis of nalidixic acid, the loss of -COOH group is observed. The photoreaction of fluoroquinolones involves heterolytic C-F bond fragmentation. The protonation and divalent cation complexation equilibria are also examined. The spectroscopic properties of these drugs are intensively investigated in biological mimetic systems such as AOT reverse micelle, and H(2)O-CH(3)OH and H(2)O-CH(3)CN mixed solvents. For ofloxacin and norfloxacin, the excited-state intramolecular charge transfer (ICT) is observed. So, fluorescence spectra exhibit reverse solvatochromism in mixed solvents. The change of radiative and non-radiative rate constant can also be explained using this ICT. The influence of dielectric effects of solvent is more significant compared with the specific hydrogen bonding interaction. Theoretical treatments support all of these results.

Irreversible photo-oxidation of propranolol triggered by self-photogenerated singlet molecular oxygen

The photochemistry of propranolol (PR), 1-isopropylamino-2-(1-naphthyloxy)propan-2-ol, has been studied by combining laser-flash photolysis and steady-state measurements. Like other compounds bearing the naphthalene chromophore, the triplet state is produced quite efficiently (phi(isc) = 0.35) and it is able to transfer its energy to oxygen, sensitizing singlet oxygen formation with phi(delta) = 0.24. PR photodecomposition takes place in an aerated aqueous medium and leads to 6-hydroxy-1,4-naphthoquinone as the sole stable photoproduct. This seems to be consistent with the intermediary formation of the highly photolabile 1,4-naphthoquinone. It is demonstrated that photodegradation occurs via a type II mechanism involving irreversible trapping of self-photogenerated singlet molecular oxygen.

Multifaceted photoreactivity of 6-fluoro-7-aminoquinolones from the lowest excited states in aqueous media: a study by nanosecond and picosecond spectroscopic techniques

Nanosecond and picosecond absorption and emission spectroscopic techniques were applied to the investigation of the reactivity from the lowest excited states of some 6-fluoro-7-piperazino-4-quinolone-3-carboxylic acids (FQs) in aqueous media at neutral pH, in the absence and presence of different sodium salts. Following the detection of various transients, we proposed a mechanism for the cleavage of the carbon-fluorine bond that proceeded through different reaction pathways, dependent on the molecular structure and the characteristics of the medium. The drug lomefloxacin (LOM), a 6,8-difluoroquinolone derivative, underwent heterolytic cleavage of the C8-F bond from the excited singlet state. With the 6-monofluoroquinolone norfloxacin (NOR) and the corresponding 1,8-naphthyridinone enoxacin (ENX), the lowest singlet state was not significantly reactive and an important deactivation channel was intersystem crossing (ISC) to the triplet manifold. The lowest triplet state underwent cleavage of the C6-F bond through a solvent mediated process possibly via a cyclohexadienyl anionic adduct. In the presence of sulfite or phosphate buffer a novel defluorination mechanism, induced by electron transfer from the inorganic anions to the FQ triplet state, was observed. The correlation between the transients observed and the final photoproducts in the different media was elucidated.

Analysis of fluoroquinolone-mediated photosensitization of 2'-deoxyguanosine, calf thymus and cellular DNA: determination of type-I, type-II and triplet-triplet energy transfer mechanism contribution

Fluoroquinolone (FQ) antibacterials are known to exhibit photosensitization properties leading to the formation of oxidative damage to DNA. In addition, photoexcited lomefloxacin (Lome) was recently shown to induce the formation of cyclobutane pyrimidine dimers via triplet-triplet energy transfer. The present study is aimed at gaining further insights into the photosensitization mechanisms of several FQ including enoxacin (Enox), Lome, norfloxacin (Norflo) and ofloxacin (Oflo). This was achieved by monitoring the formation of DNA base degradation products upon UVA-mediated photosensitization of 2'-deoxyguanosine, isolated and cellular DNA. Oflo and Norflo act mainly via a Type-II mechanism whereas Lome and, to a lesser extent, Enox behave more like Type-I photosensitizers. However, the extent of oxidative damage was found to be relatively low. In contrast, it was found that cyclobutane thymine dimers represent the major class of damage induced by Enox, Lome and Norflo within isolated and cellular DNA upon UVA irradiation. This striking observation confirms that FQ are able to promote efficient triplet energy transfer to DNA. The levels of photosensitized formation of strand breaks, alkali-labile sites and oxidative damage to cellular DNA, as measured by the comet assay, were confirmed to be rather low. Therefore, we propose that the phototoxic effects of FQ are mostly accounted for energy transfer mechanism rather than by Type-I or -II photosensitization processes.

A laser flash photolysis and pulse radiolysis study of primary photochemical processes of flumequine

The 355 nm laser flash photolysis of argon-saturated pH 8 phosphate buffer solutions of the fluoroquinolone antibiotic flumequine produces a transient triplet state with a maximum absorbance at 575 nm where the molar absorptivity is 14,000 M(-1) cm(-1). The quantum yield of triplet formation is 0.9. The transient triplet state is quenched by various Type-1 photodynamic substrates such as tryptophan (TrpH), tyrosine, N-acetylcysteine and 2-deoxyguanosine leading to the formation of the semireduced flumequine species. This semireduced form has been readily identified by pulse radiolysis of argon-saturated pH 8 buffered aqueous solutions by reaction of the hydrated electrons and the CO2*- radicals with flumequine. The absorption maximum of the transient semireduced species is found at 570 nm with a molar absorptivity of 2,500 M(-1) cm(-1). In argon-saturated buffered solutions, the semireduced flumequine species formed by the reaction of the flumequine triplet with TrpH stoichiometrically reduces ferricytochrome C (Cyt Fe3+) under steady state irradiation with ultraviolet-A light. In the presence of oxygen, O2*- is formed but the photoreduction of Cyt Fe3+ by O2*- competes with an oxidizing pathway which involves photo-oxidation products of TrpH.

Primary photophysical properties of ofloxacin

Steady-state fluorescence has been used to study the excited singlet state of ofloxacin (OFLX) in aqueous solutions. Fluorescence emission was found to be pH dependent, with a maximum quantum yield of 0.17 at pH 7. Two pKa*s of around 2 and 8.5 were obtained for the excited singlet state. Laser flash photolysis and pulse radiolysis have been used to study the excited states and free radicals of OFLX in aqueous solutions. OFLX undergoes monophotonic photoionization from the excited singlet state with a quantum yield of 0.2. The cation radical so produced absorbs maximally at 770 nm with an extinction coefficient of 5000 +/- 500 dm3 mol-1 cm-1. This is confirmed by one-electron oxidation in the pulse radiolysis experiments. The hydrated electron produced in the photoionization process reacts with ground state OFLX with a rate constant of 2.0 +/- 0.2 x 10(10) dm3 mol-1 s-1, and the anion thus produced has two absorption bands at 410 nm (extinction coefficient = 3000 +/- 300 dm3 mol-1 cm-1) and at 530 nm. Triplet-triplet absorption has a maximum at 610 nm with an extinction coefficient of 11,000 +/- 1500 dm3 mol-1 cm-1. The quantum yield of triplet formation has been determined to be 0.33 +/- 0.05. In the presence of oxygen, the triplet reacts to form both excited singlet oxygen and superoxide anion with quantum yields of 0.13 and < or = 0.2, respectively. Moreover, superoxide anion is also formed by the reaction of oxygen with the hydrated electron from photoionization. Hence the photosensitivity due to OFLX could be initiated by the oxygen radicals and/or by OFLX radicals acting as haptens.