Reactive Oxygen Species and Chromophoric Dissolved Organic Matter Drive the Aquatic Photochemical Pathways and Photoproducts of 6PPD-quinone under Simulated High-Latitude Conditions.

The photochemical degradation pathways of 6PPD-quinone (6PPDQ, 6PPD-Q), a toxic transformation product of the tire antiozonant 6PPD, were determined under simulated sunlight conditions typical of high-latitude surface waters. Direct photochemical degradation resulted in 6PPDQ half-lives ranging from 17.5 h at 20 °C to no observable degradation over 48 h at 4 °C. Sensitization of excited triplet-state pathways using Cs+ and Ar purging demonstrated that 6PPDQ does not decompose significantly from a triplet state relative to a singlet state. However, assessment of processes involving reactive oxygen species (ROS) quenchers and sensitizers indicated that singlet oxygen and hydroxyl radical do significantly contribute to the degradation of 6PPDQ. Investigation of these processes in natural lake waters indicated no difference in attenuation rates for direct photochemical processes at 20 °C. This suggests that direct photochemical degradation will dominate in warm waters, while indirect photochemical pathways will dominate in cold waters, involving ROS mediated by chromophoric dissolved organic matter (CDOM). Overall, the aquatic photodegradation rate of 6PPDQ will be strongly influenced by the compounding effects of environmental factors such as light screening and temperature on both direct and indirect photochemical processes. Transformation products were identified via UHPLC-Orbitrap mass spectrometry, revealing four major processes: (1) oxidation and cleavage of the quinone ring in the presence of ROS, (2) dealkylation, (3) rearrangement, and (4) deamination. These data indicate that 6PPDQ can photodegrade in cool, sunlit waters under the appropriate conditions: t1/2 = 17.4 h tono observable decrease (direct); t1/2 = 5.2-11.2 h (indirect, CDOM).

Light driven ultrafast electron transfer in oxidative redding of Green Fluorescent Proteins.

Fluorescent proteins undergoing green to red (G/R) photoconversion have proved to be potential tools for investigating dynamic processes in living cells and for photo-localization nanoscopy. However, the photochemical reaction during light induced G/R photoconversion of fluorescent proteins remains unclear. Here we report the direct observation of ultrafast time-resolved electron transfer (ET) during the photoexcitation of the fluorescent proteins EGFP and mEos2 in presence of electron acceptor, p-benzoquinone (BQ). Our results show that in the excited state, the neutral EGFP chromophore accepts electrons from an anionic electron donor, Glu222, and G/R photoconversion is facilitated by ET to nearby electron acceptors. By contrast, mEos2 fails to produce the red emitting state in the presence of BQ; ET depletes the excited state configuration en route to the red-emitting fluorophore. These results show that ultrafast ET plays a pivotal role in multiple photoconversion mechanisms and provide a method to modulate the G/R photoconversion process.

Photoprocesses of chloro-substituted p-benzoquinones.

The photochemistry of chloro-(ClBQ), dichloro-(2,5- and 2,6-Cl 2BQ), and trichloro-1,4-benzoquinone (Cl 3BQ) was studied in aqueous solution and/or in mixtures with acetonitrile. Final products are the corresponding hydroquinones (QH 2s) and 2-hydroxy-1,4-benzoquinones (QOHs). Three transients were detected by UV-vis absorption spectroscopy. The triplet state appears within the 20 ns 248 nm pulse and is converted within 0.1-1 micros into a photohydrate (HI aq). HI aq which is spectroscopically and kinetically separated from the triplet state decays within 5 ms, whereas the anion of the hydroxyquinone (QO (-)) grows in at ca. 500 nm in the 0.1-1 s time range. The proton formation and decay kinetics within 0.1-10 micros were observed by transient conductivity in the course of the reaction of the triplet state with water en route to HI aq at pH 4-9. Formation of QO (-) results in a permanent conductance. The efficient photoconversion of Cl n BQs at low concentrations (<0.2 mM) into QH 2s and HOQs is due to a redox reaction of Q with rearranged HI aq. The quantum yield of photoconversion at lambda irr = 254 nm is 0.8-1.2 for ClBQ or Cl 2BQs in aqueous acetonitrile and smaller (0.4) for Cl 3BQ. The yield of semiquinone radical ( (*)QH/Q (*-)) of Cl n BQs ( n = 1-4) in acetonitrile-water (1:1) is low (<20%) at low substrate concentration but is significantly increased upon addition of an H-atom donor, for example, 2-propanol. Other mechanisms involving (*)QH/Q (*-) radicals, such as quenching of the triplet state at enhanced Cl n BQ concentrations and H-atom abstraction from an organic solvent in mixtures with water, have also to be considered.

Isolation and structure elucidation of the major photodegradation products of seratrodast.

Photodegradation of seratrodast ([+/-]-7-(3,5,6-trimethyl-1,4-benzoquinone-2-yl)-7-phenyl-heptanoic acid, SD), a p-benzoquinone anti-asthmatic drug, has been investigated in different solutions. HPLC analysis showed that SD was degradated under UV irradiation at 254 nm to afford three major products (SD1, SD2 and SD3) in methanol and in acetonitrile/H2O solutions, while only two of them (SD2 and SD3) could be detected in acetonitrile solution. Furthermore, SD1 was unstable even when the samples were protected from light, so the acetylated SD1 (Ace-SD1) together with SD2 and SD3 were isolated by semi-preparative reversed-phase HPLC. The structures were elucidated as 6-(2,5-diacetoxy-3,4,6-trimethyl-phenyl)-7-phenyl-hept-6-enoic acid (Ace-SD1), 7-phenyl-6-(2,4,5-trimethyl-3,6-dioxo-cyclohexa-1,4-dienyl)-hept-6-enoic acid (SD2) and 5-(5-hydroxy-4,6,7-trimethyl-2-phenyl-benzofuran-3-yl)-pentanoic acid (SD3) based on the spectral data of MS, UV spectrum, IR spectrum and NMR spectrum. Meanwhile, a possible route for the formation of these photoproducts was proposed, which might explain the generation procedure of SD1, SD2 and SD3 during UV irradiation.

Near-UV photolysis of 2-methyl-1,4-naphthoquinone-DNA duplexes: characterization of reversible and stable interstrand cross-links between quinone and adenine moieties.

Near-UV photolysis of 2-methyl-1,4-naphthoquinone (MQ, menadione) tethered DNA induces initial charge transfer followed by either transport of the damage to G or the formation of interstrand cross-links between MQ and DNA bases. In this work, the products responsible for interstrand cross-links have been characterized by mass spectrometry, NMR, and comparison with model compounds. Three major products were formed in the photolysis of MQ-DNA duplexes. Two of the products (isomers) have a cross-link between C2 of a 2,3-saturated MQ moiety and N6 of a dAdo moiety. These products readily convert back to MQ and dAdo upon heating in neutral solution, and thus, they lead to reversible cross-links in MQ-DNA duplexes. The third product has a cross-link between C3 of a 2,3-unsaturated MQ moiety and N6 of an dAdo moiety. This product was stable in neutral solution. The formation of MQ to A cross-links in DNA may be explained by the coupling of MQ radicals that arise from the protonation of MQ radical anions, together with adenin-N6-yl radicals that arise from the deprotonation of A radical cations.

Photo-reduction of flavin mononucleotide to semiquinone form in LOV domain mutants of blue-light receptor phot from Chlamydomonas reinhardtii.

The photo-excitation dynamics of the mutants LOV1-C57S and LOV2-C250S of the LOV-domains of the phototropin photoreceptor phot from the green alga Chlamydomonas reinhardtii is investigated by absorption and fluorescence studies. The LOV domains fused to a maltose binding protein (MBP) are expressed in Escherichia coli. The mutants were studied under aerobic conditions in aqueous solution at pH 8. Blue-light exposure reduced the fully oxidized flavin mononucleotide, FMN(ox), to FMN semiquinone, FMNH*, (quantum efficiency around 1%) which further reduced to FMN hydroquinone, FMN(red)H(2) or FMN(red)H(-) (quantum efficiency ca. 3 x 10(-5)). In the dark both reduced forms recovered back to the oxidized form on a minute timescale. Besides photoreduction, blue-light photo-excitation of the mutants resulted in photoproduct formation (efficiency in the 2 x 10(-4) - 10(-3) range). Photo-reaction schemes for the mutants are discussed.

Photoreactions of p-benzo-, p-naphtho- and p-anthraquinones with ascorbic acid.

The photoreduction of 1,4-benzoquinone (BQ), 1,4-naphthoquinone (NQ), 9,10-anthraquinone (AQ) and several derivatives, e.g. dimethylBQ, trimethylBQ, duroquinone, bromoNQ, methoxyNQ, methylAQ and dimethylAQ in acetonitrile-water by ascorbate was studied by time-resolved UV-vis spectroscopy using 20 ns laser pulses at 308 nm and continuous 254 nm irradiation. The semiquinone radical (*QH/Q*(-)) is formed after H-atom transfer from ascorbate to the quinone triplet state. The rate constant for quenching is k(q)=(2-9) x 10(9) M(-1) s(-1). Termination of the radicals takes place in the micros-ms range. The results are compared with those initiated by electron transfer from DABCO under similar conditions, where the k(q) values are similar, but the termination of Q*(-) takes place by electron back transfer not yielding hydroquinones. Specific properties of the quinone triplet state, e.g. self-quenching, nucleophilic water addition and the effects of structure are discussed.

A photochemical method for patterning the immobilization of ligands and cells to self-assembled monolayers.

This work describes a chemically well defined method for patterning ligands to self-assembled monolayers (SAMs) of alkanethiolates on gold. This method begins with monolayers presenting a nitroveratryloxycarbonyl (NVOC)-protected hydroquinone which is photochemically irradiated to reveal a hydroquinone group. The resulting hydroquinone is then oxidized to the corresponding benzoquinone, providing a site for the Diels-Alder mediated immobilization of ligands. The rate constant for the photochemical deprotection is 0.032 s(-1) (with an intensity of approximately 100 mW/cm(2) between 355 and 375 nm), corresponding to a half-life of 21 s. The hydroquinone is oxidized to the benzoquinone using either electrochemical or chemical oxidation and then functionalized by reaction with a cyclopentadiene-tagged ligand. Two methods for patterning the immobilization of ligands are described. In the first, the substrate is illuminated through a mask to generate a pattern of hydroquinone groups, which are elaborated with ligands. In the second method, an optical microscope fit with a programmable translational stage is used to write patterns of deprotection which are then again elaborated with ligands. This technique is characterized by the use of well-defined chemical reactions to control the regions and densities of ligand immobilization and will be important for a range of applications that require patterned ligands for biospecific interactions.

Investigation of photoinduced electron transfer in model system of vitamin E-duroquinone by time-dependent density functional theory.

Photoinduced electron transfer of the model system composed of vitamin E and duroquinone has been investigated using time-dependent density functional theory. Calculations for the excited states tell that the photoexcitation of the model system can directly yield the charge transfer states in which the vitamin E moiety is positively charged but the duroquinone moiety is negatively charged. Our theoretical investigations indicate that the second charge transfer state of the model system can also be produced through the decay of higher locally excited state S(4). Since S(4) state in the model system corresponds to S(1) state of the isolated duroquinone used as a model for peroxyl radical, and S(2) state has the character of electron transfer from the tertiary amine group of the vitamin E moiety to the duroquinone moiety, the decay from S(4) to S(2) corresponds to the dynamic process following the photoexcitation of the duroquinone moiety of the model system, i.e., the initial stage of antioxidant reaction of vitamin E. Calculations of the kinetic parameters for the electron transfer have been carried out in the framework of the Marcus-Jortner-Levich formalism. Our calculations confirm that the electron transfer from S(4) to S(2) possesses the character of the inverted regime and the barrier is negligibly small.

Electron transport dynamics at the quinone acceptor site of bacterial photosynthetic reaction centers as probed using fast temperature changes.

Methods of laser-induced temperature jumps and fast freezing were used for testing the rates of thermoinduced conformational transitions of reaction center (RC) complexes in chromatophores and isolated RC preparations of various photosynthesizing purple bacteria. An electron transfer reaction from primary to secondary quinone acceptors was used as a probe of electron transport efficiency. The thermoinduced transition of the acceptor complex to the conformational state facilitating electron transfer to the secondary quinone acceptor was studied. To investigate the dynamics of spontaneous decay of the RC state induced by the thermal pulse, the thermal pulse was applied either before or during photoinduced activation of electron transport reactions in the RC acceptor complex. The maximum effect was observed if the thermal pulse was applied against the background of steady-state photoactivation of the RC. It was shown that neither the characteristic time of the thermoinduced transition within the temperature range 233-253 K nor the characteristic time of spontaneous decay of this state at 253 K exceeded several tens of milliseconds. Independent support of the estimates was obtained from experiments with varied cooling rates of the samples tested.

Near-IR irradiation of the S2 state of the water oxidizing complex of photosystem II at liquid helium temperatures produces the metalloradical intermediate attributed to S1Y(Z*).

Near-IR (NIR) excitation at liquid He temperatures of photosystem II (PSII) membranes from the cyanobacterium Synechococcus vulcanus or from spinach poised in the S2 state results in the production of a g = 2.035 EPR resonance, reminiscent of metalloradical signals. The signal is smaller in the spinach preparations, but it is significantly enhanced by the addition of exogenous quinones. Ethanol (2-3%, v/v) eliminates the ability to trap the signal. The g = 2.035 signal is identical to the one recently obtained by Nugent et al. by visible-light illumination of the S1 state, and preferably assigned to S1Y(Z*) [Nugent, J. H. A., Muhiuddin, I. P., and Evans, M. C. W. (2002) Biochemistry 41, 4117-4126]. The production of the g = 2.035 signal by liquid He temperature NIR excitation of the S2 state is paralleled by a significant reduction (typically 40-45% in S. vulcanus) of the S2 state multiline signal. This is in part due to the conversion of the Mn cluster to higher spin states, an effect documented by Boussac et al. [Boussac, A., Un, S., Horner, O., and Rutherford, A. W. (1998) Biochemistry 37, 4001-4007], and in part due to the conversion to the g = 2.035 configuration. Following the decay of the g = 2.035 signal at liquid helium temperatures (decay halftimes in the time range of a few to tens of minutes depending on the preparation), annealing at elevated temperatures (-80 degrees C) results in only partial restoration of the S2 state multiline signal. The full size of the signal can be restored by visible-light illumination at -80 degrees C, implying that during the near-IR excitation and subsequent storage at liquid helium temperatures recombination with Q(A-) (and therefore decay of the S2 state to the S1 state) occurred in a fraction of centers. In support of this conclusion, the g = 2.035 signal remains stable for several hours (at 11 K) in centers poised in the S2...Q(A) configuration before the NIR excitation. The extended stability of the signal under these conditions has allowed the measurement of the microwave power saturation and the temperature dependence in the temperature range of 3.8-11 K. The signal intensity follows Curie law temperature dependence, which suggests that it arises from a ground spin state, or a very low-lying excited spin state. The P1/2 (microwave power at half-saturation) value is 1.7 mW at 3.8 K and increases to 96 mW at 11 K. The large width of the g = 2.035 signal and its relatively fast relaxation support the assignment to a radical species in the proximity of the Mn cluster. The whole phenomenology of the g = 2.035 signal production is analogous to the effects of NIR excitation on the S3 state [Ioannidis, N., Nugent, J. H. A., and Petrouleas, V. (2002) Biochemistry 41, 9589-9600] producing an S2'Y(Z*) intermediate. In the present case, the intermediate is assigned to S1Y(Z*). The NIR-induced increase in the oxidative capability of the Mn cluster is discussed in relation to the photochemical properties of a Mn(III) ion that exists in both S2 and S3 states. The EPR properties of the S1Y(Z*) intermediate cannot be reconciled easily with our current understanding of the magnetic properties of the S1 state. It is suggested that oxidation of tyr Z alters the magnetic properties of the Mn cluster via exchange of a proton.

Reduction of 1,4-quinone and ubiquinones by hydrogen atom transfer under UVA irradiation.

1,4-Benzoquinone, coenzyme Q0 and Q10 were reacted with a series of hydrogen donors in the ESR cavity in the presence or absence of UVA irradiation. The signals of the radicals generated from the hydrogen donors or of those of the semiquinones were detected. The reaction mechanism was interpreted by a hydrogen atom transfer instead of the usual electron transfer mechanism on the basis of the redox potentials of the reactants and the Marcus theory. The hydrogen atom transfer is explained by the excited triplet state of quinones, which, on the basis of quantum mechanic calculations, may be reached even under visible light. In some cases, hydrogen atom transfer was also observed without irradiation, although to a lesser extent.

Photosensitization by anticancer agents--10. ortho-semiquinone and superoxide radicals produced during anthrapyrazole-sensitized oxidation of catechols.

Photosensitized oxidation of catechol, 3,4-dihydroxybenzoic acid (DHBA), 3,4-dihydroxy-dihydrocinnamic acid (DHCA), and 3,4-dihydroxy-phenylalanine (DOPA) by novel anticancer agents, anthrapyrazoles (AP), has been studied employing EPR and the spin trapping technique. The formation of o-semiquinone radicals, the one-electron oxidation products of the catechols, stabilized in the form of zinc ion complexes, has been demonstrated. Rate constants for the disproportionation of the semiquinone radical/Zn2+ complexes in (DMSO)/acetate buffer (pH 4.5, 1:1 vol/vol; 100 mM Zn2+) mixture have been determined to be 0.35 x 10(4), 14 x 10(4), 8.8 x 10(4) and 3 x 10(4) M-1 s-1 for catechol, DHBA, DHCA and DOPA respectively. The presence of oxygen enhanced rather than inhibited the photogeneration of the o-semiquinone radicals and facilitated their EPR detection. The EPR spectrum of the superoxide radical adduct with the spin trap 5,5-dimethyl-1-pyrroline-N-oxide was observed for the first time during photosensitized oxidation of the catechols in acidic aqueous solutions and in DMSO/acetate buffer mixture.