ACTINOMETRY IN MONOCHROMATIC FLASH PHOTOLYSIS: THE EXTINCTION COEFFICIENT OF TRIPLET BENZOPHENONE AND QUANTUM YIELD OF TRIPLET ZINC TETRAPHENYL PORPHYRIN

Abstract— The extinction coefficient ε_T, of triplet benzophenone in benzene has been directly determined by absolute measurements of absorbed energy and triplet absorbance, Δ D ⁰_T, under demonstrably linear conditions where incident excitation energy, E ₀, and ground state absorbance, A ₀, are both extrapolated to zero. The result, 7220 ± 320 M ^-1 cm^-1 at 530 nm, validates and slightly corrects many measurements relative to benzophenone of triplet extinction coefficients made by the energy transfer technique, and of triplet yields obtained by the comparative method.
As E ₀ and A ₀ both decrease, Δ D ⁰_T becomes proportional to their product. In this situation, the ratio R = (1/ A ₀)(dΔ D ⁰_T/d E ₀) = (ε_T - ε_G)φ_T. Measurements of R , referred to benzophenone, give (ε_T - ε_G)φ_T for any substance, without necessity for absolute energy calibration.
Both absolute and relative laser flash measurements on zinc tetraphenyl porphyrin (ε_T - ε_G at 470 nm = 7.3 × 10⁴ M ^-1 cm^-1) give φ_T= 0.83 ± 0.04.     

TRIPLET LIFETIMES OF SOME FLAVINS

Abstract— Using flash photolysis the rate constants of the triplet decay at pH 7.0 (26°C) of 10-hydroxyethyl-isoalloxazine (2700 s⁻¹), 2'-deoxyRF (2300 s⁻¹), 5'-deoxyRF (3200 s⁻¹), 8-hydroxyRF (18 000 s⁻¹) and 8-aminoRF (28 000 s⁻¹) have been measured. The results agree with other evidence that photochemical properties of flavins are influenced by the interaction of the sidechain with the isoalloxazine nucleus. In addition, our data on RF and FMN indicate that the triplet decay rates given in the literature for these compounds have to be corrected to 3200 s⁻¹ (RF) and 4900 s⁻¹ (FMN), respectively. The rate constants for the quenching of the triplet by ground state molecules for all above compounds are given.     

THE TRIPLET AND RADICAL SPECIES OF HAEMATOPORPHYRIN AND SOME OF ITS DERIVATIVES

Abstract— Nanosecond laser flash photolysis and pulse radiolysis have been used to generate and characterise the triplet state, and semioxidised and semireduced radicals of haematoporphyrin, and three 0 -acyl compounds derived from it (the monoacetate, the diacetate and the disuccinate).
After 347 nm irradiation in water containing 2% Triton X-100, haematoporphyrin forms the triplet state (φ_T= 0.92) and photoionises monophotonically (φ_I= 0.03). For the O -acyl derivatives, φ_T approaches unity and photoionisation is reduced. In acetone the triplet yield of all four compounds are close to unity. The difference and corrected spectra for the triplet species are presented and decay rates ( k ₁˜10⁴s^-1) and oxygen quenching constants ( k _Q˜1.5times10⁹ M ^-1s^-1) for the triplet state have been measured. The difference and corrected spectra for the semi-reduced species in methanol and semi-oxidised species in aqueous Triton X-100 are presented.
The photophysical characteristics in fluid solution of haematoporphyrin and its 0 -acyl derivatives are rather similar to those previously recorded for other photosensitising porphyrins.     

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).     

ELECTRON PARAMAGNETIC RESONANCE OF SOME TRIPHENYLMETHANE DYES IN THE TRIPLET STATE

Abstract— Electron paramagnetic resonance spectra of the triplet states of several triphenylmethane dyes in glassy solutions at 90 K have been measured and their zero-field splitting parameters estimated. Crystal violet and para rosaniline do not possess trigonal symmetry in their triplet states, and the unusually broad absorptions in the Δ M ₈= 1 region of the spectra have been attributed to the presence of different rotational isomers of the dye cations. A number of malachite green derivatives were investigated, but absorption of the triplet states of these dyes was only observed in the low field Δ M ₈= 2 region of the spectrum.     

THE FLUORESCENCE OF CAROTENOIDS

Abstract The carotenoids β-carotene, lutein, lutein epoxide and violaxanthin are fluorescent compounds. Upon excitation around 280 mμ, at wavelengths close to those of their ultraviolet absorption maxima, these carotenoids fluoresce within the range 300–400 mμ, with maximal fluorescence at approximately 320–340 mμ.     

SPECTRAL EVIDENCE FOR PHOTO-INDUCED ISOMERIZATION OF CAROTENOIDS IN BACTERIAL PHOTOREACTION CENTER

Abstract The aim of this work was to determine whether spirilloxanthin and sphaeroidene bound to the same site of the photoreaction center isolated from Rhodospirillum rubrum can display cis–trans isomerization under conditions that lead to formation of triplet state P^R of the primary electron donor. To this end, we monitored changes in the absorption spectrum of these bound carotenoids as induced by red light at low redox potentials. This experiment was performed both with the intact photoreaction center isolated from strain Sl and with the photoreaction center isolated from carotenoidless strain G9 reconstituted with either spirilloxanthin or sphaeroidene. In both preparations, spirilloxanthin exhibited light-induced absorption changes that can be interpreted as a cis–trans isomerization. Under our experimental conditions, the absorption changes attained their full extent in about 1 min and were not or were only partially reversed when the light was switched off. Under our experimental conditions, the extent of these changes indicate that about 15% of the bound spirilloxanthin undergoes isomerization. Sphaeroidene artificially attached to the G9 photoreaction center also undergoes light-induced absorbance changes, but these cannot easily be interpreted as a cis–trans isomerization.     

QUANTUM YIELDS OF TRIPLET FORMATION OF SOME DERIVATIVES OF ANTHRAQUINONE

Abstract— Quantum yields of triplet formation for five amino substituted anthraquinones have been determined by the comparative actinometer method using laser flash photolysis. Substitution reduces the yields to values of ˜10^--². and the requirement of low laser intensities required high sensitivity of detection of triplet absorption. The ø_T values are compared with the quantum efficiencies of fluorescence and decomposition for the compounds, and the criteria for light stability discussed.     

Cis/trans ISOMERIZATION OF CAROTENOIDS BY THE TRIPLET CARBONYL SOURCE 3-HYDROXYMETHYL-3,4,4-TRIMETHYL-1,2-DIOXETANE*

Abstract— The interaction of biological carotenoids with 3-hydroxymethyl-3,4,4-trimethyl-1,2-dioxetane (HTMD), a thermodissociable source of electronically excited ketones, was investigated using reversed-phase high-performance liquid chromatography. Incubation of the all-trans isomers of β-carotene, lycopene and canthaxanthin with HTMD led to significant trans-to-cis isomerization, with cis isomers accounting for 20–50% of products formed (the balance assigned as oxidation products). The isomers forming from all-trans-β-carotene were identified as 9-cis-, 13-cis- and 15-cis-β-carotene by cochromatography of cis isomer standards and by on-line diode array absorbance spectroscopy. An HTMD-dependent cis-to-trans isomerization was observed in incubations started with 15-cis-β-carotene, and it occurred more rapidly and to a greater extent than the isomerization of all-trans-β-carotene. The isomer patterns generated from lycopene and β-carotene are generally similar to those reported recently for various human tissues (Stahl et al, 1992, Arch. Biochem. Biophys. 294 , 173–177).     

DETERMINATION OF EXTINCTION COEFFICIENTS OF OAT PHYTOCHROME BY QUANTITATIVE AMINO ACID ANALYSES

Abstract— Previously published extinction coefficients for phytochrome were based on indirect colori-metric estimates of protein weight, mostly by the Lowry method, using bovine serum albumin as a standard. This paper reports revised values based on quantitative amino acid analyses of highly purified phytochrome. The molar extinction coefficient of the red-absorbing form of phytochrome with 120000mol. wt subunits was found to be approximately 102000cm"₁ per 120000moi. wt at 667nm. The molar extinction coefficients at 667 nm for phytochrome with 120000 mol. wt subunits and for the 60000 mol. wt chromopeptide of P_r were found to be approximately the same for samples of equivalent purity. Included in the amino acid analyses of phytochrome with 120000 mol. wt subunits are values for cysteine (11) and cystine (1), measured by two different techniques that gave the same results and agreed well with the estimate of total half-cystine (14) obtained by an independent method. Tryptophan was quantitated by analysis of a p-toluene sulfonic acid hydrolysate.     

TRIPLET STATES OF CAROTENOIDS BOUND TO REACTION CENTERS OF PHOTOSYNTHETIC BACTERIA: TIME-RESOLVED RESONANCE RAMAN SPECTROSCOPY

Time-resolved, low-temperature resonance Raman spectra of triplet states of the carotenoids specifically present in bacterial reaction centers in a strained cis conformation have been obtained, thus demonstrating the possibility of studying intermediate transient states of these structures using resonance Raman spectroscopy. Resonance Raman spectra of triplet cis spheroidene and cis methoxyneurosporene present in reaction centers of Rhodopseudomonas spheroides, (strains 2.4.1. and Ga, respectively) exhibit marked differences with those of triplet, all- trans carotenoids previously studied in vitro. These differences, together with the frequency shifts measured for the v ₁ modes, indicate that triplet carotenoids bound to reaction centers retain a cis conformation, and that probably no isomerization occurs to all- trans carotenoids upon T ← S₀ excitation. P_i electron distributions along the polyene backbone are probably less regular in the triplet state than in the singlet ground state, although probably not to the extent suggested by previous theoretical calculations. The apparently anomalous behaviour of the v ₂ bands of all- trans carotenoids upon T ← S₀ excitation is shown to result largely from the actual complexity of this region of the Raman spectra, together with a weak participation of the v _c—–c internal coordinate in the corresponding modes. Finally, the Raman scattering efficiency of triplet spheroidene bound to reaction centers is lower than that of the singlet, ground state form, under equivalent excitation conditions.     

某些单体-单体、单体-二聚体体系的活度系数

用气液色谱法测定了苯在二苯醚、联苄、二苄醚、二苯甲烷、二苯乙炔、二苯甲酮、联苯、二苯胺、α-甲基萘、α-氯萘、α-溴萘、氮萘、α-硝基萘、α-萘胺等十四种溶剂中的无限稀活度系数;用单体-二聚体的溶液模型对以上体系进行活度系数的计算,计算值与实验值接近。 用气液色谱法测定了甲苯、乙苯、邻二甲苯、对二甲苯、间二甲苯等五种溶质在环丁砜、二甲基环丁砜、N-甲基吡咯烷酮,5,N-二甲基吡咯烷酮等四种溶剂中的无限稀活度系数;用单体-单体的溶液模型计算溶剂的“溶解度参数”,计算得各溶剂的“溶解度参数”分别为一常数。     

TRIPLET STATES OF CAROTENOIDS FROM PHOTOSYNTHETIC BACTERIA STUDIED BY NANOSECOND ULTRAVIOLET AND ELECTRON PULSE IRRADIATION

Abstract— Absorptions of the triplet excited states of five carotenoids (15,15'-ds phytoene, all- trans phytoene, C-carotene, spheroidene and spirilloxanthin), extracted from the photosynthetic bacteria Rhodopseudomonas spheroides and Rhodospirillum rubrum, have been detected in solution using pulse radiolysis and laser flash photolysis. Triplet lifetimes, extinction coefficients, lowest energy levels and quantum efficiencies of formation have been determined. Comparison of the carotenoid triplet energy levels with that of O₂('Δ_g) suggests that spirilloxanthin, spheroidene and possibly alsoζ-carotene, would be expected to protect against photodynamic action caused by O₂ ('Δ_g), but not cis or trans phytoene. The S → T intersystem crossing efficiences of all five polyenes were found to be low, being a few per cent or less. In their protective role these triplet states can only therefore be effectively reached via energy transfer from another triplet, except in the case of O₂ ('Δ_g). The low crossover efficiencies also mean that light absorbed by such carotenoids in their possible role as accessory pigments would not be wasted in crossing over to the triplet state.     

某些香豆素酮类化合物三重态最低能级特性的判别

芳酮类敏化剂的最低激发三重态的特性对其光还原反应的能力往往起着决定性的作用。本文通过对五种香豆素酮类化合物发射光谱行为的研究,明确了它们激发三重态(T₁)的特性,指出化合物3-苯甲酰基香豆素、对苯二甲酰基二香豆素和3,3-二香豆素甲酮的T₁态主要为n,n^*特性,而3-呋喃甲酰基香豆素及3-噻吩甲酰基香豆素的T₁态具有n,n^*和π,π^*混合的特性。     

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.     

THE INFLUENCE OF THE NITRO GROUP ON THE DISSOCIATIONS OF SOME AROMATIC ACIDS AND BASES IN THEIR LOWEST EXCITED TRIPLET STATES*

Abstract— –Estimation of lowest excited triplet and singlet state dissociation constants of some nitro-aromatic acids and bases, from shifts in their phosphorescence and absorption spectra, respectively, indicate that intramolecular charge transfer to the nitro group is much more important in the lowest excited singlet state than in the ground or lowest excited triplet states. As a result, the effect of a nitro group on the acidity of the lowest excited singlet state of an acid or base is more exaggerated than that on the ground or lowest excited triplet state of the same compound. Furthermore, the basicity of the nitro group is greatly enhanced in the lowest excited singlet state. On this basis the increased rate of photoreduction of nitrobenzene in acidic solutions is found to be thermodynamically unfeasible in the lowest excited triplet state. Although the reaction is thermodynamically feasible in the lowest excited singlet state, the short lifetime of that state may make the reaction kinetically unfeasible. Rate-Hammett acidity profiles are therefore inadequate to alone establish the mechanism of photoreduction of nitrobenzene.     

THE EFFECT OF COORDINATING LIGANDS ON THE TRIPLET STATE OF CHLOROPHYLL

Under laser excitation at 457.9 and 514.5 nm, a frozen solution of chlorophyll a in n -octane displays fluorescence peak maxima at 2K that may be assigned to two distinct monomeric chlorophyll species. Using zero-field fluorescence-detected magnetic resonance the triplet state properties of the two chlorophyll species have been assigned to the monoligated and biligated chlorophyll monomer in which water serves as the ligand coordinated to the magnesium metal center. These triplet state properties for chlorophyll in solution are then utilized in interpreting triplet state results for in vivo chlorophylls associated with the light harvesting chlorophyll protein complex. It is shown that the triplet state data are consistent with attachment of the chlorophyll molecule to the protein site with a single ligand coordinated to the chlorophyll metal center.