WAVELENGTH DEPENDENCE OF THE QUANTUM YIELD FOR THE STRUCTURAL ISOMERIZATION OF BILIRUBIN

The quantum yield and the relative photochemical yield for lumirubin formation from bilirubin bound to human albumin were determined at eight wavelengths from 410 to 520 nm. The quantum yield averaged 0.0015 for irradiation between 450 and 490 nm. At 410 and 430 nm the quantum yield was slightly lower. At the long wavelength end of the absorption band, from 500 to 520 nm, the quantum yield averaged 0.003. The relative photochemical yield, normalized to constant fluence, was greatest at 500 nm.     

二代光致变色液晶树枝状碳硅烷的光化学研究——端基含36个丁氧基偶氮苯介晶基元

研究了新的含36个丁氧基偶氮苯介晶基元的二代光致变色液晶树枝状碳硅烷(D2)在氯仿、四氢呋喃溶液中的光强,吸收光强,摩尔消光系数,最大吸收波长,量子产率,活化能,异构转换率,光回复异构化反应平衡常数,反-顺光异构化反应速率常数,光回复异构化正/逆和热回复异构化反应速率常数.     

INVESTIGATION OF PROTRIPTYLINE PHOTOPRODUCTS WHICH CAUSE CELL MEMBRANE DISRUPTION

Abstract— Protriptyline (PTL; N-methyl-5H-dibenzo[a,d]cycloheptene-5-propylamine) hydrochloride is a skin photosensitizing agent in humans. The fluorescence and photochemical behavior of PTL varies with the solvent. In water, 40% ethanol and ethanol in the hydrochloride salt of PTL has a fluorescence quantum yield of 0.81. The fluorescence quantum yield of PTL free base in selected organic solvents is between 0.41 and 0.17; in ethanol it remains at 0.81. Photolysis of PTL in acidic aqueous solution yields at least five photoproducts which were separated by high-performance liquid chromatography. Three of these photoproducts lysed red blood cells. One of the photoproducts has been identified as a cyclobutyl photodimer of PTL based on its mass spectrum and UV absorption and its ability to undergo photore-versal with 254 nm irradiation. The others were not cyclobutyl dimers. The yield of lytic products decreased as the ethanol content was increased and were not formed from PTL free base in any solvent.     

Base pairing enhances fluorescence and favors cyclobutane dimer formation induced upon absorption of UVA radiation by DNA

The photochemical properties of the DNA duplex (dA)(20)·(dT)(20) are compared with those of the parent single strands. It is shown that base pairing increases the probability of absorbing UVA photons, probably due to the formation of charge-transfer states. UVA excitation induces fluorescence peaking at ～420 nm and decaying on the nanosecond time scale. The fluorescence quantum yield, the fluorescence lifetime, and the quantum yield for cyclobutane dimer formation increase upon base pairing. Such behavior contrasts with that of the UVC-induced processes.     

Determination of photoredox properties of individual kinetically labile complexes in equilibrium systems

The paper evaluates the possibilities and limitations in characterising photochemical properties of individual kinetically labile species being in mutual equilibrium. A model of calculating quantum yield values is elaborated and applied to azidokojic iron(III) complexes in aqueous solutions. Based on the known speciation of the complexes as a function of pH, Fe(III) and azidokojic acid concentrations, electronic absorption spectra of individual species, and the determined overall quantum yield of Fe(II) photoformation at irradiation with monochromatised radiation at 366 nm, the quantum yield of Fe(III) to Fe(II) photoreduction was obtained for each of the complexes present in the investigated systems.     

Photochemistry of 4-(2-nitrophenyl)-1,4-dihydropyridines. Evidence for electron transfer and formation of an intermediate

New evidence about the path followed in the photochemical reaction of 4-(2-nitrophenyl)-1,4-dihydropyridines such as the drugs nifedipine (Compound 1) and nisoldipine (Compound 2) to give the corresponding nitrosophenylpyridines has been found through determination of the steady-state photochemical parameters and a comparison of the photoreactions in solution and in matrix at 90 K. Additional support is given by comparison with the isomeric 4-(3-nitrophenyl)dihydropyridine as well as with simpler derivatives, such as the corresponding 4-methyldihydropyridine. In Compounds 1 and 2, the lowest lying singlet, localized on the dihydropyridine chromophore, is deactivated by (largely exothermic) electron transfer to the nitrobenzene moiety, as evidenced by the complete quenching of the blue fluorescence observed in analogues not containing the electron-accepting group. Intramolecular proton transfer ensues in the 2-nitrophenyl derivatives with a relatively medium-independent quantum yield of approximately 0.3 and leads to an aromatic zwitterion, which is detected in matrix at 90 K (photoionization of this intermediate takes place in 2-methyltetrahydrofuran secondary). The intermediate is smoothly converted into the end product upon melting the glass. The 3-nitrophenyl analog, for which such a path is not available, is less reactive by about three orders of magnitude at 366 nm, although the quantum yield arrives at approximately 0.01 by irradiation at 254 nm in MeOH, reasonably via the nitrophenyl localized triplet.     

Nitrate radical quantum yield from peroxyacetyl nitrate photolysis

Peroxyacetyl nitrate (PAN, CH3C(O)OONO2) is a ubiquitous pollutant that is primarily destroyed by either thermal or photochemical mechanisms. We have investigated the photochemical destruction of PAN using a combination of laser pulsed photolysis and cavity ring-down spectroscopic detection of the NO3 photoproduct. We find that the nitrate radical quantum yield from the 289 nm photolysis of PAN is Phi(NO3)PAN = 0.31 +/- 0.08 (+/-2 sigma). The quantum yield is determined relative to that of dinitrogen pentoxide, which is assumed to be unity, under identical experimental conditions. The instrument design and experimental procedure are discussed as well as auxiliary experiments performed to further characterize the performance of the optical cavity and photolysis system.     

Application of a "black body" like reactor for measurements of quantum yields of photochemical reactions in heterogeneous systems

We report for the first time an experimental application of the concept of a "black body" like reactor to measure quantum yields (Phi) of photochemical reactions in liquid-solid heterogeneous systems. A major advantage of this new method is its simplicity since the fractions of reflected and transmitted light are negligible due to reactor geometry and high optical density of the heterogeneous systems. The average quantum yield of a test reaction (phenol photodegradation) over TiO(2) (Degussa P25) as determined by this method was 0.14, identical to the quantum yield measured earlier for this same reaction under similar conditions by Salinaro and Serpone. We also report the quantum yield of phenol photodegradation over N-doped TiO(2) during photoexcitation at the fundamental absorption band (lambda = 365 nm; Phi = 0.12) and at the N-doping induced extrinsic absorption band (lambda = 436 nm; Phi = 0.08) of the photocatalyst.     

端基含4个丁氧基偶氮苯介晶基元光致变色液晶树状物的光化学研究

研究了新化合物含4个丁氧基偶氮苯介晶基元的零代(D0)光致变色液晶树状物在氯仿和四氢呋喃中的吸收光强、最大吸收波长、摩尔消光系数、量子产率、活化能、异构转换率、反-顺光异构化反应速率常数、热回复异构化反应速率常数、光回复异构化反应平衡常数及速率常数．D0的光致变色反应速率常数为10^-1s^-1,而含偶氮基元液晶聚硅氧烷的光致变色反应速率常数为10^-8s^-1,因此液晶树状物D0的光响应速度比后者快10^7倍．     

Flash photolysis and inactivation of aqueous lysozyme

Abstract— –Flash photolysis of aqueous lysozyme has shown that the initial photochemical products are photo-oxidized tryptophan residues (Λ_max= 500 nm), hydrated electrons (Λ_max= 720 nm), and the cystine residue electron adduct (Λ_max= 420 nm). Comparisons with mixtures of the chromophoric amino acids show that 1 to 2 tryptophan residues provide electrons at a quantum yield of 0.018 (25 per cent). Part of the ejected electrons are captured by cystine residues via a short-range, intramolecular process with essentially unit efficiency. The remainder become hydrated and back react with oxidized tryptophan residues before 10^-4sec. The cystine residue electron adduct decays with 2 msec halftime (25°C) and 1.5 kcal/mole activation energy. The surviving oxidized tryptophan residues decay with a comparable time constant in a hydroxyl ion catalyzed process. In acid solutions the oxidized tryptophan residue and long-lived H atom adduct are observed (Λ_max= 380 nm). The quantum yield of lysozyme inactivation induced by xenon flash irradiation above 250 nm is 0.023 (20 per cent), which is not sensitive to oxygen or pH. Comparison to the primary photochemical reactions indicates that electron ejection from the essential tryptophan residues inactivates the enzyme, irrespective of the electron trap and subsequent reactions. On the basis of the structure and supporting information it is proposed that the tryptophan residues of the active site are involved. Direct disruption of cystine residues does not contribute more than 10 per cent to the inactivation quantum yield in this wavelength region. Lysozyme inactivation may differ from other enzymes because the chromophores include essential residues located in the active center.     

铽-氟罗沙星配合物光化学荧光增强研究

铽 氟罗沙星 (FLRX)配合物在 36 5nm紫外光照射一定时间后 ,Tb3 的特征荧光强度大大提高。通过对该体系的荧光光谱、磷光光谱、荧光量子效率和荧光寿命等的测定 ,证实Tb3 FLRX配合物光照后发生了光化学反应 ,形成了更有利于分子内能量传递的Tb3 配合物。探讨了其荧光增敏机制。     

Rethinking Uncaging: A New Antiaromatic Photocage Driven by a Gain of Resonance Energy

Photoactivatable compounds for example photoswitches or photolabile protecting groups (PPGs, photocages) for spatiotemporal light control, play a crucial role in different areas of research. For each application, parameters such as the absorption spectrum, solubility in the respective media and/or photochemical quantum yields for several competing processes need to be optimized. The design of new photochemical tools therefore remains an important task. In this study, we exploited the concept of excited-state-aromaticity, first described by N. Colin Baird in 1971, to investigate a new class of photocages, based on cyclic, ground-state-antiaromatic systems. Several thio- and nitrogen-functionalized compounds were synthesized, photochemically characterized and further optimized, supported by quantum chemical calculations. After choosing the optimal scaffold, which shows an excellent uncaging quantum yield of 28 %, we achieved a bathochromic shift of over 100 nm, resulting in a robust, well accessible, visible light absorbing, compact new photocage with a clean photoreaction and a high quantum product (ϵ⋅Φ) of 893 M⁻¹ cm⁻¹ at 405 nm.     

QUANTUM YIELDS FOR THE CYCLIZATION AND CONFIGURATIONAL ISOMERIZATION OF 4E, 15Z-BILIRUBIN

Extraction of a solution of bilirubin configurational isomers in chloroform with an aqueous solution of human serum albumin was found to remove selectively the 4Z,15E-isomer. This phenomenon was used to develop a method for the purification of the 4E,15Z-isomer of bilirubin. The quantum yield for the cyclization and configurational isomerization of the 4E,15Z-isomer bound to a molar excess of human serum albumin was measured at 450 and 510 nm. The quantum yield for cyclization to form lumirubin was 0.12 and 0.19 at 450 and 510 nm, respectively. The quantum yield for configurational isomerization to form 4Z,15Z-bilirubin was 0.03 and 0.05 at 450 and 510 nm. An analysis of previously published data on the quantum yield for the formation of lumirubin from 4Z, 15Z-bilirubin bound to human serum albumin suggests that all of the formation of lumirubin may occur via consecutive photochemical processes with the 4E,15Z-isomer as an intermediate.     

Electron transfer mechanism and photochemistry of ferrioxalate induced by excitation in the charge transfer band

The photoredox reaction of ferrioxalate after 266/267 nm excitation in the charge transfer band has been studied by means of ultrafast extended X-ray absorption fine structure (EXAFS) analysis, optical transient spectroscopy, and quantum chemistry calculations. The Fe-O bond length changes combined with the transient spectra and kinetics have been measured and in combination with ultrahigh frequency density functional theory (UHF/DFT) calculations are used to determine the photochemical mechanism for the Fe(III) to Fe(II) redox reaction. The present data and the results obtained with 266/267 nm excitations strongly suggest that the primary reaction is the dissociation of the Fe-O bond before intramolecular electron transfer occurs. Low quantum yield electron photodetachment from ferrioxalate has also been observed.     

A CALORIMETRIC DETERMINATION OF THE QUANTUM YIELD FOR THE JONIZATION OF MALACHITE GREEN CYANIDE BY ULTRAVIOLET RADIATION

Abstract The quantum yield for the conversion of malachite green cyanide (MGL) to the oxidized form MG⁺ has been measured over the wavelength range from 225 to 289 nm using a total absorption aluminium calorimeter to measure the flux of photons. The number of molecules of MGL converted was determined from the increase in absorbance of the solution at 622 nm. MG⁺ was found to have a maximum extinction coefficient of 10.63 × 10⁴ at 622 nm. The quantum yield for the conversion of MGL to MG⁺ is constant over the wavelength range with a value of 0.91 ± 0.01. The use of MGL as an actinometer for photochemical studies is described.     

三苯基硫鎓盐衍生物激光闪光光解的研究

The photochemical and photophysical behaviors of tri(4-tert-butoxycarbonyl oxyphenyl) sulphonium salts have been investigated. In argon-satureted acetonitrile, the quantum yields of Bronsted acid formed during photolysis of these compounds was abount 0.5. In the transient absorption spectrum excited by 266nm in mathanol and dioxane an evident absorption peak at 360 nm decayed in accordance with pseudo-first-order reaction was observed. In the presence of poly-p-hydroxystyrene or diphenyl sulphide, the apparent second-order reaction decay rate constsnts were 107 and 108 L﹒mo-1﹒s-1, respectively. Results indicated that the transient absorption peak at 360nm was attributed to the diphenyl sulphide radical cations formed in photolysis of sulphonium salts, which were proposed by abstraction of hydrogen from solvent or polymer to yield the photonic acid and diphenyl sulphide as listed in eqns. (1) and (2). The influence of non-nuncleophilic anions of title compounds on their photochemical behavior was just less important.     

Photochemical method of silver deposition on a quartz surface modified by polybutoxytitanium

The results of silver photochemical deposition on a surface of titanium-containing coatings are presented. The formation of the particles occurs in a near-surface layer with a direct involvement of the coating which gains an anatase structure under the action of UV light. The deposition of silver begins with the formation of low-atomic clusters which reach particle sizes of 300 nm in diameter and 35–40 nm in thickness under long irradiation. The quantum yield of the reduction of Ag(I) ions is 0.001.     

The photochemical characteristics of aromatic enediyne compounds substituted with electron donating and electron withdrawing groups

trans- and cis-1-(4-Dimethylaminophenyl)-6-(4-nitrophenyl)hex-3-ene-1,5-diynes (trans- and cis-DANE) were synthesized and their photochemical properties were studied. The absorption spectra of trans-DANE red-shifted compared with the parent compound bisphenylethynylethene (BEE) due to intramolecular charge transfer. The fluorescence spectra, Stokes shift, fluorescence lifetime, fluorescence quantum yield, and quantum yield of trans-to-cis photoisomerization of trans-DANE showed strong dependence upon the solvent polarity in the less-polar region. No fluorescence emission from trans-DANE was observed in medium-polar and polar solvents. The quantum yield of cis-to-trans isomerization was almost solvent independent. The donor-acceptor substituents shifted the equilibrium between the trans perpendicular triplet state and the trans planar triplet state to the trans triplet state, and resulted in an increase in the triplet lifetime. Comparison of the photochemical properties of trans-DANE with trans-4-dimethylamino-4'-nitrostilbene (DANS) suggests that trans-DANE is a possible fluorescent probe in the non-polar region.     

LASER FLASH PHOTOLYSIS AND PHOTOINACTIVATION OF SUBTILISIN BPN'

Abstract— Laser flash photolysis of subtilisin BPN'at 265 nm has shown that photoionization of tryptophanyl (Trp) and tyrosinyl (Tyr) residues are the principal initial photochemical reactions. The initial products are the corresponding oxidized radicals. Trp and Tyr, and hydrated electrons (e_aq) which react with the enzyme at: k (e_aq+ subt. BPN') = 2.1 × 10¹⁰ M⁻¹ s⁻¹. The photoionization quantum yield was 0.032 ± 0.005 at 265 nm, which was enhanced 3.5-fold by simultaneous excitation at 265 and 530 nm. The photoionization yields were unchanged by 3 M bromide ion and 8 M urea. which did affect the enzyme fluorescence excited at 265 and 295 nm. A similar lack of correlation between the effects of perturbants on the photionization yields and fluorescence yields was found for subtilisin Carlsherg. The results indicate that the monophotonic and biphotonic ionization of the Trp residues does not involve the thermally-equilibrated. lowest excited singlet state and that singlet energy transfer from Tyr to Trp does not contribute to Trp photoionization. The photoinactivation quantum yield was 0.014 for 265 nm laser excitation. which was not changed by simultaneous 530 nm excitation. The corresponding quantum yield was 0.009 for low intensity 254 nm radiation, indicative of a biphotonic contribution to photoinactivation. The results are explained by postulating that photolysis of Trp-113 leads to disruption of hydrogen bonding to Asn-117 and a shift in the primary chain sequence associated with the aromatic substrate binding sites. The photoionization quantum yields in subtilisin BPN'and subtilisin Carlsberg agree with a model based on the assumption that exposed Trp and Tyr residues contribute independently at intrinsic photoionization efficiencies characteristic of the chromophores.     

Interactive effects of ultraviolet radiation and nutrient addition on growth and photosynthesis performance of four species of marine phytoplankton

Experiments (6-8 days) were carried out during the austral summer of 2005 in Chubut, Argentina (43 degrees S, 65 degrees W) to determine the interactive effects of solar UVR (280-400 nm) and nutrient addition on growth and chlorophyll fluorescence of four species of marine phytoplankton--the diatoms Thalassiosira fluviatilis Hustedt and Chaetoceros gracilis Schütt, and the dinoflagellates Heterocapsa triquetra (Ehrenberg) Stein and Prorocentrum micans (Ehrenberg). Samples were incubated under three radiation treatments (two sets of each radiation treatment): (a) samples exposed to full solar radiation (PAR+UVR, PAB treatment, 280-700 nm); (b) samples exposed to PAR and UV-A (PA treatment, 320-700 nm) and (c) samples exposed only to PAR (P treatment, 400-700 nm). At the beginning of the experiments, nutrients (i.e., NaPO(4)H(2) and NaNO(3)) were added to one set of samples from each radiation treatment ("N" cultures) whereas in the other, the nutrients concentration was that of the culture medium. At all times, the lowest growth rates (mu) were determined in the PAB treatments, where enriched cultures had significantly higher mu (P<0.05) than non-enriched cultures. Daily cycles of photochemical quantum yield (Y) displayed a pattern of relatively high values early in the morning with a sharp decrease at noon; recovery was observed late in the afternoon. In general, higher Y values were determined in enriched cultures than in non-enriched cultures. As the experiments progressed, acclimation (estimated as the difference between Y at noon and that at time zero) was observed in all species although in variable degree. All species displayed some degree of UVR-induced decrease in the photochemical quantum yield, although it was variable among treatments and species. However, this effect decreased with time, and this pattern was more evident in the dinoflagellates, as the concentration of UV-absorbing compounds increased. Thus, under conditions of nutrient enrichment as may occur by river input or by re-suspension by mixing, dinoflagellates outcompete with diatoms because they may have a higher fitness under UVR stress.