Photocatalytic degradation of aniline at the interface of TiO2 suspensions containing carbonate ions

The degradation of aniline has been investigated using aqueous TiO2 suspensions containing carbonate ions as photocatalyst. The addition of carbonate to Degussa P-25 increased the number of active adsorption sites at its surface. For the TiO2 suspensions containing carbonate ions the intensity of adsorption of aniline increased to 6.9 x 10(2) from 5.5 x 10(2) mol(-1) dm(3) in case of bare TiO2 suspensions. This in turn results in the increased interfacial interaction of the photogenerated charge carriers with the adsorbed aniline and thus enhancing the rate of its photodecomposition to 6.5 x 10(-6) mol dm(-3) s(-1) compared to 2.7 x 10(-6) mol dm(-3) s(-1) in the absence of Na(2)CO(3). The maximum efficiency of this photocatalyst has been obtained upon addition of 0.11 mol dm(-3) of Na(2)CO(3) at pH 10.8. The photocatalytic action is understood by the simultaneous interaction of intermediates, *OH and CO*-(3), and their reactivity with aniline. Azobenzene, p-benzoquinone, nitrobenzene, and NH(3) have been identified as the major products of the photooxidation of aniline. Both the reactant and products have been followed kinetically. The photodegradation follows Langmuir-Hinshelwood Model. The mechanism of the occurring reactions has been analyzed and discussed. In the presence of Na(2)CO(3), 3 x 10(-3) mol dm(-3) of aniline could be photodegraded completely in about 6 h while all organic intermediates decomposed completely within about 10 h.     

Active oxygen intermediates in the degradation of hematoporphyrin derivative in tumor cells subjected to photodynamic therapy

Hematoporphyrin derivative (HPD), a sensitizer used in photodynamic therapy (PDT) of malignancies, is progressively destroyed during the treatment. Prior studies suggested that upon PDT the photobleaching of HPD in tumor tissues is largely mediated by self-sensitized singlet oxygen. However, little is known about the role of other reactive oxygen species (ROS). The main aim of this work was to clarify the significance of H2O2, superoxide (O2.(-)) and hydroxyl (OH.) radicals in bleaching of HPD in tumor cells subjected to PDT. Experiments were performed on Ehrlich ascites carcinoma (EAC) cells, which were loaded with HPD in PBS and then irradiated with red light at 630 nm in the same buffer. Studies showed that photosensitization of EAC cells by HPD led to the formation of significant amounts of H2O2, O2.(-) and OH., and that these ROS could be involved in the photobleaching of HPD during PDT. In fact, we found that addition of catalase (CAT, a scavenger H2O2), Cu/Zn-superoxide dismutase (Cu/Zn-SOD) and Tiron (scavengers of O2.(-)), Na-benzoate, mannitol and deferoxamine (scavengers of OH.) caused a substantial decrease in the rate of HPD photobleaching in EAC cells. In these cells, the inhibitory effects of Na-benzoate, mannitol and deferoxamine on the photodegradation of HPD correlated well with suppression of the OH. generation, a highly active oxidizer. In EAC cells, the glutathione redox cycle and CAT (scavengers of H2O2) as well as Cu/Zn-SOD was found to suppress the photoinduced degradation of HPD. It was also established that HPD can directly scavenge H2O2 and oxygen free radicals; in a phosphate buffer its second-order rate constants were measured as 5.51+/-0.32 x 10(3)M(-1)s(-1) (for the reaction with O2.(-)), 5.08+/-0.31 x 10(4)M(-1)s(-1) (for H2O2), and 3.44+/-0.08 x 10(10)M(-1)s(-1) (for OH.). Thus, our data suggest that OH. could be one of the main oxidants mediating the photobleaching behavior of HPD in malignancies. Studies showed that photoexcited moieties of HPD can oxidize cell proteins with the formation of protein peroxides (PPO), which currently are regarded as a new form of ROS. Model experiments suggest that PPO could also participate in bleaching of HPD in tumors treated with PDT. It was found that HPD may destroy in tumor cells after cessation of photoirradiation and that this event is largely mediated by the presence of H2O2, a precursor of OH(.).     

The effects of some operational parameters in photodegradation of benzylamine and aniline and their kinetics in aqueous suspension of TiO2 and Pt -loaded TiO2

Photocatalytic degradation of benzylamine and aniline on TiO2, Pt-modified TiO2, ZnO and ZnS in aqueous solution has been investigated. The degradation of the compounds follows a pseudo-first-order kinetics according to Langmuir-Hinshelwood model. The degradation process of benzylamine and aniline was evaluated by ninhydrin spectrophotometric method using UV-visible spectrophotometer in lambda(max) = 538 and 525 nm, respectively. The results showed the order of Pt/TiO2 > TiO2 > ZnO > ZnS for photocatalytic activity. In addition increasing of the Pt-loading was found to enhance the degradation rate of the compounds up to the optimal amount of 5 wt. % onto the surface of TiO2 so that the rates of degradation were increased about two times. Rate constants for photodegradation of benzylamine and aniline were found to be 1.4 x 10(-3) min(-1) and 0.7 x 10(-3) min(-1) for TiO2 as photocatalyst, while 2.7 x 10(-3) min(-1) and 1.7 x 10(-3) min(-1) for (5 wt.%) Pt/TiO2 as photocatalyst. Running the reactions in various pH (5-11), indicated that the pH = 8 and 10 or Higher are the optimum pH for photocatalytic degradation of benzylamine and aniline respectively. The effects of some other parameters such as amount of photocatalyst, flux of oxygen and irradiation time were evaluated. Furthermore, the Langmuir-Hinshelwood rate constant k(r) and adsorption constant K(A) for the titled compounds are reported.     

Time-resolved investigations of singlet oxygen luminescence in water, in phosphatidylcholine, and in aqueous suspensions of phosphatidylcholine or HT29 cells

Singlet oxygen was generated by energy transfer from the photoexcited sensitizer, Photofrin or 9-acetoxy-2,7,12,17-tetrakis-(beta-methoxyethyl)-porphycene (ATMPn), to molecular oxygen. Singlet oxygen was detected time-resolved by its luminescence at 1270 nm in an environment of increasing complexity, water (H2O), pure phosphatidylcholine, phosphatidylcholine in water (lipid suspensions), and aqueous suspensions of living cells. In the case of the lipid suspensions, the sensitizers accumulated in the lipids, whereas the localizations in the cells are the membranes containing phosphatidylcholine. By use of Photofrin, the measured luminescence decay times of singlet oxygen were 3.5 +/- 0.5 micros in water, 14 +/- 2 micros in lipid, 9 +/- 2 micros in aqueous suspensions of lipid droplets, and 10 +/- 3 micros in aqueous suspensions of human colonic cancer cells (HT29). The decay time in cell suspensions was much longer than in water and was comparable to the value in suspensions of phosphatidylcholine. That luminescence signal might be attributed to singlet oxygen decaying in the lipid areas of cellular membranes. The measured luminescence decay times of singlet oxygen excited by ATMPn in pure lipid and lipid suspensions were the same within the experimental error as for Photofrin. In contrast to experiments with Photofrin, the decay time in aqueous suspension of HT29 cells was 6 +/- 2 micros when using ATMPn.     

Induced and photoinduced DNA damage by quinolones: ciprofloxacin, ofloxacin and nalidixic acid determined by comet assay

Quinolones are degraded by light with loss of their antimicrobial activity, generating active species or radicals. Evidence exists that some fluoroquinolones (lomefloxacin, fleroxacin and enoxacin) induce damage to the cellular membrane and DNA cleavage by photosensitization. In this study, the genotoxic potential of the quinolones ofloxacin, nalidixic acid and ciprofloxacin (three antimicrobials frequently used in therapy) was evaluated upon irradiation with UV light by using the comet assay on cells of the Jurkat line. The results demonstrate that there are significant differences between the control groups (positive control with 50 microM H2O2, negative controls without drug and with and without irradiation) and the groups of irradiated quinolones (ofloxacin 2.76 x 10(-5) M, nalidixic acid 2.15 x 10(-4) M and ciprofloxacin 2.01 x 10(-5) M), indicating that, at the dose of irradiation employed (necessary to produce 50% photodegradation), the photodecomposition of the quinolones enhanced DNA damage. The unirradiated drugs also exhibited genotoxicity significantly different from that of the negative control.     

A kinetic study on the inhibitory action of sympathomimetic drugs towards photogenerated oxygen active species. The case of phenylephrine

Kinetics and mechanism of the aerobic Riboflavin (Rf, vitamin B2) sensitized photodegradation of Phenylephrine (Phen), a phenolamine belonging to the sympathomimetic drugs family, has been studied in water, employing continuous photolysis, polarographic detection of oxygen uptake, steady-state and time-resolved fluorescence spectroscopy, time-resolved IR-phosphorescence and laser flash photolysis. Results indicate the formation of a weak dark complex Rf-Phen, with an apparent association constant of 5.5+/-0.5M(-1), only detectable at Phen concentrations much higher than those employed in the photochemical experiments. Under irradiation, an intricate mechanism of competitive reactions operates. Phen quenches excited singlet and triplet states of Rf, with rate constants of 3.33+/-0.08 and 1.60+/-0.03x10(9)M(-1)s(-1), respectively. With the sympathomimetic drug in a concentration similar to that of dissolved molecular oxygen in water, Phen and oxygen competitively quench triplet excited Rf, generating superoxide radical anion and singlet molecular oxygen (O2((1)Deltag)) by processes initiated by electron- and energy-transfer mechanisms respectively. As a global result, the photodegradation of the vitamin, a known process taking place from its excited triplet state, is retarded, whereas the phenolamine, practically unreactive towards these oxidative species, behaves as a highly efficient physical deactivator of O2((1)Deltag). The phenolamine structure in Phen appears as an excellent scavenger of activated oxygen species, comparatively superior, in kinetic terms, to some commercial phenolic antioxidants.     

Dark and photoinduced interactions between Trolox, a polar-solvent-soluble model for vitamin E, and riboflavin

The aerobic riboflavin (Rf)-sensitized visible-light irradiation of Trolox (TX), a polar-solvent-soluble model for vitamin E, has been studied employing stationary photolysis, polarographic detection of oxygen uptake, stationary and time-resolved fluorescence spectroscopy, and laser flash photolysis. Results indicate that in methanolic solution, no dark complexation exists between Rf and TX. The latter quenches singlet and triplet states of Rf, with rate constants of 6.2 x 10(9) M(-1) s(-1) and 4.7 x 10(9) M(-1) s(-1), respectively. The photodecomposition of Rf, a known process taking place from triplet Rf, has been found to depend on the concentration of dissolved TX: at >/=30 mM very slight Rf photodecomposition occurs due to the massive quenching of excited singlet Rf, while at TX concentrations < or =1 mM triplet Rf is photogenerated and subsequently quenched either by oxygen, giving rise to O(2)((1)Delta(g)), or by TX, yielding semireduced Rf through an electron transfer process. Complementary experiments performed in pure water employing superoxide dismutase and sodium azide inhibition of the oxygen uptake, in coincidence with flash photolysis data, indicate that superoxide anion and singlet molecular oxygen are generated, likely by the reaction of the anion radical from Rf with dissolved oxygen, also yielding neutral, ground state Rf or by energy transfer from triplet Rf to ground-state oxygen, respectively. The final result is that both TX and Rf are photodegraded, likely through oxidation with activated oxygen species. In the absence of oxygen no degradation of TX can be detected, but Rf photodegradation is favoured because Rf regeneration is avoided.     

Kinetics Study on Reaction of Atenolol with Singlet Oxygen by Directly Monitoring the 1O2 Phosphorescence

The pharmaceutically active compound atenolol, a kind of $\beta$-blockers, may result in adverse effects both for human health and ecosystems if it is excreted to the surface water resources. To effectively remove atenolol in the environment, both direct and indirect photodegradation, driven by sunlight play an important role. Among indirect photodegradation, singlet oxygen (¹O₂), as a pivotal reactive species, is likely to determine the fates of atenolol. Nevertheless, the kinetic information on the reaction of atenolol with singlet oxygen has not been well investigated and the reaction rate constant is still ambiguous. Herein, the reaction rate constant of atenolol with singlet oxygen is investigated directly through observing the decay of the ¹O₂ phosphorescence at 1270 nm. It is determined that the reaction rate constant between atenolol and ¹O₂ is 7.0×10⁵ (mol/L)$^{-1}\cdot$s^-1 in D₂O, 8.0×10⁶ (mol/L)$^{-1}\cdot$s^-1 in acetonitrile, and 8.4×10⁵ (mol/L)$^{-1}\cdot$s^-1 in EtOH, respectively. Furthermore, the solvent effects on the title reaction were also investigated. It is revealed that the solvents with strong polarity and weak hydrogen donating ability are suitable to achieve high rate constant values. These kinetics information on the reaction of atenolol with singlet oxygen may provide fundamental knowledge to the indirect photodegradation of $\beta$-blockers.     

A novel chemiluminescent method for the determination of salicylic acid in bactericidal solutions

A new flow-injection procedure has been developed for the determination of salicylic acid based on the enhancement of the chemiluminescence from the cerium(IV)-Tween 20 reaction by salicylic acid in acidic medium. The method is simple, selective and sensitive with a detection limit of 2.5x10(-9) g mL(-1). It is applicable to the determination of salicylic acid in the concentration range of 4.0x10(-9)-1.1x10(-6) g mL(-1). The relative standard deviation (RSD) is 0.85% for 4.0x10(-7) g mL(-1) salicylic acid (n=11). The method has been successfully applied to the determination of salicylic acid in bactericidal solutions. Furthermore, it is suggested that light emission from cerium(IV)-Tween 20 reaction is probably because of the formation of singlet oxygen 1O2* and the emitter is excited oxygen molecular pairs O2(1delta(g))O2(1sigma(g)-).     

Cyclometalated iridium and platinum complexes as singlet oxygen photosensitizers: quantum yields, quenching rates and correlation with electronic structures

Photophysical properties are reported for a series of cyclometalated platinum and iridium complexes that can serve as photosensitizers for singlet oxygen. The complexes have the formula (C;N)(2)Ir(O;O) or (C;N)Pt(O;O) where C;N is a monoanionic cyclometalating ligand such as 2-(phenyl)pyridyl and 2-(phenyl)quinolyl, and O;O is the ancillary ligand acetylacetonate (acac) or dipivaloylmethane (dpm). Also examined were a series of (N;N)PtMe(2) complexes where N;N is a diimine such as 2,2'-bipyridyl. In general, the cyclometalated complexes are excellent photosensitizers for the production of singlet oxygen, while the (N;N)PtMe(2) complexes were ineffective at this reaction. Quantum yields of singlet oxygen production range from 0.9-1.0 for the cyclometalated Pt complexes and 0.5-0.9 for Ir complexes. Luminescence quenching and singlet oxygen formation of the Ir complexes occurs from a combination of electron and energy transfer processes, whereas the Pt complexes only react by energy transfer. For Ir complexes with low emission energy, physical deactivation of the triplet excited state becomes competitive with energy transfer to ground state dioxygen. The rates of singlet oxygen quenching for the complexes presented here are in the range 6 x 10(6)-2 x 10(7) M(-1) s(-1) for Pt complexes and 2 x 10(5)-2 x 10(7) M(-1) s(-1) for Ir complexes, respectively. Differences in the efficiency of both forming and quenching singlet oxygen between the Ir and Pt cyclometalates are believed to come about from the more exposed coordination geometry in the latter species.     

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.     

The role of A2E in prevention or enhancement of light damage in human retinal pigment epithelial cells

The process of sight (photostasis) produces, as a by-product, a chromophore called 2-[2,6-dimethyl-8-(2,6,6-trimethyl-1-cyclohexen-1-yl)-1E,3E, 5E,7E-octatetraenyl]-1-(2-hydroxyethyl)-4-[4-methyl-6-(2,6,6-trimethyl-1-cyclohexen-1-yl)-1E, 3E, 5E-hexatrienyl]-pyridinium (A2E), whose function in the eye has not been defined as yet. In youth and adulthood, A2E is removed from human retinal pigment epithelial (h-RPE) cells as it is made, and so it is present in very low concentrations, but with advanced age, it accumulates to concentrations reaching 20 microM. In the present study we have used photophysical techniques and in vitro cellular measurements to explore the role of A2E in h-RPE cells. We have found that A2E has both pro- and antioxidant properties. It generated singlet oxygen (phiso = 0.004) much less efficiently than its precursor trans-retinal (phiso = 0.24). It also quenched singlet oxygen at a rate (10(8) M(-1) s(-1)) equivalent to two other endogenous quenchers of reactive oxygen species in the eye: alpha-tocopherol (vitamin E) and ascorbic acid (vitamin C). The endogenous singlet oxygen quencher lutein, whose quenching rate is two orders of magnitude greater than that of A2E, completely prevented light damage in vitro, suggesting that singlet oxygen does indeed play a role in light-induced damage to aged human retinas. We have used multiphoton confocal microscopy and the comet assay to measure the toxic, phototoxic and protective capacity of A2E in h-RPE cells. At 1-5 microM, A2E protected these cells from UV-induced breaks in DNA; at 20 microM, A2E no longer exerted this protective effect. These results imply that the role of A2E is not simple and may change over the course of a lifetime. A2E itself may play a protective role in the young eye but a toxic role in older eyes.     

Singlet Oxygen-mediated Photobleaching of the Prosthetic Group in Hemoglobins and C-Phycocyanin

Proteins bearing colored prosthetic groups, such as the heme group in hemoglobin or the bilin group in c-phycocyanin, quench singlet oxygen by interactions at the apoprotein and the prosthetic group levels. In both proteins, chemical modification of the chromophore constitutes only a minor reaction pathway. While total deactivation of singlet oxygen takes place with rate constants of 4.0 x 10(9) and 4.2 x 10(8) M-1 s-1 for hemoglobin and phycocyanin, respectively, the bleaching of the chromophore takes place with rate constants of 3.2 x 10(6) and approximately 1 x 10(7) M-1 s-1. Irradiation of phycocyanin with red light bleaches the chromophore with low yields (approximately 0.8 x 10(-4)). Part of this bleaching is mediated by singlet oxygen produced by the irradiation of the bilin group. The low relevance of the singlet oxygen pathway is compatible with a low quantum yield (approximately 10(-3)) of free singlet oxygen production after irradiation of the protein.     

Calculation of singlet oxygen dose from photosensitizer fluorescence and photobleaching during mTHPC photodynamic therapy of MLL cells

Predicting the therapeutic outcome of photodynamic therapy (PDT) requires knowledge of the amount of cytoxic species generated. An implicit approach to assessing PDT efficacy has been proposed where changes in photosensitizer (PS) fluorescence during treatment are used to predict treatment outcome. To investigate this, in vitro experiments were performed in which Mat-LyLu cells were incubated in meta-tetra(hydroxyphenyl)chlorin (mTHPC) and then irradiated with 652 nm light. PS concentration, fluence rate and oxygenation were independently controlled and monitored during the treatment. Fluorescence of mTHPC was monitored during treatment and, at selected fluence levels, cell viability was determined using a colony-formation assay. Singlet oxygen dose was calculated using four different models and was compared with cell survival. For the dose metric based on singlet oxygen-mediated PS photobleaching, a universal relationship between cell survival and singlet oxygen dose was found for all treatment parameters. Analysis of the concentration dependence of bleaching suggests that the lifetime of singlet oxygen within the cell is 0.05-0.25 micros. Generation of about 9 x 10(8) molecules of singlet oxygen per cell reduces the surviving fraction by 1/e.     

The photophysics of monomeric bacteriochlorophylls c and d and their derivatives: properties of the triplet state and singlet oxygen photogeneration and quenching

Measurements of pigment triplet-triplet absorption, pigment phosphorescence and photosensitized singlet oxygen luminescence were carried out on solutions containing monomeric bacteriochlorophylls (Bchl) c and d, isolated from green photosynthetic bacteria, and their magnesium-free and farnesyl-free analogs. The energies of the pigment triplet states fell in the range 1.29-1.34 eV. The triplet lifetimes in aerobic solutions were 200-250 ns; they increased to 280 +/- 70 microseconds after nitrogen purging in liquid solutions and to 0.7-2.1 ms in a solid matrix at ambient or liquid nitrogen temperatures. Rate constants for quenching of the pigment triplet state by oxygen were (2.0-2.5) x 10(9) M-1 s-1, which is close to 1/9 of the rate constant for diffusion-controlled reactions. This quenching was accompanied by singlet oxygen formation. The quantum yields for the triplet state formation and singlet oxygen production were 55-75% in air-saturated solutions. Singlet oxygen quenching by ground-state pigment molecules was observed. Quenching was the most efficient for magnesium-containing pigments, kq = (0.31-1.2) x 10(9) M-1 s-1. It is caused mainly by a physical process of singlet oxygen (1O2) deactivation. Thus, Bchl c and d and their derivatives, as well as chlorophyll and Bchl a, combine a high efficiency of singlet oxygen production with the ability to protect photochemical and photobiological systems against damage by singlet oxygen.     

Kinetics and mechanism of the sensitized photodegradation of uracil--modeling the fate of related herbicides in aqueous environments

The dye-sensitized photodegradation of uracil (UR), the parent compound of several profusely employed herbicides, has been studied as a model of their environmental fate. In order to mimic conditions frequently found in nature, aqueous solutions of UR have been irradiated with visible light in the presence of the natural sensitizer riboflavin (Rf). The results indicate that UR is photostable in acid media, but is quickly degraded in pH 7 or pH 9 solutions, where singlet molecular oxygen [O2(1Delta(g))] and, to a lesser extent, superoxide radical anion (O2*-)-both species photogenerated from triplet excited Rf, 3Rf*-participate in the photodegradation. At pH 7, UR is slowly degraded through an O2*- -mediated mechanism, whereas Rf disappears through its reaction with O2(1Delta(g)) and, in the form of 3Rf*, with UR. On the contrary, at pH 9 Rf is photoprotected through two processes: its regeneration from the formed Rf radical species-a back electron transfer that also produces O2*- -and the elimination from the medium of O2(1Delta(g)) by its reaction with UR. The overall result of the preservation of ground state Rf is the continuity of the photosensitized process and, hence, of the UR degradation. Media with higher pH values could not be employed due to the fast photodegradation of Rf. With rose bengal (RB) as photosensitizer, the rate constants found for the overall interaction between UR and the photogenerated O2(1Delta(g)) were in the range 5 x 10(5) M(-1) s(-1) (at pH 7) to 1.3 x 10(8) M(-1) s(-1) (in 1 M NaOH aqueous solution, mainly physical quenching). The maximum O2(1Delta(g)0-mediated photooxidation efficiencies with RB were reached at pH 11, where only the O2(1Delta(g)0-reactive quenching with UR was observed.     

Singlet oxygen photogeneration at surface modified titanium dioxide

Substitution of surface -OH groups of TiO2 may influence competition between photoinduced energy and electron-transfer processes and lead to improved singlet oxygen generation. In contrast to neat TiO2, surface modified titanium dioxide can photocatalyze degradation of a very stable substance, cyanuric acid. Presented results are in contradiction to usually accepted hypothesis of the mechanism of cyanuric acid photodegradation in the presence of fluorinated TiO2 (F-TiO2) in which "bulk" hydroxyl radicals play a key role. It seems plausible that the difference of activity observed for TiO2 and F-TiO2 is not related to various types of generated hydroxyl radicals, but rather to competition between electron-transfer processes (formation of radicals) and energy-transfer processes (formation of singlet oxygen).     

Mechanisms of photoinitiated cleavage of DNA by 1,8-naphthalimide derivatives.

Using water-soluble 1,8-naphthalimide derivatives, the mechanisms of photosensitized DNA damage have been elucidated. Specifically, a comparison of rate constants for the photoinduced relaxation of supercoiled to circular DNA, as a function of dissolved halide, oxygen and naphthalimide concentration, has been carried out. The singlet excited states of the naphthalimide derivatives were quenched by chloride, bromide and iodide. In all cases the quenching products were naphthalimide triplet states, produced by induced intersystem crossing within the collision complex. Similarly, the halides were found to quench the triplet excited state of the 1,8-naphthalimide derivatives by an electron transfer mechanism. Bimolecular rate constants were < 10(5) M-1 s-1 for quenching by bromide and chloride. As expected from thermodynamic considerations quenching by iodide was 6.7 x 10(9) and 8.8 x 10(9) M-1 s-1 for the two 1,8-naphthalimide derivatives employed. At sufficiently high ground-state concentration self-quenching of the naphthalimide triplet excited state also occurs. The photosensitized conversion of supercoiled to circular DNA is fastest when self-quenching reactions are favored. The results suggest that, in the case of 1,8-naphthalimide derivatives, radicals derived from quenching of the triplet state by ground-state chromophores are more effective in cleaving DNA than reactive oxygen species or radicals derived from halogen atoms.     

Solvents effects in the photodegradation and reactivity of the various ionic forms of haematoporphyrin

The dependence of the ionic forms of haematoporphyrin(LX) dihydrochloride (HpdiCl) on solvent composition was investigated. In 2.8 x 10(-4) M solutions of HpdiCl in apolar (C6H6) and polar (CH3CN) solvents, HpdiCl exists in dicationic form. In hydrogen-bonding solvents, such as CH3OH, HpdiCl can exist in neutral, monocationic and dicationic forms. In C6H6-CH3OH solvent mixtures, the ionic forms in which HpdiCl is present depend on the composition of the solvent and on the acidity of the solution. The rate of oxidative photodegradation of HpdiCl excitation in its Q bands (WBI) and the ability to produce free radicals are different for the different ionic species. The highest values correspond to the dicationic form of HpdiCl and the lowest values correspond to the neutral species. In the absence of oxygen, the formation of free radicals due to the reaction of 3(Hp dication) is detected in the following solvent mixtures: CH3OH-toluene, CH3OH-ethylbenzene, CH3OH-hexane. The data obtained indicate that interaction of 3(Hp dication) with methine groups is an intermediate step in the formation of free radicals. In the HpdiCl concentration range studied, the presence of a phenolic antioxidant, such as beta-naphtol, inhibits the oxidative photodegradation of the dicationic form in a treated solution, but has little effect on the oxidative photobleaching of the monocation. The rate of oxidative photodegradation of the monocationic form increases with the addition of propionic acid to the solution.     

Merocyanine 540 Solubilized as an Ion Pair with Cationic Surfactant in Nonpolar Solvents: Spectral and Photochemical Properties

Merocyanine 540 (MC) is an anionic dye that is used to photopurge the bone marrow of leukemia cells. Under these conditions it is localized mostly in cell membranes, which may affect its photochemical reactivity. We investigated the photochemistry of MC dissolved as a hydrophobic ion pair with a hexadecyltrioctadecylammonium cation in cyclohexane, trimethylpentane and toluene as well as in propylene carbonate, CH3CN, C2H5OH and D2O. In organic solvents, the absorption and fluorescence spectra of MC were strongly red-shifted compared with aqueous solutions. The fluorescence was also more intense despite aggregation that occurred in some solvents. Aggregation strongly affects the spectral and photochemical properties of MC, especially in aliphatic hydrocarbons in which distinctive H-type aggregates are formed. Hydrophobic MC is a moderate photosensitizer of singlet molecular oxygen (1O2). The following values for 1O2 quantum yields were calculated based on 1O2 phosphorescence relative to 1O2 generation by Rose Bengal: approximately 0.12 in trimethylpenthane, approximately 0.13 in cyclohexane, 0.045 in EtOH, 0.039 in toluene, 0.007 in CH3CN and approximately 3 x 10(-4) in D2O. The H-aggregates of MC in cyclohexane and trimethylpentane are better 1O2 producers than monomeric MC. The above 1O2 quantum yields are corrected for self-quenching because MC is an efficient 1O2 quencher (17 x 10(7) M-1 s-1 in CH3CN, 6.8 x 10(7) M-1 s-1 in D2O, 5.2 x 10(7) M-1 s-1 in EtOH, and 1.4 x 10(7) M-1 s-1 in toluene). Merocyanine undergoes photodegradation, a solvent-dependent process that proceeds faster when the dye is aggregated. The initial photodegradation rate is much slower in organic solvents than in water, but photodegradation products accumulated during longer irradiation may increase the rate in most solvents. Higher photostability and better photosensitization by MC in hydrophobic nonpolar solvents suggest that the killing of leukemia cells via a photodynamic mechanism may operate mostly in cell membranes. In contrast, any cytotoxic products from photodecomposition may be important in hydrophilic cell compartments. Our data show the spectral and photochemical properties of MC in a pure hydrophobic environment.