STUDIES ON THYMINE AND URACIL TRIPLET EXCITED STATE IN ACETONITRILE AND WATER

Abstract— Thymine and uracil triplet-triplet absorption spectra and triplet excited state lifetimes have been observed in acetonitrile and water by nanosecond laser flash spectroscopy. A study of triplet energy transfer from these pyrimidines to retino! has also allowed an estimation of the triplet extinction coefficient ε_TT of thymine and uracil. These ε_TT were then used to determine the triplet quantum yields Ø_T of both pyrimidines in acetonitrile and water.     

TRIPLET EXCITED STATE OF COUMARIN AND 4'5' DIHYDROPSORALEN: REACTION WITH NUCLEIC ACID BASES AND AMINO ACIDS

Abstract—The triplet-triplet absorption spectra of coumarin, 5.7 dimethoxycoumarin and the furocoumarin 4'5' dihydropsoralen. a model for 4'5' psoralen-pyrimidine mono adducts, have been determined by the techniques of pulse radiolysis and laser flash photolysis. The extinction coefficients of the triplet transitions have been measured and used to determine the singlet → triplet intersystem crossing quantum yields for 347 nm excitation in water. Reaction rate constants for coumarin and 4'5' dihydropsoralen triplets with various pyrimidine and purine nucleic acid bases, and amino acids, have been measured. Long-lived transient absorptions detected after quenching coumarin and 4'5' dihydropsoralen triplets with tryptophan are assigned to mixtures of the corresponding coumarin radical anion and the tryptophan radical cation. The spectra of the radical anions of coumarin and 4'5' dihydropsoralen were established using pulse radiolysis of the coumarins in aqueous formate. It is suggested that coumarins and furocoumarin triplets are quenched by nucleic acid bases and amino acids via a chargetransfer mechanism.     

STUDIES ON THE TRIPLET EXCITED STATE OF 4-THIOURIDINE

Abstract— The absorption spectra, lifetimes, extinction coefficients and intersystem crossing quantum yields of the lowest triplet T ₁ of 4-thiouridine have been determined both in acetonitrile and in water. An ordering of ^1,3(n,π)* and ^1,3(π,π)* states is suggested. Triplet quenching rate constants with various pyrimidine bases or amino acids are reported.     

DECAY KINETICS OF THE TRIPLET EXCITED STATE OF LUMIFLAVIN

Abstract— The rate of decay of the triplet state of lumiflavin in deaerated phosphate buffer at pH 6.9, has been studied using the flash photolysis technique, at various concentrations and flash energies. The decay of the transient appears as a mixture of first and second order processes, the relative importance of which depends on the experimental conditions.
The following competitive reactions have been found to explain the course of the reaction of the triplet excited state of lumiflavin.     

SOME PROPERTIES OF THE TRIPLET EXCITED STATE OF THE PHOTOSENSITIZING FUROCOUMARIN: 3-CARBETHOXYPSORALEN

Abstract— 3-Carbethoxypsoralen (3-CPs) has been tested in the photochemotherapy of psoriasis. It only forms monoadducts with DNA and is being considered as a non-carcinogenic alternative to 8-MOP which itself forms DNA crosslinks that arc difficult to repair. Using laser flash photolysis or pulse radiolysis, the triplet state of 3-CPs, a possible intermediate in photosensitization, has been generated in several solvents: ethanol, water and benzene. The triplet lifetime, extinction coefficient and quantum efficiency of formation have been measured. Triplet reactivities towards (i) the solvents used, (ii) 3-CPs, (iii) oxygen, (iv) tryptophan and (v) tyrosine, leading, respectively, to photoadditions with water, ethanol and 3-CPs, to ¹O₂, semioxidized tryptophan and semioxidized tyrosine, (vi) thymine and (vii) uracil have been investigated. The dark binding of 3-CPs to DNA has been studied by comparing the reactivity of e_aq^- with free 3-CPs, free DNA and the 3-CPs DNA complex. Some photophysical and photochemical properties of 4',5'di-hydro-3-carbethoxypsoralen(DH–3-CPs), model of the main fluorescent photo-product of 3-CPs, have also been investigated. Biological consequences of the photochemical properties of 3-CPs andDH–3-CPs have been studied in a cellular system (haploid yeast).     

THE TRIPLET EXCITED STATES OF BILIVERDIN AND BILIVERDIN DIMETHYL ESTER

Abstract— The triplet states of biliverdin and biliverdin dimethyl ester have been generated using pulse radiolysis excitation. Biliverdin triplet was formed by energy transfer from biphenyl triplet in acetone, absorbed throughout the wavelength range studied (380–1000 nm) and had a half-life of 11.7μs under the cpnditions chosen. Biliverdin dimethyl ester triplet was formed by energy transfer from biphenyl triplet in benzene, likewise absorbed throughout the wavelength range studied (360–1000 nm) and had a half-life of 6.7μs under the conditions used. Both biliverdin and biliverdin dimethyl ester efficiently quench anthracene, naphthacene, but not μ-carotene, triplet states. On the other hand. neither μ-carotene nor oxygen were found to quench the triplet states of biliverdin or biliverdin dimethyl ester. Estimates or limits for the rate constants of all these quenching reactions were obtained. These reactivities suggest that the triplet levels of both biliverdin and biliverdin dimethyl ester lie around 90 kJ mol^-1. The triplet energy transfer rate from bilirubin to biliverdin dimethyl ester in benzene was measured to be 1.9 × 10⁹ M^-1 s^-1. The singlet-triplet intersystem crossing efficiencies of both molecules were very low, limits of 0.004 and 0.001 being found for biliverdin and biliverdin dimethyl ester, respectively, using 347 nm laser excitation.     

FLAVIN-PHOTOREDUCTION BY CYANIDE: NUCLEOPHILIC ADDITION IN THE EXCITED TRIPLET STATE

Abstract— Flavin photochemistry as well as biochemistry consists of competitive 1e⁻ - and 2e⁻ -reduction pathways, depending on the nature of the substrate. We show that cyanide ion is a photosubstrate which suppresses 1e⁻-oxidoreduction and leads to exclusive formation of 6- and 9-cyano-1,5-dihydroflavin. The photoreduction mechanism is thus revealed as a nucleophilic addition of cyanide ion at the excited flavin triplet. Preparative photochemistry and isolation and characterization of cyanoflavins have been done, as well as thorough mechanistic studies by conventional flash photolysis. In contrast, nitrite ion is shown to be a normal photosubstrate for flavins leading to exclusive 1e⁻ -transfer followed by back donation.     

TRIPLET EXCITED STATE OF FUROCOUMARINS: REACTION WITH NUCLEIC ACID BASES AND AMINO ACIDS

Abstract. The triplet-triplet absorption spectra of psoralen, xanthotoxin, angelicin and bergapten have been determined using the technique of pulse radiolysis and laser flash photolysis in benzene and water. The extinction coefficients of the triplet transitions have been measured and used to determine the singlet → triplet intersystem crossing quantum yields for 353 nm excitation. These yields vary considerably in passing from benzene to water as solvent. The highest yields were obtained for psoralen and angelicin in water, being 0.45 and 0.33, respectively. Reaction rate constants of the psoralen and angelicin triplets with various pyrimidine and purine nucleic acid bases and amino acids have been measured. The high reactivities support the suggestion that the photosensitising properties of furocou-marins are mediated by the corresponding furocoumarin triplet states. The similarly high S → T quantum yields, and triplet reactivities with pyrimidines of psoralen and angelicin, also support the suggestion that the observed differences in photosensitising properties of these two psoralens may be due to their differing geometries, which allow psoralen itself to intercalate DNA more easily than angelicin. The efficient reactions also found for psoralen and angelicin triplets with amino acids suggest that such triplets in vivo may also react with these components. Significant damage may thus be caused to protein as well as nucleic acid components of the cell.     

ACID-BASE EQUILIBRIA OF THE EXCITED SINGLET AND TRIPLET STATES AND THE SEMI-REDUCED FORM OF ACRIDINE ORANGE

Abstract— The acid-base equilibria of the excited singlet and triplet states of acridine orange (AO) were studied by flash-photolysis and fluorometric methods. The dye is a stronger base in the first excited singlet state (pK_s= 13.3) than in the triplet and ground states (pK_r= 10.3: pK_c, = 10.2); acridine orange follows the trend observed with some other heterocyclic compounds, viz. pK_s > pK_r= pK,_c. At room temperature, an anomalous fluorescence occurs from the dye in basic media: the assignment of this emission is discussed.
The semi-reduced dye was studied as a function of pH. In a large pH range (3–14), only the protolytic equilibrium between the cationic (AOH₂⁺) and the neutral (AOH) radicals was observed; the pK value corresponding to this equilibrium was found to be in the range of pH 5–6.     

TRIPLET EXCITED STATES OF PYRIMIDINE NUCLEOSIDES AND NUCLEOTIDES

Abstract— The triplet absorption spectra, lifetimes, extinction coefficients, e_TT, and intersystem crossing quantum yields to the lowest triplet T₁, ø_T1, of thymidine, thymidine monophosphate, uridine and uridine monophosphate, have been determined in various solvents at 300 K.
The effect of H-bonding on ø_T1, of these nucleosides and nucleotides and also of uracil has been determined and discussed. This effect allows, an ordering of ^l,3 n, π* and ^1,3 π, π* states in protic and aprotic solvents.     

THE TRIPLET STATE OF 8-METHOXYPSORALEN

Abstract— Transient absorption spectra produced by laser flash photolysis of an aqueous solution of 8-methoxypsoralen (8-MOP) have been studied. The biphotonic production of hydrated electrons and of the radical ions, 8-MOP ⁺ and 8-MOP^- is reported. The hydrated electron was found to react with ground state 8-MOP with k ˜ 3 × 10¹⁰ M ^-1 s^-1. In order to obtain a true triplet-triplet absorption spectrum. contributions from the radical ions were subtracted from the overall transient absorption. In addition, contributions from e^-_aq to the transient spectrum were removed by using N₂O, low laser intensity to minimize photoionization or by measuring the transient O.D. after the electron has decayed. These three methods each produced the same triplet-triplet spectrum which differs in the red region from previously reported spectra.     

MONOFUNCTIONAL 4',5'-PHOTOCYCLOADDUCT BETWEEN 4,5'-DIMETHYLANGELICIN AND THYMINE

Abstract A new fluorescent compound has been isolated from the products of hydrolysis of DNA irradiated in the presence of 4,5'-dimethylangelicin, a monofunctional photosensitizing furocoumarin. The marked similarity of the UV absorption and fluorescence spectra of this photoproduct to those of 4',5'-dihydro-4,5'-dimethylangelicin, as well as its behaviour on photodissociation (254 nm) yielding thymine and 4,5'-dimethylangelicin in equimolecular amounts, are consistent with a cycloadduct between one molecule of 4,5'-dimethylangelicin and one of thymine, and the C₄-cycloaddition occurs through 4',5'–double bond of the furocoumarin and the 5,6-double bond of the pyrimidine.     

QUENCHING OF OXONINE EXCITED STATES BY EDTA VIA ELECTRON TRANSFER AND DEACTIVATION PROCESSES. A COMPARATIVE STUDY OF EXCITED SINGLET AND TRIPLET STATE REACTIVITY

The quenching of excited singlet oxonine by EDTA in aqueous solution leads mainly to deactivation of the dye to the ground state and, to a lesser extent, to electron abstraction. The rate constants for these processes have been measured and compared to those for the same reactions involving the oxonine triplet state. The rate constant of electron abstraction is about ten times greater via the singlet state than via the triplet state. However, the rate constant of deactivation to the ground state is 10³-10⁴ times greater for the excited singlet state than for the triplet state, so that the efficiency of electron transfer is much smaller for the singlet state.     

EXCITED SINGLET AND TRIPLET PRODUCTS GENERATED FROM OXIDATION OF THE -CO-CH- CONTAINING MOLECULES

Abstract— In a previous study. we found indirect chemiluminescence from air oxidation of various simple ketones in alkaline aprotic solvents containing 9,10-diphenylanthracene and 9,10-dibromoanthracene. By summarizing the experimental results. it was concluded that excited products. mainly in the triplet state. were generated from air oxidation of those simple compounds having a -CO-CH-group via dioxetane intermediates.
Recently. we have succeeded in observing an intense direct chemiluminescence due to the generation of the excited singlet product by the air oxidation of such anthracene derivatives having a -CO-CH-group as 9,10-diisobutyrylanthracene and related compounds. The finding lends strong support to the hypothesis that dioxetane or dioxetanone may be the critical intermediate to give fluorescent singlet products.     

PHOTOTAUTOMERISM OF HARMALINE AND HARMALOL IN THE EXCITED SINGLET STATE

Unusual behaviour of harmaline and harmalol by excited state proton transfer has been studied and the acidity constants for the different prototropic equilibria in the ground and excited singlet states have been reported.     

THE LOWEST EXCITED TRIPLET STATES OF CODEINE AND MORPHINE AND OF THEIR MOLECULAR SUBUNITS VERATROLE AND GUAIACOL

Abstract— The lowest excited triplet T₁ states of the two alkaloids, morphine and codeine, have been investigated by ODMR and emission spectroscopy. The electronic character of their T₁ states could be determined by comparing the T₁ properties with those of the constituent parts guaiacol and veratrole. The T₁ properties, e.g. phosphorescence spectra, zero-field splitting parameters and triplet sublevel selective decay rates are very similar for the constituent parts and the alkaloids. Thus, in analogy to veratrole and guaiacol, the T₁ state of codeine and morphine is assigned to a ππ* state mainly localized on the benzene ring and the attached alkoxy and hydroxy groups. There is no evidence for a contribution from the other subunits, trialkylamine and unsaturated alcohol, to the T₁ state of codeine and morphine.     

碘鎓盐/胺体系基态和激发态的光谱研究

研究了碘鎓盐与胺在基态和激发态的相互作用,观察到它们形成以鎓盐为电子受体和胺为电子给体的基态和激发态电荷转移络合物。测定了二苯基碘鎓盐和三乙胺形成的络合物组成为1:1分子比,其缔合常数和克分子吸收系数分别为2.2×10mol^-1和2.8×10³mol^-1·1·cm^-1。实验结果表明,随着胺的给电性增加,电荷转移络合物的吸收波长红移,胺的空间体积增大,由于空间障碍,电荷转移络合物的克分子吸收系数下降。