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…     

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.     

萘酰亚胺及其含二茂铁二元化合物的分子间(内)三重态能量传递

近年来萘酰亚胺类化合物作为超分子体系中的功能单元逐渐引起了人们的重视,如快速响应的光开关体系[1]、生物荧光探针体系[2]、激光染料天线分子[3]等.     

Formation and decay of a triplet state of dipyrido[3,2-a:2′,3′-c]phenazine

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Photoinduced electron transfer from electron donor to bis-carbocyanine dye in excited triplet state

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C-O-bond cleavage of esters with a naphthyl group in the higher triplet excited state during two-color two-laser flash photolysis

A C-O-bond cleavage of esters having a naphthyl group, NpCO-OR and RCO-ONp (Np=alpha- and beta-naphthyl ((alpha)Np and (beta)Np, respectively), R=Ph and Me), was found during the two-color two-laser flash photolysis in acetonitrile. The C-O-bond cleavage occurred when NpCO-OR and RCO-ONp were excited to the singlet excited states (S1). On the other hand, no reaction occurred from the lowest triplet excited states (T1). When NpCO-OR(T1) and RCO-ONp(T1) were excited to the higher triplet excited states (Tn) using the second laser during the two-color two-laser flash photolysis, the C-O-bond cleavage occurred. The C-O-bond cleavage quantum yield (Phi) was estimated from the plots of the T1-state esters disappeared within a laser flash versus the second laser intensities. The C-O-bond cleavage in (beta)NpCO-OPh(Tn) occurred more efficiently than in (alpha)NpCO-OPh(Tn) and that in PhCO-O(beta)Np(Tn) occurred more efficiently than in PhCO-O(alpha)Np(Tn). The Phi value for ester with Ph and beta-Np groups was larger than that for ester with Ph and alpha-Np groups. The Phi value for MeCO-O(alpha)Np(Tn) was similar to those for PhCO-ONp(Tn), while that for MeCO-O(beta)Np(Tn) was much smaller than those for PhCO-ONp(Tn) and MeCO-O(alpha)Np(Tn). On the other hand, no C-O-bond cleavage was observed in NpCO-OMe(Tn). The Phi value depended on the characters of the groups (Np, Ph, and Me) on the ester. Whether R is Ph or Me with or without pi electron, respectively, is important for the C-O-bond cleavage. In other words, electronic delocalization of the T(n) state including Np and ester groups is necessary for the occurrence of the C-O-bond cleavage in NpCO-OR(Tn) and RCO-ONp(Tn).     

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.     

Photosensitized oxidation of tryptophan and tyrosine by aromatic ketones: A laser flash photolysis study

Photochemical processes of benzophenone (BP) and xanthone (XT) with tryptophan (TrpH) and tyrosine (TyrOH) were studied using the laser flash photolysis technique.It has been observed that the triplet state of BP and XT reacted with TrpH and TyrOH by hydrogen transfer with the formation of BP and XT ketyl radicals and oxidized radicals of TrpY and TyrOY.The related rate constants of these reactions were determined in this paper.The free energy changes (G) of these reactions suggested that the proposed hydrogen transfer mechanism was thermodynamically feasible.These results provide theoretical foundation for further studying structural effects on the photochemical behaviors of proteins with triplet state BP and XT.     

二苯基卡宾反应性的取代基效应

用激光闪光光解法对室温溶液中产生的三线态二苯基卡宾进行了直接观察,并通过对产生的三线态二苯基卡宾寿命的测定以及与氧气反应速率常数的考察,就邻位及对位的取代基效应进行了初步探讨。     

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…     

Kinetics and mechanism of reactions of photoexcited kynurenine with molecules of some natural compounds

Photochemical reactions involving kynurenines, viz., molecules present in the eye lens, can result in modifications of the lens proteins and cause a development of a cataract. The rate constants of the reactions of photoexcited kynurenine with several amino acids and antioxidants contained in the lens were measured. The most efficient quenchers of triplet kynurenine are amino acids tryptophan and tyrosine, as well as antioxidant ascorbate. In all cases, the quenching reaction proceeds by the electron transfer mechanism, except for the reaction with oxygen where transfer of the triplet energy to the oxygen molecule occurs. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 704–710, April, 2007.     

One-electron oxidation pathways during beta-cyclodextrin-modified TiO2 photocatalytic reactions

The photocatalytic one-electron oxidation reactions of aromatic sulfides using the carboxymethyl-beta-cyclodextrin (CM-beta-CD)-modified TiO(2) nanoparticles (TiO(2)/CM-beta-CD) were investigated by using nano- and femtosecond transient absorption spectroscopies. The one-electron oxidation processes of the substrate (S) by the valence band hole (h(VB) (+)) at the TiO(2) surface and the trapped hole at the adsorption site of the CM-beta-CD (h(CD) (+)) were examined. The transient absorption spectra and time traces observed for the charge carriers and the radical cation of S (S(.+)) revealed that the one-electron oxidation reaction of S during the nano- and femtosecond laser flash photolyses of TiO(2)/CM-beta-CD is significantly enhanced relative to bare TiO(2). The kinetics of the decay and the dimerization processes between S(.+)s are discussed on the basis of the results obtained by the pulse radiolysis technique.     

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.     

Photo- and Radiation-Induced One-Electron Oxidation of Methionine in Various Structural Environments Studied by Time-Resolved Techniques

Oxidation of methionine (Met) is an important reaction that plays a key role in protein modifications during oxidative stress and aging. The first steps of Met oxidation involve the creation of very reactive and short-lived transients. Application of complementary time-resolved radiation and photochemical techniques (pulse radiolysis and laser flash photolysis together with time-resolved CIDNP and ESR techniques) allowed comparing in detail the one-electron oxidation mechanisms initiated either by ^●OH radicals and other one-electron oxidants or the excited triplet state of the sensitizers e.g., 4-,3-carboxybenzophenones. The main purpose of this review is to present various factors that influence the character of the forming intermediates. They are divided into two parts: those inextricably related to the structures of molecules containing Met and those related to external factors. The former include (i) the protection of terminal amine and carboxyl groups, (ii) the location of Met in the peptide molecule, (iii) the character of neighboring amino acid other than Met, (iv) the character of the peptide chain (open vs cyclic), (v) the number of Met residues in peptide and protein, and (vi) the optical isomerism of Met residues. External factors include the type of the oxidant, pH, and concentration of Met-containing compounds in the reaction environment. Particular attention is given to the neighboring group participation, which is an essential parameter controlling one-electron oxidation of Met. Mechanistic aspects of oxidation processes by various one-electron oxidants in various structural and pH environments are summarized and discussed. The importance of these studies for understanding oxidation of Met in real biological systems is also addressed.     

Triplet states of humic acids studied by laser flash photolysis using different excitation wavelengths

The spectra and kinetics of short-lived intermediates formed from aqueous (0.1 N NaOH) solutions of the natural mixture of humic and fulvic acids (HFA) were studied by laser flash photolysis using excitation wavelengths of 337, 390, 470, and 520 nm. Laser photolysis of HFA with light of 520 and 470 nm results in the formation of triplet excited states (T_HFA) characterized by the broad absorption spectrum with a maximum near 630 nm and lifetimes of 0.15 ms in deoxygenated solutions. The formation of two types of T_HFA with lifetimes of 0.1 and 2 ms and absorption spectra with maxima at 570 nm is observed under photolysis with light of 337 and 390 nm. The estimation of quantum yields of T_HFA gives 1 and 0.3% under photolysis with excitation wavelengths of 337 and 520 nm, respectively. The rate constants of T_HFA quenching by molecular oxygen are equal to (7—8)·10⁸ L mol^–1 s^–1.     

Pulse radiolysis study of one-electron oxidation of thionine in aqueous solutions

One-electron oxidation of thionine has been studied using specific oxidizing radicals such as Cl⁻Tl(II) and N₃ generated by pulse radiolysis of aqueous solutions. The semioxidized thionine exhibited threepK’s indicating four conjugate acid-base forms. N₃ radicals were found to be less efficient in oxidizing thionine as compared to Cl ₂ ⁻ , Tl²⁺ and Tl(OH)⁺. The rate constants for electron abstraction from thionine by Cl ₂ ⁻ , Tl²⁺, Tl(OH)⁺, Tl(OH)₂ and N₃ were evaluated. The spectra of different protonated forms of semioxidized thionine and the extinction coefficients at λ_max are presented. Reaction of OH radicals with thionine gave transient products whose spectra and acid-base properties were different from those of semioxidized thionine. The rate constant for formation of the product transient agrees well with competition kinetic value for reaction of OH with thionine reported earlier.     

Aromatic Monochlorination Photosensitized by DDQ with Hydrogen Chloride under Visible‐Light Irradiation

Photochlorination of aromatic substrates by hydrogen chloride with 2,3‐dichloro‐5,6‐cyano‐p‐benzoquinone (DDQ) occurs efficiently to produce the corresponding monochlorinated products selectively under visible‐light irradiation. The yields for the chlorination of phenol were 70 % and 18 % for p‐ and o‐chlorophenol, respectively, without formation of further chlorinated products. The photoinduced chlorination is initiated by electron transfer from Cl^? to the triplet excited state of DDQ. The radical intermediates involved in the photochemical reaction have been detected by time‐resolved transient absorption measurements.     

Oxidizing mechanism of podophyllotoxin and its derivatives by sodium persulfate

The oxidizing mechanism of podophyllotoxin and its derivatives by sodium persulfate has been studied by laser flash photolysis and pulse radiolysis. The formation and decay processes of transient species of podophyllotoxin and its derivatives have been observed, and a series of rate constants for the formation and decay of transient species have been determined. It has been found that there is a light (UV) sensitive group on the C-4 of podophyllotoxin and two light (UV) sensitive groups on the C-4 and C-4' of 4'-demethylepipodophyllotoxin.     

Kinetics of hydrated electron reactions with phosphate anions: a laser photolysis study

The decay kinetics of hydrated electron (e_aq ⁻) formed upon photolysis of aqueous solutions of sodium pyrene-1,3,6,8-tetrasulfonate at λ = 337 nm in the presence of phosphate anions (up to 2 mol L⁻¹) was studied by nanosecond laser-pulse photolysis in a wide range of pH (3.5–10) and ionic strength (I, up to 2 mol L⁻¹) values. At high pH values, where the HPO₄ ²⁻ ions dominate, the e_aq ⁻ decay kinetics depends only slightly on phosphate concentration (rate constant for the reaction is at most 2·10⁵ L mol⁻¹ s⁻¹). The H₂PO₄ ⁻ ions react with e_aq ⁻ at a rate constant of 2.8·10⁶ L mol⁻¹ s⁻¹ (I = 0), which increases linearly with the parameter in accordance with the Debye-Hückel theory. The rate constant for quenching of e_aq ⁻ by H₃PO₄ at pH ≤ 4 decreases linearly with the parameter due to the secondary salt effect and equals 1.6·10⁹ L mol⁻¹ s⁻¹ at I = 0. The logarithm of the rate constant for quenching of e_aq ⁻ by phosphates is linearly related to the number of the O-H bonds in the phosphate molecule. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1277–1280, July, 2007.