丙酮三重态与含芳香氨基酸肽的物理化学过程

用激光闪光光解瞬态吸收光谱研究了水溶液中含芳香氨基酸残基肽的光敏化反应过程.结果表明,在丙酮存在的含色氨酸残基肽(Trp-Gly,n-f-Met-Trp,Trp-Phe)体系的光解,丙酮三重态与Trp分别通过三重态-三重态(T-T)激发能转移和电子转移生成Trp激发三重态和N中心自由基(Trp/N^·);丙酮三重态仅与含酪氨酸残基肽(Phe-Tyr)通过电子转移生成Tyr酚氧自由基(Tyr/O^·).在色氨酰酪氨酸(Trp-Tyr)与丙酮的光解体系中,观察到分子内的电子转移,即由Trp/N^·-Tyr→Trp-Tyr/O^·自由基的生成过程     

Flavin-sensitized Photo-oxidation of Lysozyme and Serum Albumin

The excited state processes of riboflavin, flavin mononucleotide and flavin adenine dinucleotide in argon-saturated aqueous solution were studied in the presence of lysozyme or bovine serum albumin (BSA). UV–Vis absorption and fluorescence spectroscopy indicates that the noncovalent flavin-protein binding is relatively weak. Quenching of the flavin triplet state by BSA, observed by time-resolved photolysis, is less efficient than by lysozyme. Light-induced oxidation of the two proteins and reduction of the three flavins were studied. The quantum yields of the former and latter in the absence of oxygen are up to 0.1 and 0.04, respectively. The effects of pH and sensitizer and protein concentrations were examined in greater detail. The proposed reaction is electron transfer from the tryptophan moiety to the flavin triplet rather than excited singlet state.     

Photophysical and photochemical processes of riboflavin (vitamin B_2) 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, both 3RF* and hydrated electron (eaq) formed from photoionizationcould be detected. Photobiological implications have been inferred on the basis of reactivity of 3RF* 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.     

An electron paramagnetic resonance study of the influence of spermine on the photophysical reactions of tryptophan in aqueous solution at 77–200 K

The influence of the antioxidant spermine of the UV-induced formation of free radicals from tryptophan in frozen aqueous solutions was studied by electron paramagnetic resonance (EPR) instrumentation, and the stability of the radicals was investigated in the range 95–200 K. Without spermine, the tryptophan cation and neutral tryptophan radical were stabilized at 77 K; cations were formed by electron ejection from an excited singlet state, and neutral radicals by hydrogen donation from tryptophan in the triplet state. When present, spermine trapped the ejected photoelectrons; the rates of the two photoreactions of tryptophan were also influenced by spermine. Firstly, at low tryptophan concentrations, the yield of cations was reduced, due to diminished charge transfer from the excited singlet state to the solvation shell. Secondly, at high concentrations, minute additions of spermine enhanced intersystem crossing (which is quenched, in the absence of spermine, by dimerization) and, consequently, the yield of neutral radicals was increased. At 180 K, the electrons trapped by spermine were released and reacted with molecular oxygen to form the superoxide radical; at 190 K, the tryptophan radicals were thermally annealed.     

没食子酸的激光光解研究

The transient species of gallic acid(GA)have been studied by 266 nm nanosecond laser flash photolysis inaqueous solution and acetonitrile.The intermediate with absorption at 320 nm was identified as excited triplet state(~3GA~*),the decay rates of which were obtained in aqueous solution and acetonitrile respectively.Energy transferfrom ~3GA~* to β-carotene was observed and the energy transfer rate constant k_(ent)was determined to be 2.2×10~9mol~(-1)·L·s~(-1).GA underwent photoionization during photolysis and the quantum yield of photoionization was de-termined to be 0.12 at room temperature with KI as a reference.     

Reactions of excited states of phenoxazin-3-one dyes with amino acids

The interaction with amino acids of the excited states of the N-oxide resazurin and its deoxygenation product resorufin, has been studied in aqueous solution at pH 7.5. Steady-state and time-resolved studies show that the fluorescence is quenched by amino acids. Complexation of the dyes in the ground state with aromatic amino acids was also observed. The singlet quenching is attributed to electron transfer from the amino acids to the excited dye based on the dependence of the bimolecular rate constants with the ionization potential of quenchers. Flash photolysis experiments allowed determination of the quenching rate constants for the triplet deactivation of dyes by several amino acids, as well as the characterization of the transients formed in the process. These data show that the triplet is also deactivated by an electron transfer process. However, the deactivation of the N-oxide dye by tryptophan can be described by a hydrogen atom transfer. The protolytic dissociation constants of the dye radical ions are reported. The irradiation of rezasurin in the presence of amino acids leads to deoxygenation of the dye to give resorufin. This process involves the triplet excited state of resazurin and is efficient only in the presence of amino acids containing the -SH group.     

Primary Photoprocesses in a Fluoroquinolone Antibiotic Sarafloxacin†

The photophysical properties of the fluoroquinolone antibiotic sarafloxacin (SFX) were investigated in aqueous media. SFX in water, at pH 7.4, shows intense absorption with peaks at 272, 322 and 335 nm, (? = 36800 and 17000 dm³ mol^?1 cm^?1, respectively). Both the absorption and emission properties of SFX are pH‐dependent; pK_a values for the protonation equilibria of both the ground (5.8 and 9.1) and excited singlet states (5.7 and 9.0) of SFX were determined spectroscopically. SFX fluoresces weakly, the quantum yield for fluorescence emission being maximum (0.07) at pH 8. Laser flash photolysis and pulse radiolysis studies have been carried out in order to characterize the transient species of SFX in aqueous solution. Triplet–triplet absorption has a maximum at 610 nm with a molar absorption coefficient of 17,000 ± 1000 dm³ mol^?1 cm^?1. The quantum yield of triplet formation has been determined to be 0.35 ± 0.05. In the presence of oxygen, the triplet reacts to form excited singlet oxygen with quantum yield of 0.10. The initial triplet (³A*) was found to react with phosphate buffer to form triplet ³B* with lower energy and longer lifetime and having an absorption band centered at 700 nm. SFX triplet was also found to oxidize tryptophan to its radical with concomitant formation of the anion radical of SFX. Hence the photosensitivity of SFX could be initiated by the oxygen radicals and/or by SFX radicals acting as haptens.     

THE PHOTOREDUCTION OF FLAVINS BY AMINO ACIDS AND EDTA. A CONTINUOUS AND FLASH PHOTOLYSIS STUDY

Abstract— Primary and secondary photochemical processes in oxygen-free aqueous solution have been characterised for FMN alone and in the presence of EDTA and four amino acids using nanosecond and microsecond flash photolysis and continuous photolysis techniques. The relative contributions of oneelectron and two-electron (group or hydride transfer) reactions to the deactivation of the triplet has been determined by comparing the radical concentration (560 nm) with the bleaching of the ground state (446 nm). It was concluded that one-electron reactions (hydrogen atom or electron abstraction) are the major mode of reactivity of the flavin triplet state with all the suhstrates studied.
The nature of the reactions of the flavin semiquinone radical have been studied quantitatively by microsecond flash photolysis. These secondary reactions consist of either a 'back reaction' between the flavin and substrate radicals (tryptophan or glycyl-tyrosine) or the transfer of a second electron (or hydrogen atom) from the substrate radical to the flavin radical (EDTA, methionine and possibly cysteine) to form reduced flavin and oxidised substrate. From a comparison of the quantum yields of formation of reduced flavin using 'flash' and continuous irradiation, an additional pathway for the decay of the flavin radical is suggested to occur at low light intensities in the presence of glycyl-tyrosine or histidine.     

Time resolve study on isopropyl-ether porphyrindiol

In the past years extensive studies have been conducted on porphyrin-type photosensitizers because of their photosensitive activity. With regard to their interaction with many important macromolecules such as nucleic acids, proteins and lipids, porphyrin-type photosensitizers are capable of damaging numerous cells. They damage DNA via oxidation of four bases, especially guanine and cytosine pairs[1], damage protein by oxidation of (at least) two amino acids——cysteine and tryptophan residues…     

Comparison of intermediates of tryptophan, tyrosine and their dipeptide induced by UV light and SO. 4 –

The chemical processes of tryptophan (Trp), tyrosine (Tyr) and a dipeptide Trp-Tyr, which are induced by UV radiation and one-electron oxidation of SO^. ₄ ^–, have been investigated in aqueous solution by KrF (248 nm) laser flash photolysis. On the basis of optical studies, the photoionization of Trp and Tyr produces the tryptophan indolyl radical and tyrosine phenoxyl radical, respectively, and these are different from the intermediates resulting from interaction of Trp and Tyr with SO^. ₄ ^–. In the case of Trp, SO^. ₄ ^– would attack the indole moiety to produce a C(2)-yl sulphate radical adduct, and Tyr is oxidized to produce mainly the corresponding one-electron oxidized radical, which deprotonates rapidly to form the phenoxyl radical in neutral solution, and a possible sulphate radical adduct. From transient absorption spectra of photoionization of Trp-Tyr, an intramolecular electron transfer, Trp/N^.-Tyr Trp-Tyr/O^., has been observed, but there was no observation of the process of one-electron oxidation of Trp-Tyr by SO^. ₄ ^–.     

二氟沙星激发态氧化损伤氨基酸和脱氧鸟苷酸的激光光解研究

The reactions of triplet-state difloxacin (DFX) with various amino acids and deoxyguanylic acid in aqueous media were studied using laser flash photolysis. Tryptophan, tyrosine, cysteine, and 2'-deoxyguanosine-5'-monophosphate (dGMP) were found to completely quench the triplet state of DFX in aqueous solution, the corresponding second-order rate constants being 1.97×10⁸, 1.48×10⁸, 1.72×10⁸, and 6.92×10⁷ dm³·mol^－1·s^－1. The quenching mechanism involves electron transfer to the photoexcited triplet state of DFX from the tryptophan, tyrosine, cysteine, and dGMP moieties, followed by fast protonation of the resulting DFX anion radical.     

CHLOROPHYLL PHOTOCHEMISTRY IN CONDENSED MEDIA—I. TRIPLET STATE QUENCHING AND ELECTRON TRANSFER TO QUINONE IN CELLULOSE ACETATE FILMS*

Chlorophyll-a was incorporated into cellulose acetate films and the triplet state decay kinetics and electron transfer from triplet to p-benzoquinone in aqueous solution was studied using laser flash photolysis and EPR. The triplet was found to decay by first order kinetics with a rate constant which was independent of Chl concentration. The triplet yield, however, was concentration dependent. These properties are due to quenching which occurs only at the singlet state level. In the presence of quinone, the triplet is quenched and, when the quinone is in an aqueous solution in contact with the film, Chl cation radical (C^±) as well as the semiquinone anion radical (Q^±) can be observed. The C decays by second order kinetics with a rate constant of 1.5 × 10⁶M^-1 s^-1. Although triplet conversion to radicals is slightly lower in the films as compared to fluid solutions (? 3 times), the lifetimes of the radicals are greatly increased (? 10³ times).     

Primary photophysical properties of ofloxacin

Steady-state fluorescence has been used to study the excited singlet state of ofloxacin (OFLX) in aqueous solutions. Fluorescence emission was found to be pH dependent, with a maximum quantum yield of 0.17 at pH 7. Two pKa*s of around 2 and 8.5 were obtained for the excited singlet state. Laser flash photolysis and pulse radiolysis have been used to study the excited states and free radicals of OFLX in aqueous solutions. OFLX undergoes monophotonic photoionization from the excited singlet state with a quantum yield of 0.2. The cation radical so produced absorbs maximally at 770 nm with an extinction coefficient of 5000 +/- 500 dm3 mol-1 cm-1. This is confirmed by one-electron oxidation in the pulse radiolysis experiments. The hydrated electron produced in the photoionization process reacts with ground state OFLX with a rate constant of 2.0 +/- 0.2 x 10(10) dm3 mol-1 s-1, and the anion thus produced has two absorption bands at 410 nm (extinction coefficient = 3000 +/- 300 dm3 mol-1 cm-1) and at 530 nm. Triplet-triplet absorption has a maximum at 610 nm with an extinction coefficient of 11,000 +/- 1500 dm3 mol-1 cm-1. The quantum yield of triplet formation has been determined to be 0.33 +/- 0.05. In the presence of oxygen, the triplet reacts to form both excited singlet oxygen and superoxide anion with quantum yields of 0.13 and < or = 0.2, respectively. Moreover, superoxide anion is also formed by the reaction of oxygen with the hydrated electron from photoionization. Hence the photosensitivity due to OFLX could be initiated by the oxygen radicals and/or by OFLX radicals acting as haptens.     

Photophysical and photochemical studies of 2-phenylbenzimidazole and UVB sunscreen 2-phenylbenzimidazole-5-sulfonic acid

The sunscreen agent 2-phenylbenzimidazole-5-sulfonic acid (PBSA) and its parent 2-phenylbenzimidazole (PBI) cause DNA photodamage via both Type-I and Type-II mechanisms when UVB irradiated. We have studied the photophysical and photochemical properties of these compounds and their ability to photogenerate reactive oxygen species including free radicals. PBI and PBSA exhibit both oxidizing and reducing properties in their excited state. The absorption and fluorescence properties of PBSA depend strongly upon pH, and hence the photochemistry of PBSA was studied in both neutral and alkaline solutions. PBSA showed strong oxidizing properties when UV irradiated in neutral aqueous solution (pH 7.4) in the presence of cysteine, glutathione and azide, as evidenced by the detection of the corresponding S-cysteinyl, glutathiyl and azidyl radicals with the aid of the spin trap, 5,5-dimethyl-1-pyrroline N-oxide (DMPO). However, when an aqueous anaerobic solution (pH 10) of PBSA and either nitromethane (NM) or 4-nitrobenzoic acid (4-NBA) were irradiated, the corresponding nitro anion radicals were observed. This finding suggests that both NM and 4-NBA are reduced by direct electron transfer from the excited state PBSA. During UV irradiation of an aerobic solution of PBSA, O2*- and *OH radical were generated and trapped by DMPO. Further, PBI (in ethanol) and PBSA (in ethylene glycol : water 2: 1 mixture) showed low temperature (77 K) phosphorescence (lambdamax = 443, 476 and 509 nm) and also an electron paramagnetic resonance half-field transition (deltaMs = +/-2), which is evidence for a triplet state. This triplet produced singlet oxygen (1O2) with quantum yields 0.07 and 0.04 in MeCN for PBI and PBSA, respectively. These studies demonstrate that UV irradiation of PBSA and PBI generates a variety of free radicals and active oxygen species that may be involved in the photodamage of DNA.     

Photochemistry of a naphthalene-thymine dyad in the presence of acetone

Irradiation of dyad 1 in aqueous acetone leads to the introduction of an acetonyl substituent at the naphthalene 5-position, to give photoproduct 2. The proposed reaction mechanism involves electron transfer from the naphthalene excited singlet state to the ketone. Neither thymine dimers, nor acetone photoadducts involving the thymine ring were detected. These photoproducts would arise from the thymine triplet excited state, which in dyad 1 must be efficiently depopulated via a fast intramolecular energy transfer to the naphthalene chromophore, due to the lower energy of its excited triplet state.     

Photoinduced decarboxylative benzylation of phthalimide triplets with phenyl acetates: a mechanistic study

The photodecarboxylative benzylation of N-alkyl, N-arylalkyl, and N-aryl phthalimides with arylacetic acids in aqueous solution proceeds via electron transfer from the arylalkanoate to the excited triplet state of the phthalimide, either formed directly or upon sensitization with acetone. The rate constant for triplet quenching of N-methylphthalimide is k(q) < 10(7) M(-1) s(-1) for 2-phenylacetic acid and k(q) = (1-3) x 10(9) M(-1) s(-1) for its mono-, di- and trimethoxy-substituted derivatives, suggesting a change of the mechanism for the primary oxidation step from a Photo-Kolbe type reaction yielding an acyloxy radical to a pseudo-Photo-Kolbe process involving the formation of resonance-stabilized zwitterion radicals as intermediates.     

Photolysis of methylcobalamin: identification of the relevant excited states involved in Co-C bond scission

The relevant excited states involved in the photolysis of methylcobalamin (MeCbl) have been examined by means of time-dependent density functional theory (TD-DFT). The low-lying singlet and triplet excited states have been calculated along the Co-C bond at the TD-DFT/BP86/6-31g(d) level of theory in order to investigate the dissociation process of MeCbl. These calculations have shown that the photodissociation is mediated by the repulsive 3(sigmaCo-C --> sigma*Co-C) triplet state. The key metastable photoproduct involved in Co-C bond photolysis was identified as an S1 state having predominantly dCo --> pi*corrin metal-ligand charge transfer (MLCT) character.     

Reactive intermediates in laser photolysis of guanosine

The photochemical reactions of aqueous solution guanosine with acetone as photosensitizer have been investigated by laser flash photolysis. From detailed kinetic analysis, the reaction mechanism has been derived: dehydrogenated guanosine radical G(-H)* and triplet guanosine ³G* were produced via electron transfer and triplet-triplet energy transfer respectively. The half-life time of ³G* has been determined to be 7.4 μs, and the quenching rate constant of Mn²⁺ on it was obtained to be 1.9 × 10⁸ M^-1 s^-1. Based on pH titration, the pKa value of ³G* was also obtained to be 8.7.     

吡啶离子液体[BPy][BF4]与乙腈混合体系中杜醌的光化学反应动力学

用激光闪光光解方法研究了杜醌(DQ)在吡啶型离子液体N-丁基吡啶四氟硼酸盐([BPy][BF₄])与乙腈(MeCN)组成的共混溶剂中的光化学反应机理与动力学. 实验结果表明, 离子液体[BPy][BF₄]对混合体系中杜醌激发三线态(³DQ^*)的瞬态吸收峰位置和吸光度大小都没有产生明显影响. 在N₂饱和条件下, 无论是在乙腈溶液中还是在[BPy][BF₄]/MeCN混合溶液中³DQ^*的衰减都遵循一级反应动力学规律. 而[BPy][BF₄]的存在对³DQ^*与三乙胺(TEA)之间的电子转移影响显著. 随着[BPy][BF₄]/MeCN 体系中离子液体比例的增加, 杜醌三线态³DQ^*与TEA间的瞬态反应机理没有改变, 但它们之间的光诱导电子转移反应速率和生成自由基的量子产额逐渐降低, 通过改变离子液体的比例可以调节该体系中光诱导电子转移反应的速率和效率.     

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.