Photophysical and photochemical processes of riboflavin (vitamin B2) by means of the transient absorption spectra in aqueous solution

Using time-resolved techniques of 337 and 248 nm laser flash photolysis, the photo physical and photochemical processes of riboflavin (RF, vitamin B₂) were studied in detail in aqueous solution. The excited triplet state of riboflavin (³RF^*) was produced with 337 nm laser, while under 248 nm irradiation, both³RF^* and hydrated electron (e_aq) formed from photoionization could be detected. Photobiological implications have been inferred on the basis of reactivity of³RF^* including energy transfer, electron transfer and hydrogen abstraction. The RF^·+ was generated by oxidation of SO₄ ^·-radical with the aim of confirming the results of photolysis.     

Photophysical and photochemical processes of riboflavin (vitamin B2) by means of the transient absorption spectra in aqueous solution

Using time-resolved techniques of 337 and 248 nm laser flash photolysis, the photo physical and photochemical processes of riboflavin (RF, vitamin B₂) were studied in detail in aqueous solution. The excited triplet state of riboflavin (³RF^*) was produced with 337 nm laser, while under 248 nm irradiation, both³RF^* and hydrated electron (e_aq) formed from photoionization could be detected. Photobiological implications have been inferred on the basis of reactivity of³RF^* including energy transfer, electron transfer and hydrogen abstraction. The RF^·+ was generated by oxidation of SO₄ ^·-radical with the aim of confirming the results of photolysis.     

亚油酸与核黄素或FAD激发态之间电荷转移的直接证据

根据荧光显微镜方法,我们首次发现核黄素(维生素B2)主要分布在细胞核的膜上和核的内部,故核黄素光敏化与辐射化的靶位置主要集中在细胞核内;当核黄素的浓度较大时,细胞膜上也有药物的分布,即在高浓度时,细胞膜也是光敏化与辐射敏化的作用位点一。应用308nn激光光解时间分辨吸收方法,以亚油酸作为脂质的模型化合物,研究了亚油酸与核黄素和黄素腺嘌呤二核苷酸(FAD)的激发三重态之间的电荷转移过程,首次给出了电荷转移的直接证据。     

Characterization of transient species in laser photolysis of aromatic amino acids using acetone as photosensitizer

The photochemical processes of aromatic amino acids were investigated in aqueous solution using acetone as photosensitizer by KrF (248 nm) laser flash photolysis. Laser-induced transient species were characterized according to kinetic analysis and quenching experiments. The intermediates recorded were assigned to the excited triplet state of tryptophan, the radicals of tryptophan and tyrosine. The excited triplet state of tryptophan produced via a triplet-triplet excitation transfer and the radicals arising from electron transfer reaction has been identified. Neither electron transfer nor energy transfer between triplet acetone and phenylalanine can occur in photolysis of phenylalanine aqueous solution which contains acetone. Furthermore, triplet acetone-induced radical transformation: Trp/N-Tyr→Trp-Tyr/O was observed directly in photolysis of dipeptide (Trp-Tyr) aqueous solution containing acetone, and the transformation resulting from intramolecular electron transfer was suggested.     

Transient species and its properties of melatonin

Reactive oxygen species (ROS) are generated dur- ing radiation, respiratory burst, normal metabolic processes and so on. There are enzymatic and non-enzymatic antioxidants such as superoxide dis- mutase (SOD), glutathione peroxidase (GSH-Px), vi- tamin E (VE) and carotenoids that can either inhibit or repair the ROS-induced damage. ROS is essential to maintain physiological homeostasis. However, exces- sive ROS give rise to oxidative damage to proteins, lipids and DNA which related t…     

Study of the interaction between triplet riboflavin and the alpha-, betaH- and betaL-crystallins of the eye lens

Time-resolved photolysis studies of riboflavin (RF) were carried out in the presence and absence of alpha-, betaH- and betaL-crystallins of bovine eye lens. The transient absorption spectra, recorded 5 micros after the laser pulse, reveal the presence of the absorption band (625-675 nm) of the RF neutral triplet state (tau = 42 micros) accompanied by the appearance of a long-lived absorption (tau = 320 micros) in the 500-600 nm region due to the formation of the semireduced RF radical. The RF excited state is quenched by the crystallin proteins through a mechanism that involves electron transfer from the proteins to the flavin, as shown by the decrease of the triplet RF band with the concomitant increase of the band of its semireduced form. Tryptophan loss on RF-sensitized photooxidation of the crystallins when irradiated with monochromatic visible light (450 nm) in a 5% oxygen atmosphere was studied. A direct correlation was found between the triplet RF quenching rate constants by the different crystallin fractions and the decomposition rate constants for the exposed and partially buried tryptophans in the proteins. The RF-sensitized photooxidation of the crystallins is accompanied by the decrease of the low molecular weight constituents giving rise to its multimeric forms. A direct correlation was observed between the initial rate of decrease of the low molecular weight bands corresponding to the irradiated alpha-, betaH- and betaL-crystallins and the quenching constant values of triplet RF by the different crystallins. The correlations found in this study confirm the importance of the Type-I photosensitizing mechanism of the crystallins, when RF acts as a sensitizer at low oxygen concentration, as can occur in the eye lens.     

Photolytic dechlorination of 4-chlorophenol in O2-saturated aqueous solutions in the absence of photocatalysts or additives using ArF (193 nm) and KrF (248 nm) excimer lasers

One of the toxic products from chlorine bleaching of wood pulp is 4-chlorophenol (4-CP). Detoxification of such compounds usually requires their dechlorination. The present study involves a fairly detailed comparison of the attempt and success of the photolytic dechlorination of 4-CP in O₂-saturated aqueous solutions using ArF^* (193 nm) and KrF^* (248 nm) excimer laser radiation at higher (1.1 × 10⁻² M) and lower (4.5 × 10⁻⁴ M) starting substrate concentrations and comparisons of quantum yield (φ), product distributions, etc.

At the higher starting substrate concentration (1.1 × 10⁻² M) the average initial quantum yields, i.e. early in the reaction process, for the disappearance of 4-CP (φ ≈2 0.30) and for the generation of chloride ions (φ ≈2 0.25) were about the same for both 193 and 248 nm radiation. However, when the number of photons absorbed (n) became greater than about 3 × 10²⁰, more chloride ions (higher φ) were generated with the 193 nm radiation than with the 248 nm radiation. Oligomers were the major products for both wavelengths of radiation, but the quantity of oligomers generated was greater using the 193 nm radiation. At 248 nm a significant amount of hydroquinone was generated, whereas hydroquinone was not detected with the 193 nm radiation. In addition, a significant amount of 4-chlorocatechol was generated during the direct photolysis of 4-CP using either wavelength of radiation. This is a new result for the photolysis of 4-CP at a wavelength longer than 193 nm in the absence of added H₂O₂.

At the lower starting substrate concentration (4.5 × 10⁻⁴ M) the average initial quantum yields for the disappearance of 4-CP (φ ≈2 0.55) and for the generation of chloride ions (φ ≈2 0.45) were both much greater using the 193 nm radiation than with the 248 nm radiation and 0.10 respectively). At 193 nm oligomers were still the major products generated, but the fraction of oligomeric products generated was less than the fraction at the higher substrate concentration. This is consistent with the fact that at the lower substrate concentration a significant amount of hydroquinone was generated at 193 nm, whereas it was not detected at this wavelength for the higher substrate concentration. During the 248 nm photolysis of 4-CP at this lower starting substrate concentration, hydroquinone was the only major product generated in these experiments. Oligomers were not observed at this lower substrate concentration using 248 nm radiation, whereas oligomers were generated at the higher substrate concentration.

The results demonstrate the utility of using an excimer laser for the photolytic dechlorination of 4-CP without added photocatalysts or additives. It is also possible to suggest a number of explanations, given in the text, which are consistent with our findings.     

Photolysis of 4-oxo-2-pentenal in the 190-460 nm region

We have studied the gas-phase photolysis of 4-oxo-2-pentenal by laser photolysis combined with cavity ring-down spectroscopy. Absorption cross sections of cis- and trans-4-oxo-2-pentenal have been measured in the 190-460 nm region. The product channel following 193, 248, 308, and 351 nm photolysis of 4-oxo-2-pentenal was investigated. The HCO radical is a photodissociation product of 4-oxo-2-pentenal only at 193 and 248 nm. The HCO quantum yields from the photolysis of a mainly trans-4-oxo-2-pentenal sample are 0.13 +/- 0.02 and 0.014 +/- 0.003 at 193 and 248 nm, where errors quoted (1sigma) represent experimental scatter. The HCO quantum yields from the photolysis of a mainly cis-4-oxo-2-pentenal sample are 0.078 +/- 0.012 and 0.018 +/- 0.007 at 193 and 248 nm, where errors quoted (1sigma) represent experimental scatter. The end-products from 193, 248, 308, and 351 nm photolysis of 4-oxo-2-pentenal (the 4-oxo-2-pentenal sample had a tran/cis ratio of 1.062:1) have been determined by FTIR. Ethane, methyl vinyl ketone, and 5-methyl-3H-furan-2-one have been observed, suggesting the occurrence of 4-oxo-2-pentenal photolysis pathways such as CH(3)COCH=CHCHO + hnu --> CH(3) + COCH=CHCHO, CH(3)COCH=CHCHO + hnu --> CH(3)COCH=CH(2) + CO, and CH(3)COCH=CHCHO + hnu --> 5-methyl-3H-furan-2-one. The estimated yields for the CH(3) + COCH=CHCHO channel are about 25%, 33%, 31%, and 23% at 193, 248, 308, and 351 nm, respectively. The absolute uncertainties in the determination of CH(3) + COCH=CHCHO yields are within 55% at 193 nm, and 65% at 248, 308, and 351 nm. The estimated yields for the CH(3)COCH=CH(2) + CO channel are about 25%, 23%, 40%, and 33% at 193, 248, 308, and 351 nm, respectively. The absolute uncertainties in the determination of CH(3)COCH=CH(2) yields are within 80% at 193 and 248 nm and 65% at 308 and 351 nm. The estimated yields for the 5-methyl-3H-furan-2-one channel are about 1.2%, 2.1%, 5.3%, and 5.5% at 193, 248, 308, and 351 nm, respectively. The absolute uncertainties in the determination of 5-methyl-3H-furan-2-one yields are about 23%, 86%, 40%, and 46% at 193, 248, 308, and 351 nm. Results from our investigation indicate that photolysis is a dominant removal pathway for 4-oxo-2-pentenal degradation in the atmosphere.     

Determination of the photochemical efficiency of o-quinodimethane ring closure in room-temperature solutions by using time-delayed, two-color photolysis technique

Photochemical efficiency of o-quinodimethane (3) ring closure at room temperature was determined by using a time-delayed, two-color photolysis technique. o-Quinodimethane (3) was generated by the photolysis of 1,2-bis[(phenylseleno)methyl]benzene (1) by a KrF (248 nm) laser pulse and thus-generated 3 was photolyzed by a subsequent XeCl (308 nm)/XeF (351 nm) laser pulse with varying delay time of 0 to 3 s. The time profile of 3 was monitored by the chemical analyses of benzocyclobutene (5) (a photochemical product of 3), which was formed by a one-photon process, and the spiro dimer of 3 (4) (a thermal product of 3) in the two-color photolysis experiments. The time profile of 3 followed a second-order decay kinetics. The photochemical efficiency was obtained by the analysis of the delay-time dependence of the product yields; those of the consumption of 3 and the conversion 3-->5 by a single pulse of the excimer laser were 81% and 5.7% for the XeCl laser, and 73% and 2.3% for the XeF laser. This difference was attributed to the different excited states involved in the photolysis. In contrast to the photolysis of 3 in argon or rigid organic matrixes, it was revealed that photochemical conversion 3-->5 was not the main path in the solutions, and intermolecular reactions predominated.     

Riboflavin-sensitized photochemical radical ion chain reaction of sulfo-group substitution for halogen in halogenated hydroxynaphthalenes

It was shown that riboflavin (RF) is a sensitizer for the radical ion chain reaction of sulfo group substitution for halogen in halogenated hydroxynaphthalenes. The initaition mechanism involves the electron transfer reaction between the sulfite ion and excited riboflavin. The quantum yields of RF radical anions from the singlet and the triplet states upon excitation in an aqueous sodium sulfite solution are 0.01 and 0.15, respectively, as determined by means of flash photolysis. The principal decay reaction for RF radical anions is their recombination with sulfite radical anions in the bulk of solution at a rate constant of (3.8 ± 0.5) × 10⁹ dm³ mol^?1 s^?1. The quantum yields of the riboflavin-sensitized substitution reaction increases in the presence of electron scavengers (chloranil, dinitrobenzene) and inorganic salts in the system.     

1,2-和1,4-萘醌248 nm激光光解的瞬态吸收光谱研究

利用纳秒级激光光解动态吸收光谱装置,研究了1,2-和1,4-萘醌中性水溶液的瞬态吸收光谱.发现1,2-萘醌及1,4-萘醌被光电离后形成的阳离子自由基在380nm均有最大吸收,但1,4-萘醌阳离子自由基在衰变过程中又形成了两种新的活性粒子,它们的最大吸收分别位于410和580nm,分析表明:410nm属于1,4-萘醌脱氢自由基的吸收,而580nm很可能归属由于电子转移而形成的瞬态产物.进一步研究发现,1,2-萘醌在中性水溶液中能被248nm激光单光子电离.     

Photo-detrapping of solvated electrons in an ionic liquid

We studied the dynamics of photo-detrapped solvated electrons in the ionic liquid trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide (TMPA-TFSI) using laser flash photolysis. The solvated electrons were produced by the electron photodetachment from iodide via a 248 nm KrF excimer laser. The solvated electron decayed by first-order kinetics with a lifetime of about 240 ns. The spectrum of the solvated electron in the ionic liquid TMPA-TFSI is very broad with a peak around 1100 nm. After the 248 nm pulse, a 532 nm pulse was used to subsequently detrap the solvated electrons. After the detrapping pulse, quasi-permanent bleaching was observed. The relative magnitude of the bleaching in the solvated electron absorbance was measured from 500 to 1000 nm. The amount of bleaching depends on the probe wavelength. The fraction of bleached absorbance was larger at 500 nm than that at 1000 nm, suggesting that there are at least two species that absorb 532 nm light. We discuss the present results from viewpoint of the heterogeneity of ionic liquids.     

Laser photolysis of interaction of poly-guanylic acid (5'''') with anthraquinone-2-sulfonate

The electron transfer reaction between triplet anthraquinone-2-sulfonate and poly-guanylic acid (5') in CH3CN-H2O (97 : 3) has been investigated by 248 nm (KrF) laser flash photolysis. The transient absorption spectra and kinetics obtained from the interaction of triplet anthraquinone-2-sulfonate and poly[G] demonstrate that the primary ionic radical pair, radical cation of poly[G] and radical anion of anthraquinone-2-sulfonate have been detected simultaneously. The free energy changes in the process of the electron transfer were also calculated.     

Further investigation of the photodissociation dynamics of dichlorocarbene near 248 nm

A further investigation of the 248 nm photodissociation of CCl(2), which expands upon our original study of this process [S. K. Shin and P. J. Dagdigian, Phys. Chem. Chem. Phys. 8, 3446 (2006)], is presented. The CCl(2) parent molecule and the CCl photofragment were detected by laser fluorescence excitation in a molecular beam experiment. From the dependence of the CCl(2) signals on the photolysis laser fluence, attenuation cross sections of the 0(0), 1(1), and 2(1) vibrational levels were determined; the cross sections for the excited vibrational levels were found to be significantly smaller than those for the ground vibrational level. The previously observed fragment CCl bimodal rotational state distribution was found to arise from the photolysis of more than one parent molecule. At low CHCl(3) mole fractions in the gas supplied to the pyrolysis beam source, it was concluded that CCl(2) is the photolysis precursor for both low-J and high-J CCl fragments. On the basis of the dependence of the CCl signals on the photolysis laser fluence, ground and vibrationally excited CCl(2), respectively, were assigned as the precursors to these two classes of fragments. The photofragment excitation spectra for low-J and high-J CCl fragments from the photolysis of CCl(2) were recorded in the wavelength range around 248 nm; both were found to be structureless. The 248 nm photodissociation dynamics of CCl(2) is discussed in light of our experimental observations and quantum chemical calculations of the CCl(2) excited electronic states.     

Laser photolysis of interaction of poly-guanylic acid (5′) with anthraquinone-2-sulfonate

The electron transfer reaction between triplet anthraquinone-2-sulfonate and poly- guanylic acid (5′) in CH3CN-H2O (97:3) has been investigated by 248 nm (KrF) laser flash photolysis. The transient absorption spectra and kinetics obtained from the interaction of triplet anthraquinone-2-sulfonate and poly[G] demonstrate that the primary ionic radical pair, radical cation of poly[G] and radical anion of anthraquinone-2-sulfonate have been detected simultaneously. The free energy changes in the process of the electron transfer were also calculated.     

Electron spin resonance study of radicals generated in cellulose/N-methylmorpholine solutions after flash photolysis at 77 K

The processes of radical formation in N-methylmorpholine-N-oxide monohydrate (NMMO) and cellulose/NMMO solutions were studied by ESR at 77 K under high-power UV (lambda = 248 nm) excimer laser flash photolysis. Radicals mainly generated were attributed to the nitroxide type radicals -CH2-NO*-CH2- and -CH2-NO*-CH3 at the first step and methyl *CH3 and formyl *CHO radicals at the second step of the photoreaction. Kinetic studies of these radicals revealed that formation and recombination rates of the radicals depend on the cellulose concentration in cellulose/NMMO solutions and the concentration of additional ingredients, e.g. Fe(II) and propyl gallate. Even at frozen state temperature, acceleration or quenching of radical reaction processes was found. The proposed scheme of UV light-induced NMMO degradation during irradiation based on ESR data correlates well with independently obtained results based on high-performance liquid chromatography (HPLC). The analysis of degradation products by HPLC, e.g. aminoethanol and acetaldehyde, supports the assumption concerning a radical-initiated ring opening of NMMO.     

Cl2O photochemistry: ultraviolet/vis absorption spectrum temperature dependence and O(3P) quantum yield at 193 and 248 nm

The photochemistry of Cl(2)O (dichlorine monoxide) was studied using measurements of its UV/vis absorption spectrum temperature dependence and the O((3)P) atom quantum yield, Φ(Cl(2)O)(O)(λ), in its photolysis at 193 and 248 nm. The Cl(2)O UV/vis absorption spectrum was measured over the temperature range 201-296 K between 200 and 500 nm using diode array spectroscopy. Cl(2)O absorption cross sections, σ(Cl(2)O)(λ,T), at temperatures <296 K were determined relative to its well established room temperature values. A wavelength and temperature dependent parameterization of the Cl(2)O spectrum using the sum of six Gaussian functions, which empirically represent transitions from the ground (1)A(1) electronic state to excited states, is presented. The Gaussian functions are found to correlate well with published theoretically calculated vertical excitation energies. O((3)P) quantum yields in the photolysis of Cl(2)O at 193 and 248 nm were measured using pulsed laser photolysis combined with atomic resonance fluorescence detection of O((3)P) atoms. O((3)P) quantum yields were measured to be 0.85 ± 0.15 for 193 nm photolysis at 296 K and 0.20 ± 0.03 at 248 nm, which was also found to be independent of temperature (220-352 K) and pressure (17 and 28 Torr, N(2)). The quoted uncertainties are at the 2σ (95% confidence) level and include estimated systematic errors. ClO radical temporal profiles obtained following the photolysis of Cl(2)O at 248 nm, as reported previously in Feierabend et al. [J. Phys. Chem. A 114, 12052, (2010)], were interpreted to establish a <5% upper-limit for the O + Cl(2) photodissociation channel, which indicates that O((3)P) is primarily formed in the three-body, O + 2Cl, photodissociation channel at 248 nm. The analysis also indirectly provided a Cl atom quantum yield of 1.2 ± 0.1 at 248 nm. The results from this work are compared with previous studies where possible.     

Pressure dependence for the CO quantum yield in the photolysis of acetone at 248 nm: a combined experimental and theoretical study

The quantum yield of CO in the laser pulse photolysis of acetone at 248 nm and at 298 K in the pressure range 20-900 mbar (N2) has been measured directly using quantitative infrared diode laser absorption of CO. It is found that the quantum yield of CO shows a significant dependence on total pressure with Phi(CO) decreasing with pressure from around 0.45 at 20 mbar to approximately 0.25 at 900 mbar. From a combination of ab initio quantum chemical calculations on the molecular properties of the acetyl (CH3CO) radical and its unimolecular fragmentation as well as the application of statistical (RRKM) and dynamical calculations we show that CO production results from prompt secondary fragmentation (via(2a)) of the internally excited primary CH3CO* photolysis product with an excess energy of approximately 62.8 kJ mol(-1). Hence, our findings are consistent with a consecutive photochemically induced decomposition model, viz. step (1): CH3COCH3+hv--> CH3CO*+ CH3, step (2a): CH3CO*--> CH3+ CO or step (2b) CH3CO*-(+M)--> CH3CO. Formation of CO via a direct and/or concerted channel CH3COCH3+hv--> 2CH(3)+ CO (1') is considered to be unimportant.     

4-硝基喹啉氧化物激发三重态与甲硫氨酸二肽之间的电子转移

4 硝基喹啉氧化物 ( 4ＮＱＯ)在紫外光的作用下 ,发生电子跃迁生成较高量子产额的激发三重态 ,这一性质在光化学研究中可作为光敏剂 ,同时又是一种典型的致癌物 .在水溶液中 ,具有与ＤＮＡ通过电荷转移作用连接到碱基上的性质 ,这一性质可能与其致癌作用有关[1～ 4].由于ＤＮＡ具有的遗传功能通过染色体来实现 ,染色体是ＤＮＡ与蛋白质形成的配合物 ,这种蛋白质结合在ＤＮＡ上 ,同ＤＮＡ碱基一样参与化学反应诱导的致癌过程 .因此 ,研究光引发 4ＮＱＯ与氨基酸及相关肽的反应对于了解光诱导 4ＮＱＯ致癌作用的分子机制有重要意义 .Ｓｅｋｉ…     

Photodissociation dynamics of dichlorocarbene at 248 nm

The dynamics of the 248 nm photodissociation of the CCl(2) molecule have been investigated in a molecular beam experiment. The CCl(2) parent molecule was generated in a molecular beam by pyrolysis of CHCl(3), and both CCl(2) and the CCl photofragment were detected by laser fluorescence excitation. The 248 nm attenuation cross sections was estimated from the reduction of the CCl(2) signal as a function of the photolysis laser fluence. The internal state distribution of the CCl photofragment was derived from analysis of laser fluorescence excitation spectra in the A (2)Delta- X (2)Pi band system. The CCl(X (2)Pi, nu = 0) rotational state distribution was found to be bimodal, with maximum populations at N approximately 10 and 85, and was dependent upon the source backing pressure, and hence upon the internal state distribution of the CCl(2) precursor. The 248 nm photodissociation dynamics appears to involve two separate channels, namely nearly impulsive rotational energy release and predissociation with little rotational energy imparted to the CCl fragment.