Photochemistry of phytoalexins containing phenalenone-like chromophores: photophysics and singlet oxygen photosensitizing properties of the plant oxoaporphine alkaloid oxoglaucine

Oxoglaucine (OG) is an oxoaporphine alkaloid, which has been linked to plant defense mechanisms. It contains a phenalenone (PN)-like chromophore, which suggests a role as singlet oxygen ((1)O(2)) photosensitizer. Indeed, OG is able to photosensitize (1)O(2) with 100% efficiency in nonpolar environments, similar to PN. However, this efficiency decreases in polar and protic media, although (1)O(2) is formed in all environments ranging from benzene to water. OG is a rather inefficient (1)O(2) quencher (k(q) = 8 x 10(5) M(-1) s(-1)) unlike the related alkaloids boldine and glaucine, for which an antioxidant role has been suggested. The results of this study contribute to the view that plant defense mediated by PN-like secondary metabolites may have a photochemical component.     

Applied biological and physicochemical activity of isoquinoline alkaloids: oxoisoaporphine and boldine

The aim of this study was to determine the electronic influence of substituent groups and annelated rings such as oxazole-oxazinone on the physicochemical and photoprotection, antioxidant capacity, toxicity and singlet oxygen photosensitization biological properties of isoquinoline alkaloid frameworks. Thus, oxoisoaporphine derivatives 1-5 and 3-azaoxoisoaporphine (6), some of them with phenolic structures, did not present any antioxidant capacity, possibly either by formation of keto-enol tautomerism species or the formation of unstable free radicals. Due to the singlet oxygen quantum yields (F_D) near to unity, and greater photostability than phenalenone, oxoisoaporphines 4-6 may be considered as photosensitizers for singlet oxygen production and can be used as new universal study tools. The biological application as antibacterial agents is an important and possible tool in the study of compounds with low cytotoxicity and high reactivity in antineoplastic chemotherapy. On the other hand, when boldine and its annelated derivatives B1-4 are irradiated, a photoprotector effect is observed (SPF = 2.35), even after 30 minutes of irradiation. They also act as photoprotectors in cell fibroblast cultures. No hemolysis was detected for boldine hydrochloride and its salts without irradiation. In solutions irradiated before incubation (at concentrations over 200 ppm) photoproducts were toxic to the nauplii of Artemia salina.     

Effect of borate buffer on the photolysis of riboflavin in aqueous solution

The photolysis of riboflavin (RF) in the presence of borate buffer (0.1-0.5M) at pH 8.0-10.5 has been studied using a specific multicomponent spectrophotometric method for the determination of RF and photoproducts, formylmethylflavin (FMF), lumichrome (LC) and lumiflavin (LF). The overall first-order rate constants for the photolysis of RF (1.55-4.36 x 10(-2)min(-1)) and the rate constants for the formation of FMF (1.16-3.52 x 10(-2)min(-1)) and LC (0.24-0.84 x 10(-2)min(-1)) have been determined. The values of all these rate constants decrease with an increase in buffer concentration suggesting the inhibition of photolysis reaction by borate species. The kinetic data support the formation of a RF-borate complex involving the ribityl side chain to cause the inhibition of photolysis. The second-order rate constants for the borate inhibited reaction range from 1.17-3.94 x 10(-2)M(-1)min(-1). The log k-pH profiles for the reaction at various buffer concentrations indicate a gradual increase in rate, with pH, up to 10 followed by a decrease in rate at pH 10.5 probably due to ionization of RF and quenching of fluorescence by borate species. A graph of second-order rate constants against pH is a sigmoid curve showing that the rate of photolysis increases with an increase in pH. The results suggest the involvement of excited singlet state, in addition to excited triplet state, in the formation of LC.     

Direct aqueous photochemistry of isoprene high-NO(x) secondary organic aerosol

Secondary organic aerosol (SOA) generated from the high-NO(x) photooxidation of isoprene was dissolved in water and irradiated with λ > 290 nm radiation to simulate direct photolytic processing of organics in atmospheric water droplets. High-resolution mass spectrometry was used to characterize the composition at four time intervals (0, 1, 2, and 4 h). Photolysis resulted in the decomposition of high molecular weight (MW) oligomers, reducing the average length of organics by 2 carbon units. The average molecular composition changed significantly after irradiation (C(12)H(19)O(9)N(0.08) + hν → C(10)H(16)O(8)N(0.40)). Approximately 65% by count of SOA molecules decomposed during photolysis, accompanied by the formation of new products. An average of 30% of the organic mass was modified after 4 h of direct photolysis. In contrast, only a small fraction of the mass (<2%), belonging primarily to organic nitrates, decomposed in the absence of irradiation by hydrolysis. Furthermore, the concentration of aromatic compounds increased significantly during photolysis. Approximately 10% (lower limit) of photodegraded compounds and 50% (upper limit) of the photoproducts contain nitrogen. Organic nitrates and multifunctional oligomers were identified as compounds degraded by photolysis. Low-MW 0N (compounds with 0 nitrogen atoms in their structure) and 2N compounds were the dominant photoproducts. Fragmentation experiments using tandem mass spectrometry (MS(n), n = 2-3) indicate that the 2N products are likely heterocyclic/aromatic and are tentatively identified as furoxans. Although the exact mechanism is unclear, these 2N heterocyclic compounds are produced by reactions between photochemically-formed aqueous NO(x) species and SOA organics.     

N-hydroxy-4-(4-chlorophenyl)thiazole-2(3H)-thione as a photochemical hydroxyl-radical source: photochemistry and oxidative damage of DNA (strand breaks) and 2'-deoxyguanosine (8-oxodG formation)

On irradiation of N-hydroxythiazole-2(3H)-thione 3 at 300 nm, the photoproducts disulfide 4, bisthiazole 5 and thiazole 6 are formed. During this photolysis, hydroxyl radicals are released, which have been detected by spin trapping with 5,5-dimethyl-1-pyrroline N-oxide (DMPO), coupled with electron paramagnetic resonance spectroscopy. In the presence of supercoiled pBR322 DNA, irradiation of thiazolethione 3 induces strand breaks through the photogenerated hydroxyl-radicals, as confirmed by control experiment with the hydroxyl-radical scavenger isopropanol. Singlet oxygen appears not to be involved, as attested by the lack of a D2O isotope effect. During the photoreaction of thiazolethione 3 in the presence of 2'-deoxyguanosine (dG), the latter is photooxidized (ca 10% conversion after 2 h of irradiation) to the 7,8-dihydro-8-oxo-2'-deoxyguanosine as the main oxidation product. The dG conversion levels off after complete consumption of thiazolethione 3 and is suppressed by the addition of the hydroxyl-radical scavenger 2,6-di-tert-butylcresol or DMPO. Since the photoproducts 4-6 are ineffective as sensitizers for the photooxidation of dG and DNA, the hydroxyl radicals released in the photolysis of thiazolethione 3 are the oxidizing species of DNA and dG. These results suggest that the thiazolethione 3 may serve as a novel and effective photochemical hydroxyl-radical source for photobiological studies.     

Photochemistry of neutral isonitrile gold(I) complexes: modulation of photoreactivity by aurophilicity and pi-acceptance ability

This work demonstrates for the first time that aurophilicity and ligand pi-acceptance ability sensitize the photoreactivity of Au(I) complexes. Photolysis of LAu(I)Cl (L = RNC or CO) complexes leads to free L, Au(III), and Au(0) photoproducts. Solutions of (p-tosyl)CH(2)NCAuCl in dichloromethane undergo significant oligomerization leading to dimers and trimers with formation constants of 1.61 x 10(3) and 6.61 x 10(3) M(-1), respectively, representing the highest values reported to date for complexes that exhibit aurophilic association in solution. The photoproduct quantum yield (Phi) varies with the LAu(I)Cl concentration in solution. For (p-tosyl)CH(2)NCAuCl, metallic gold forms with Phi = 0.0065 and 0.032 in 4.0 x 10(-5) and 4.0 x 10(-3) M dichloromethane solutions, respectively. Meanwhile, irradiation of t-BuNCAuCl primarily produces t-BuNCAuCl(3) with Phi = 0.0045 and 0.013 for 5.0 x 10(-5) and 5.0 x 10(-3) M dichloromethane solutions, respectively. For Au(CO)Cl, metallic gold forms with Phi = 0.013 and 0.065 upon irradiation of 8.0 x 10(-5) and 8.0 x 10(-3) M dichloromethane solutions, respectively. Hence, *[LAuX](n) oligomeric species are more photoreactive than monomeric species. The results also demonstrate intuitive control of Phi via modulation of the pi-acceptance ability of L, as both follow CO > (p-tosyl)CH(2)NC > (alkyl)NC in LAuCl, a trend that is also commensurate with the relative long-term photosensitivity of the corresponding solids and solutions. A new method for preparing stable small gold nanoparticles is described based on the fundamental findings above. Thus, photolysis of different concentrations of LAuX in solutions containing a primary amine-terminated dendrimer leads to clear solutions exhibiting tunable visible plasmon absorptions of gold nanoparticles; these solutions maintain their colors and stability indefinitely. TEM measurements for representative samples prepared by photolysis of (p-tosyl)CH(2)NCAuCl solutions give rise to spherical nanoparticles as small as 5 nm.     

A FACILE METHOD FOR THE SYNTHESIS OF GLAUCINE AND NORGLAUCINE FROM BOLDINE

ABSTRACT

A large-scale preparation of glaucine was achieved by reacting boldine with trimethylphenylammonium chloride and potassium t-butoxide in high yield. Treatment of glaucine with 30% H₂O₂, subsequent with hydrated ferrous sulfate, afforded norglaucine in an overall yield of 40%.     

On the photochemistry of IONO2: absorption cross section (240-370 nm) and photolysis product yields at 248 nm

The absolute absorption cross section of IONO(2) was measured by the pulsed photolysis at 193 nm of a NO(2)/CF(3)I mixture, followed by time-resolved Fourier transform spectroscopy in the near-UV. The resulting cross section at a temperature of 296 K over the wavelength range from 240 to 370 nm is given by log(10)(sigma(IONO(2))/cm(2) molecule(-1)) = 170.4 - 3.773 lambda + 2.965 x 10(-2)lambda(2)- 1.139 x 10(-4)lambda(3) + 2.144 x 10(-7)lambda(4)- 1.587 x 10(-10)lambda(5), where lambda is in nm; the cross section, with 2sigma uncertainty, ranges from (6.5 +/- 1.9) x 10(-18) cm(2) at 240 nm to (5 +/- 3) x 10(-19) cm(2) at 350 nm, and is significantly lower than a previous measurement [J. C. M?ssinger, D. M. Rowley and R. A. Cox, Atmos. Chem. Phys., 2002, 2, 227]. The photolysis quantum yields for IO and NO(3) production at 248 nm were measured using laser induced fluorescence of IO at 445 nm, and cavity ring-down spectroscopy of NO(3) at 662 nm, yielding phi(IO) 相似文献   

Cold-surface photochemistry of primary and tertiary alkyl nitrites

Reflection-absorption infrared spectroscopy (RAIRS) is used to explore the photochemistry of primary and tertiary alkyl nitrites deposited on a gold surface. The primary alkyl nitrites examined for this study were n-butyl, isobutyl, and isopentyl nitrite. These compounds showed qualitatively similar spectra to those observed in previous condensed-phase measurements. The photolysis of the primary nitrites involved the initial formation of an alkoxy radical and NO, followed by production of nitroxyl (HNO) and an aldehydic species. In addition, the formation of nitrous oxide, identified from its distinctive transition near 2230 cm(-1), was observed to form from the self-reaction of nitroxyl. The reaction rates for cis and trans conformer decay, as tracked through their intense N═O stretching modes, were found to be significantly different, potentially due to a structural bias that favors HNO formation for the initial trans conformer photoproducts over recombination. Tert-butyl nitrite demonstrates only the trans conformer in the RAIRS spectra prior to photolysis; however, recombination of the initial NO and RO(?) photoproducts was observed to produce the cis conformer in the photolyzed samples. The primary photoproducts from tert-butyl nitrite can also react to form acetone and nitrosomethane, but the absence of HNO prohibits the formation of N(2)O that was observed for the primary alkyl nitrites. Additionally, the RAIRS spectrum of isobutyl nitrite co-deposited with water was measured to examine the photolysis of this species on a water-ice surface. No change in the identity of the photoproducts was observed in this experiment, and minimal frequency shifting (1-3 cm(-1)) of the vibrational modes occurred. In addition to being a known atmospheric source of NO and various aldehydes, our results point to cold surface processing of alkyl nitrites as a potential environmental source of nitrous oxide.     

The atmospheric photolysis of E-2-hexenal, Z-3-hexenal and E,E-2,4-hexadienal

The atmospheric photolysis of E-2-hexenal, Z-3-hexenal and E,E-2,4-hexadienal has been investigated at the large outdoor European Photoreactor (EUPHORE) in Valencia, Spain. E-2-Hexenal and E,E-2,4-hexadienal were found to undergo rapid isomerization to produce Z-2-hexenal and a ketene-type compound (probably E-hexa-1,3-dien-1-one), respectively. Both isomerization processes were reversible with formation of the reactant slightly favoured. Values of j(E-2-hexenal)/j(NO(2)) = (1.80 +/- 0.18) x 10(-2) and j(E,E-2,4-hexadienal)/j(NO(2)) = (2.60 +/- 0.26) x 10(-2) were determined. The gas phase UV absorption cross-sections of E-2-hexenal and E,E-2,4-hexadienal were measured and used to derive effective quantum yields for photoisomerization of 0.36 +/- 0.04 for E-2-hexenal and 0.23 +/- 0.03 for E,E-2,4-hexadienal. Although photolysis appears to be an important atmospheric degradation pathway for E-2-hexenal and E,E-2,4-hexadienal, the reversible nature of the photolytic process means that gas phase reactions with OH and NO(3) radicals are ultimately responsible for the atmospheric removal of these compounds. Atmospheric photolysis of Z-3-hexenal produced CO, with a molar yield of 0.34 +/- 0.03, and 2-pentenal via a Norrish type I process. A value of j(Z-3-hexenal)/j(NO(2)) = (0.4 +/- 0.04) x 10(-2) was determined. The results suggest that photolysis is likely to be a minor atmospheric removal process for Z-3-hexenal.     

THE PHOTOCHEMISTRY OF 5-METHYLCYTOSINE AND 5-METHYL-2''-DEOXYCYTIDINE IN AQUEOUS SOLUTION

The nucleobase 5-methylcytosine (I) is a minor component of eukaryotic DNA thought to be important in regulation of gene expression. The photochemical reactions of this nucleobase and its 2'-deoxyribonucleoside, 5-methyl-2'-deoxycytidine (II), in water have been studied. These reactions lead, respectively, to 3-amino-2-methylacrylamidine (Ib) and 3-(2-erythro-D-pentopyranos-1-yl)amino-2-methylacrylamidine (IIb) as the main photoproducts. The structure of the photoproducts was established by spectroscopic methods (1H and 13C NMR, UV spectroscopy, electron impact and liquid secondary ion mass spectrometry); in the case of Ib, confirmatory evidence was obtained by chemical methods (photolysis of 5-methyl[2-13C]cytosine, hydrolysis of N-carbomethoxy-3-amino-2-methylacrylamidine and reaction of Ib with 1,1'-carbonyldiimidazole to give I). The quantum yield for formation of Ib was determined to be 1.8 x 10(-3) at pH 7.5 while the quantum yield for formation of IIb has a lower value of 0.2 x 10(-3) at pH 7.5. These quantum yields depend strongly on pH and reach maximum values of 2.0 x 10(-3) at pH 7.0 (Ib) and 0.6 x 10(-3) at pH 5.0 (IIb). The mechanism of formation of Ib (or IIb) is proposed to involve nucleophilic attack of water on the C-2 position of photoexcited I (or II), followed by ring opening and decarboxylation of an intermediate carbamic acid.     

Photohydroxylation of 1,4-benzoquinone in aqueous solution revisited

In water, photolysis of 1,4-benzoquinone, Q gives rise to equal amounts of 2-hydroxy-1,4-benzoquinone HOQ and hydroquinone QH(2) which are formed with a quantum yield of Phi=0.42, independent of pH and Q concentration. By contrast, the rate of decay of the triplet (lambda(max)=282 and approximately 410 nm) which is the precursor of these products increases nonlinearly (k=(2-->3.8) x 10(6) s(-1)) with increasing Q concentration ((0.2-->10) mM). The free-radical yield detected by laser flash photolysis after the decay of the triplet also increases with increasing Q concentration but follows a different functional form. These observations are explained by a rapid equilibrium of a monomeric triplet Q* and an exciplex Q(2)* (K=5500+/-1000 M(-1)). While Q* adds water and subsequent enolizes into 1,2,4-trihydroxybenzene Ph(OH)(3), Q(2)* decays by electron transfer and water addition yielding benzosemiquinone (.)QH and (.)OH adduct radicals (.)QOH. The latter enolizes to the 2-hydroxy-1,4-semiquinone radical (.)Q(OH)H within the time scale of the triplet decay and is subsequently rapidly (microsecond time scale) oxidized by Q to HOQ with the concomitant formation of (.)QH. On the post-millisecond time scale, that is, when (.)QH has decayed, Ph(OH)(3) is oxidized by Q yielding HOQ and QH(2) as followed by laser flash photolysis with diode array detection. The rate of this pH- and Q concentration-dependent reaction was independently determined by stopped-flow. This shows that there are two pathways to photohydroxylation; a free-radical pathway at high and a non-radical one at low Q concentration. In agreement with this, the yield of Ph(OH)(3) is most pronounced at low Q concentration. In the presence of phosphate buffer, Q* reacts with H(2)PO(4) (-) giving rise to an adduct which is subsequently oxidized by Q to 2-phosphato-1,4-benzoquinone QP. The current view that (.)OH is an intermediate in the photohydroxylation of Q has been overturned. This view had been based on the observation of the (.)OH adduct of DMPO when Q is photolyzed in the presence of this spin trap. It is now shown that Q*/Q(2)* oxidizes DMPO (k approximately 1 x 10(8) M(-1) s(-1)) to its radical cation which subsequently reacts with water. Q*/Q(2)* react with alcohols by H abstraction (rates in units of M(-1) s(-1)): methanol (4.2 x 10(7)), ethanol (6.7 x 10(7)), 2-propanol (13 x 10(7)) and tertiary butyl alcohol ( approximately 0.2 x 10(7)). DMSO (2.7 x 10(9)) and O(2) ( approximately 2 x 10(9)) act as physical quenchers.     

Probing lipid peroxidation by using linoleic acid and benzophenone

A thorough mechanistic study has been performed on the reaction between benzophenone (BZP) and a series of 1,4-dienes, including 1,4-cyclohexadiene (CHD), 1,4-dihydro-2-methylbenzoic acid (MBA), 1,4-dihydro-1,2-dimethylbenzoic acid (DMBA) and linoleic acid (LA). A combination of steady-state photolysis, laser flash photolysis (LFP), and photochemically induced dynamic nuclear polarization (photo-CIDNP) have been used. Irradiation of BZP and CHD led to a cross-coupled sensitizer-diene product, together with 6, 7, and 8. With MBA and DMBA as hydrogen donors, photoproducts arising from cross-coupling of sensitizer and diene radicals were found; compound 7 was also obtained, but 6 and o-toluic acid were only isolated in the irradiation of BZP with MBA. Triplet lifetimes were determined in the absence and in the presence of several diene concentrations. All three model compounds showed similar reactivity (k(q) ≈10(8) M(-1) s(-1)) towards triplet excited BZP. Partly reversible hydrogen abstraction of the allylic hydrogen atoms of CHD, MBA, and DMBA was also detected by photo-CIDNP on different timescales. Polarizations of the diamagnetic products were in full agreement with the results derived from LFP. Finally, LA also underwent partly reversible hydrogen abstraction during photoreaction with BZP. Subsequent hydrogen transfer between primary radicals led to conjugated derivatives of LA. The unpaired electron spin population in linoleyl radical (LA(.)) was predominantly found on H(1-5) protons. To date, LA-related radicals were only reported upon hydrogen transfer from highly substituted model compounds by steady-state EPR spectroscopy. Herein, we have experimentally established the formation of LA(.) and shown that it converts into two dominating conjugated isomers on the millisecond timescale. Such processes are at the basis of alterations of membrane structures caused by oxidative stress.     

Photochemical and photophysical studies of guanine derivatives: intermediates contributing to its photodestruction mechanism in aqueous solution and the participation of the electron adduct

The low-intensity steady-state (254 nm), microsecond flash and nanosecond (266 nm) laser photolysis of some guanine (Gua) derivatives in aqueous solution were studied. A photodestruction yield between 10(-3) and 10(-2) at a base concentration of 75 microM was determined for 254 nm irradiation at room temperature using high-performance liquid chromatography. This yield decreases with increasing purine concentration. For a similar concentration of the purine bases (2 +/- 1) x 10(-5) M, the yield increases as follows: Gua approximately 9-ethylguanine < deoxyguanosine approximately guanosine (Guo) < guanosine 5'-monophosphate. At concentrations higher than 2 x 10(-4) M the Gua derivatives' photodestruction yield seems to converge to a limiting value of the order of 10(-4). This behavior is explained in terms of self-quenching and aggregation effects which deactivate the excited states of the bases. The yields of electron photoejection have been determined in the nanosecond laser photolysis (0.083) and in the low-intensity steady-state (5.8 x 10(-3)) for Guo. Competition experiments using electron scavengers suggest that the electron adducts of the bases are one of the principal species participating in the photodestruction mechanism of these monomeric Gua. Close to 75% of the total destruction yield has contributions from initial reactions of the photojected electron at neutral pH. The quantum yield of photodestruction of Guo increases when the pH is increased as follows: 4.7 x 10(-3) (pH 1.1), 6.5 x 10(-3) (pH 2.9), 7.7 x 10(-3) (pH 7.5) and 8.1 x 10(-3) (pH 11.9). This dependence on pH and the electron scavenger experiments provide further evidence for the radical anion or its protonated form as one of the principal species involved in the photodestruction of the bases at the different pH. Under oxygen saturated conditions a 22% increase in the destruction yield is observed for Guo. However, for the dinucleotides adenylyl (3'-->5')-guanosine and thymidylyl (3'-->5')2'-deoxyguanosine, the participation of the electron is 41 and 36%, respectively, suggesting that going into a more DNA or RNA-like structure, the participation of the electron adducts species in the photodamage of DNA and RNA decreases. A mechanism of photodestruction for the Gua derivatives is proposed which takes into account these findings.     

Triplet-sensitized photolysis of the photoisomer of a 2,11-diaza[3,3](9,10)anthracenoparacyclophane: an adiabatic cycloreversion and a [2pia + 2pia + 2sigmas] rearrangement in a triplet state of the biplanophane system

The triplet-sensitized photoreactions of the title biplanophane system 6, the photoisomer of a 2,11-diaza[3,3](9,10)anthracenoparacyclophane derivative 5, were investigated by stationary and laser-flash photolyses using xanthone (XT) and benzophenone (BP) as triplet sensitizers. When photoisomer 6 underwent XT-sensitized irradiation, a triplet cyclophane 5 and a novel polycyclic product 7 were obtained via an adiabatic cycloreversion and a formal [2pia + 2pia + 2sigmas] rearrangement, respectively. The maximum quantum yield for the formation of cyclophane 5 (0.69) and the upper-limit efficiency for the formation of polycycle 7 (0.31) were determined by laser photolysis techniques. For BP-sensitized photolysis of photoisomer 6, oxetane 8, in addition to triplet cyclophane 5 and polycycle 7, was formed by a Paterno-Buchi reaction. The quenching rate constant (k(q)) of triplet BP by photoisomer 6 (3.4 x 10(8) dm(3) mol(-)(1) s(-)(1)) was found to be 1 order of magnitude smaller than that for XT (5.0 x 10(9) dm(3) mol(-)(1) s(-)(1)). On the basis of the relationship between k(q) and the triplet donor-acceptor energy gap, the triplet energy level of photoisomer 6 was estimated to be approximately 71 kcal mol(-)(1). The photochemical and the photophysical processes involved in the sensitized photolyses are summarized in an energetic reaction diagram and discussed in detail.     

THE USE OF SPECIFIC RADIOIMMUNOASSAYS TO DETERMINE ACTION SPECTRA FOR THE PHOTOLYSIS OF (6-4) PHOTOPRODUCTS

Abstract. A radioimmunoassay (RIA) was developed which specifically detects a photoproduct produced by the near-UV photolysis of pyrimidine(6-4)pyrimidone photoproducts. This assay was used in conjunction with a previously characterized RIA which specifically detects (6-4) photoproducts to determine the relative efficiency of wavelengths between 265 and 435 nm for photolysing these lesions. The rate of loss of antibody-binding sites associated with (6-4) photoproducts correlates with the production of those associated with its photolysis product. Action spectra for both the loss of (6-4) photoproducts and the induction of the photolysis product parallel the absorption spectrum of the (6-4) photoproduct.     

The primary photodynamics of aqueous nitrate: formation of peroxynitrite

We have examined the photochemical reactions occurring after irradiation at 200 nm of the aqueous nitrate ion, NO3(-)(aq). Using femtosecond transient absorption spectroscopy over the range 194-388 nm, we have characterized the formation and subsequent relaxation of the primary photoproducts of nitrate photolysis. The dominant photoproduct is the cis-isomer of peroxynitrite, which accounts for 48% of the excited state molecules initially produced. A slightly smaller fraction, 44%, of the excited molecules return to the electronic ground state of NO3(-) and relax to the vibrational ground state in 2 ps. The remaining 8% of the molecules initially excited react via the *NO + *O2(-) or the NO- + O2 dissociation channels. Formation of NO2(-) and *NO2 is not observed, suggesting that the previous observations of these species in steady-state photolysis are caused by reactions occurring on a longer time scale.     

Differences between protein dynamics of hemoglobin upon dissociation of oxygen and carbon monoxide

Protein dynamics of human adult hemoglobin (HbA) upon ligand photolysis of oxygen (O(2)) and carbon monoxide (CO) was investigated using time-resolved resonance Raman (TR(3)) spectroscopy. The TR(3) spectra of the both photoproducts at 1-ns delay differed from that of the equilibrium deligated form (deoxy form) in the frequencies of the iron-histidine stretching [ν(Fe-His)] and methine wagging (γ(7)) modes, and the band intensity of pyrrole stretching and substituent bending (ν(8)) modes. Spectral changes of the O(2) photoproduct in the submicrosecond region were faster than those of the CO photoproduct, indicating that the structural dynamics following the photodissociation is ligand dependent for HbA. In contrast, no ligand dependence of the dynamics was observed for myoglobin, which has a structure similar to that of the subunit of HbA. The structural dynamics and relevance to the functionality of HbA also are discussed.     

Substrate and Solvent Influence on the Photochemical C-H Bond Activation Reactivity of (HBPz'(3))Rh(CO)(2) (Pz' = 3,5-Dimethylpyrazolyl)

The photochemically-induced intermolecular C-H bond activation reaction of (HBPz'(3))Rh(CO)(2) (Pz' = 3,5-dimethylpyrazolyl) has been investigated in various hydrocarbon solutions at 293 K following excitation at 366 and 458 nm. UV-visible and FTIR spectra recorded throughout photolysis illustrate that the dicarbonyl complex can be converted readily to the corresponding (HBPz'(3))Rh(CO)(R)H derivatives at each of the excitation wavelengths. The photochemistry proceeds without interference from secondary photoprocesses or thermal reactions and the reactivity has been measured quantitatively with the determination of absolute quantum efficiencies for intermolecular C-H bond activation (phi(CH)). These measurements indicate that the C-H activation reaction proceeds very efficiently (phi(CH) = 0.13-0.32) on excitation at 366 nm but is much less effective (phi(CH) = 0.0059-0.011) on photolysis at 458 nm for each of the hydrocarbon substrates. The observed dependence of phi(CH) on irradiation wavelength is consistent with different reactivities from two rapidly dissociating low-energy ligand field (LF) excited states and the generation of monocarbonyl (HBPz'(3))Rh(CO) and ligand-dechelated (eta(2)-HBPz'(3))Rh(CO)(2) intermediates upon UV and visible excitation, respectively. The former species is attributed to be responsible for the unusually efficient C-H bond activation, whereas it is suggested that the latter complex effectively lowers the quantum efficiency by undergoing a facile eta(2)-->eta(3) ligand rechelation process. Significantly, the photoefficiencies are found to be unaffected on increasing the dissolved CO concentration, illustrating that the monocarbonyl reaction intermediate is extremely short-lived and is solvated before CO is able to coordinate. Additionally, the lack of a [CO] dependence on phi(CH) indicates that this solvated intermediate is not subject to a competitive back-reaction with CO prior to the C-H activation step, illustrating that the quantum efficiencies in (HBPz'(3))Rh(CO)(2) appear to be solely determined by the branching ratio between the dissociative and nondissociative routes. At any particular excitation wavelength the photoefficiencies are observed to be similar across the series of alkanes but are significantly reduced for the aromatic solvents, even though the aryl hydrido photoproducts are found to be more thermodynamically stable. These phi(CH) differences are also rationalized in terms of photophysical effects on the upper LF level and are related to variations in the nonradiative relaxation rates for the excited (HBPz'(3))Rh(CO)(2) complex in the hydrocarbon solutions.     

Unrepaired cyclobutane pyrimidine dimers do not prevent proliferation of UV-B-irradiated cultured human fibroblasts

Mutagenic and carcinogenic UV-B radiation is known to damage DNA mostly through the formation of bipyrimidine photoproducts, including cyclobutane dimers (CPD) and (6-4) photoproducts ((6-4) PP). Using high-performance liquid chromatography coupled to tandem mass spectrometry, we investigated the formation and repair of thymine-thymine (TT) and thymine-cytosine (TC) CPD and (6-4) PP in the DNA of cultured human dermal fibroblasts. A major observation was that the rate of repair of the photoproducts did not depend on the identity of the modified pyrimidines. In addition, removal of CPD was found to significantly decrease with increasing applied UV-B dose, whereas (6-4) PP were efficiently repaired within less than 24 h, irrespective of the dose. As a result, a relatively large amount of CPD remained in the genome 48 h after the irradiation. Because the overall applied doses (<500 J m(-2)) were chosen to induce moderate cytotoxicity, fibroblasts could recover their proliferation capacities after transitory cell cycle arrest, as shown by 5-bromo-2'-deoxyuridine (BrdUrd) incorporation and flow cytometry analysis. It could thus be concluded that UV-B-irradiated cultured primary human fibroblasts normally proliferate 48 h after irradiation despite the presence of high levels of CPD in their genome. These observations emphasize the role of CPD in the mutagenic effects of UV-B.