PHOTOCHEMISTRY OF FLAVINS—I. CONVENTIONAL AND LASER FLASH PHOTOLYSIS STUDY OF ALLOXAZINE

Abstract— The photobleaching of alloxazine in buffered aqueous solution has been studied by means of flash photolysis using conventional and laser excitation sources. Several transient species have been characterized. The alloxazine triplet state (Λ_max 420 nm and 550 nm, times; = 9 μs) was identified with the aid of low-temperature comparison experiments in ethanol. Transient absorption with Λ_max 440 nm, which appears after decay of the triplet state, and whose second-order decay is pH-dependent, is postulated to be due to the semiquinone radical (AH₂*) and a radical derived from alloxazine by addition of water and loss of a hydrogen atom (HAOH*), which are in equilibrium with their conjugate cation radicals. The results of experiments in the presence of oxygen indicate that these species are not primarily formed from the triplet state. The enhanced second-order decay of the flavin radicals in oxygen-containing solutions is interpreted in terms of their reaction with the peroxy radicals. The proposed mechanisms account for the production of hydroxylated alloxazines.     

LUMINESCENCE AND PHOTOCHEMISTRY OF 4-THIOURIDINE IN AQUEOUS SOLUTION

Abstract— In bidistilled water, 4-thiouridine (4TU) exhibits a weak unusual luminescence, the quantum yield of which is 3 × 10^-4 at 25°C. The excitation spectrum corresponds well to the 4TU absorption spectrum. The emission lies at longer wavelengths (Λ_max 550 nm) than the 4TU phosphorescence observed at 77 K (Λ_max, 470–480 nm). From the emission signal obtained after an excitation flash of 3 ns half-width, an "apparent" rate constant for the radiative deactivation process, shorter than 5 × 10⁶ s, can be inferred. The 300 K emission is efficiently quenched by halides and by oxygen: quenching involves a long-lived intermediate (⋍ 200 ns).
Clearly the emissive state X is populated through the S₀-S₁ electronic transition π→π* of 4TU. The nature of X cannot be unambiguously determined: it cannot be an excimer but can be either the 4TU triplet state or another chemical state distinct from the 4TU excited singlet or triplet states.
An interesting finding is that the 300 K emission and the ability of 4TU to photoreact are related: they are quenched with the same efficiency by halide anions. This indicates that quenching occurs at the same long-lived intermediate species , which is either a precursor of the emitter or the emitter itself.     

A FLASH PHOTOLYSIS STUDY OF 1-METHYLINDOLE

Abstract— The conventional flash photolysis of 1-methylindole in aqueous media was studied at Λ_excitation≥290 nm. The transients observed 20 μs after excitation consisted mainly of the radical cation (R⁺). the hydrated electron (e^-_aq) and the triplet state (T). Electron counting experiments indicate that photoionization is the only source of R⁺ with e^-_aq/R⁺= 1.07±0.09 in neutral media. Quenching of the R⁺ yield with H⁺ indicates that the fluorescent state is the precursor to 80% of the photoionization events with the remainder probably arising from a prefluorescent state. The triplet decays with a lifetime of 29 μs in deaerated neutral media. This decay is unchanged by N₂O saturation, but T reacts with acrylamide with k ≥2.8 × 10⁹ M ^-1. In 2 M Br^-, R⁺ and T yields are increased by factors of 2–3. Consideration of fluorescence quenching and T enhancement by Br-permits an estimate of φ_Isc between 0.33 and 0.49. The increased R⁺yield at high Br-concentrations cannot be accounted for by induced photoionization or triplet state reactions.     

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.     

ON THE PHOTOIONIZATION ENERGY THRESHOLD OF TRYPTOPHAN IN AQUEOUS SOLUTIONS

Abstract— To investigate the existence and energy position of a photoionization threshold. tryptophan (Trp) has been photoionized in desecrated neutral aqueous or alcoholic solution under monochromatic light of variable frequency, in presence of N₂O to scavenge the photoelectron.
Present findings and some literature data converge to show the existence of a threshold for the one photon ionization process. This threshold is located at 4.5 ± 0.1 eV and 4.85 ± 0.1 cV for Trp in aqueous and ethanol solutions. respectively, which corresponds to a lowering with respect to the gas phase ionization potential of 3.4 and 3.0 eV.
The photoionization quantum yields for Trp is found about 4 times greater at 250 nm than at Λ_cx= 265 nm, where φe^-_4M=0.080±0.025. In such spectral range. at most one photoelectron out of 4–5 escaping geminate recombination would lead to Trp photodegradation in acrated solutions.
These results also point out that the neutral radical Trp. which has been previously observed for Λ_cx > 275 nm, i.e. below the ionization threshold energy—would not necessarily derive from Trp ⁺ deprotonation or cation-electron dissociative recombination. Similarly, the opening of the indole ring with formylkynurenine (FK) formation which is observed under aerobic conditions and Λ_cx >, 280 nm would not imply an electron attachment on O₂ but reactions such as Trp +³O₂ or Trp*+³O₂ or else
     

265 NM LASER FLASH PHOTOLYSIS OF SOME ORTHO-SUBSTITUTED ANILIDES AND RELATED N-FORMYLKYNURENINE DERIVATIVES

Abstract— The physical and chemical properties of the triplet state of eight ortho-substituted anilides including N -formylkynurenine (FK), the major trp UV-photooxidation product and a remarkable photodynamic agent, have been investigated using both pulse radiolysis and 265 nm laser flash photolysis techniques. The molar extinction coefficient, the intersystem-crossing quantum yield and the oscillator strength of the T ₁→ T _n absorption band (Λ_max˜ 450 nm) have been determined. It is shown that anilides having n,π* triplets readily react with most solvents whereas those having π ,π* triplets slowly react with alcohols. In both cases, the semi-reduced species are formed. In water, the formation of the semi-reduced. species most probably involves the first excited singlet state. The triplet state properties of the FK derivatives (i.e. ortho-substituted anilides having a side chain bearing charged groups such as carboxylic or amino groups) are strongly modified by the ionization state of the charged side chain. In the case of the FK derivatives possessing an uncharged amino group, quenching of the triplet state occurs via a fast reversible electron transfer reaction from the NH₂ to the triplet anilide.     

A PULSE RADIOLYSIS and PULSED LASER STUDY OF THE VITAMIN D3 TRIPLET STATE: LIFETIME, RELAXATION and 'NONVERTICAL'EXCITATION

Abstract— The triplet state of vitamin D3 in benzene has been characterised in terms of its absorption spectrum, Λ_max 315 nm, its lifetime, 300 ns, its rate constant for reaction with oxygen, 4.2 times 10⁹mol⁻¹ s⁻¹ and the efficiency with which it sensitizes the formation of singlet oxygen, 25%. There is a large difference in the electronic excitation energies of the spectroscopic and relaxed triplets, ˜ 237 kJ mol⁻¹ and147–168 kJ mol⁻¹, respectively. It has been shown that, in the endothermic situation, the vitamin D3 molecule is a 'nonvertical'acceptor of triplet energy. This is in accord with the non-planar character of its acyclic conjugated Il-system.     

REFLECTANCE SPECTROSCOPY FOR THE CHARACTERIZATION OF PHOTOCHROMISM IN THE CRYSTALLINE STATE

Abstract— The Kubelka-Munk theory for diffuse reflectance has been applied to a quantitative study of photochromism in the crystalline state. For three systems investigated it was found possible to assign first order rate constants to the thermal relaxation process and estimate the pre-exponential factor A and the activation energy E_a in Arrhenius equation. For the fading of the red photocolored form, Λ_max=490 mμ, of benzaldehyde phenylhydrazone A = 1.4×10⁸ min^-1 and E_a= 15.7 kcal mole^-1. For the fading of the blue photocolored form, Λ_max=590 mμ, of 2–(2,4-dinitrobenzyl)pyridine A= 5×10¹⁴ min^-1 E_a =23.3 kcal mole^-1, Cinnamaldehyde semicarbazone showing 'reversed phototropy' has a photoactivated state, Λ_max=400 mμ, which in dark is transformed into a strongly absorbing yellow species, Λ_max= 430 mμ with A = 14 × 10¹⁰ min^-1 and E_a= 18.7 kcal mole-¹.     

QUANTIFICATION OF THE SOLVENT EFFECTS ON THE TRIPLET QUANTUM YIELD OF PSORALEN BY THE "LINEAR SOLVATION ENERGY RELATIONSHIP"

Triplet formation quantum yields (Φ_τ) of psoralen in a set of 17 pure solvents ranging from n -hexane to water and in dioxane: water mixtures were obtained by nanosecond laser flash photolysis. The triplet yield increases with solvent polarity. The extremum values are 0.009 and 0.545 in n -hexane and water, respectively. Good correlations of the experimental Φ_τ values with empirical "polarity" scales (Dimroth/Reichardt's E_T [30], Kamlet/Taft's solva-tochromic parameters β, and α, and Swains acity/basity A_S/B_S) were obtained: Ln(φ_T^-1 - 1) = 8.86 - 0.143E_T(30) Ln(φ_T^-1 - 1) = 4.40 - 2.34_τ - 1.70α Ln(φ_T^-1 - 1) = 4.65 - 3.72A_s - 1.12B_s The results are discussed in terms of the sensitivity of psoralen triplet quantum yield to solvent polarity and hydrogen-bonding abilities.     

THE CHARACTERISATION OF THE TRIPLET STATE OF CROCETIN, A WATER SOLUBLE CAROTENOID, BY NANOSECOND LASER FLASH PHOTOLYSES

Abstract— The triplet state of crocetin, which is a water soluble carotenoid, has been sensitized by psoralen. The triplet extinction coefficient, ε_T (73000 dm³ mol^-1 cm^-1 at 470 nm), the triplet-triplet spectrum and the quantum yield of triplet formation, φ_T (less than 1%) are reported in aqueous solution.
In order to calculate the extinction coefficient of crocetin it was necessary to obtain ε_T for psoralen in water (10000dm³ mol^-1 cm^-1 at 450 nm). This latter value was obtained using the complete conversion technique and is reported with the triplet-triplet spectrum.     

POTENTIAL PHOTOSENSITIZERS FOR PHOTODYNAMIC TUMOR THERAPY—I. PHOTOPHYSICAL PROPERTIES OF TWO CHLORIN DERIVATIVES

Abstract— Photophysical properties of two chlorin type molecules (CHLI) and (CHLII) were investigated in different solvents. Quantum yields of fluorescence φ_F of S, → T, intersystem crossing φ_T, and of singlet oxygen (¹Δ_g) formation φ_Δ, as well as the Stern-Volmer constants for the quenching of the S, states by oxygen and the bimolecular rate constants of quenching of ¹Δ_g by the chlorins were measured. The values of φ_T and φ_Λ can be given as 0.57 and 0.58 for CHLI and 0.69 and 0.58 for CHLII. The values of the fluorescence quantum yields, the strong absorption of the chlorins in the red (Λ > 630 nm) and the high values of the quantum yields for ¹Δ_g formation recommend the chlorin derivatives as potential markers and photosensitizers for tumor therapy.     

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.     

A LASER FLASH PHOTOLYSIS STUDY OF PYRENE-1-ALDEHYDE. INTERSYSTEM CROSSING EFFICIENCY, PHOTOREACTIVITY AND TRIPLET STATE PROPERTIES IN VARIOUS SOLVENTS

Abstract— Triplet-and singlet-related photoprocesses of pyrene-1-aldehyde (PA) in various solvents have been investigated in detail using 337.1 and 355 nm laser flash photolysis in conjunction with time-correlated determination of fluorescence lifetimes (τ_F) and steady-state photochemical and absorption-emission spectral measurements. In benzene, the lowest triplet of PA (43 < E_T < 46 kcal/mol) has a lifetime of about 50 µs (τ_T) and displays the absorption maximum at 443 nm with a maximum extinction coefficient (ε_max) of 21000 M ^-1cm^-1; the corresponding ketyl radical has a sharp absorption maximum at 428 nm (ε_max≥ 25000 M ^-1cm^-1). The quantum yields (φ_T) of lowest triplet occupation are high in nonprotic solvents (0.6–0.8), decrease in protic solvents (alcohols) as the polarity of the latter is increased, and maintain a complementary relationship with the quantum yields (φ_F) of fluorescence. Quantum yields (φ_PC) of loss of PA due to photoreactions in some solvents have also been determined under conditions of steady irradiation at 366 nm; φ_PC is in the range 0.1–0.2 in electron-rich olefinic solvents such as cyclohexene and tetramethylethylene. These results concerning τ_F, τ_T, φ_F. φ_T and φ_PC as well as the effects of 1,2,4-trimethoxybenzene and 2,5-dimethyl-2,4-hexadiene as quenchers for fluorescence, triplet yield, and photochemistry are discussed in the light of possible state orders for PA in polar and nonpolar environments.     

TRIPLET STATES AND cis-trans PHOTOISOMERIZATION PROCESSES IN THE SCHIFF BASES OF RETINAL ISOMERS

Abstract —The triplet states of the n -butyl-amine Schiff bases of 11- cis , 9- cis , 13- cis and all- trans retinal are produced via triplet-triplet energy transfer. Their absorption spectra, peaking around 435 nm, and their decay kinetics are recorded using pulsed-laser photolysis. Direct-excitation (φ^D_ISO) and triplet-sensitized (φ^T_ISO) photoisomerization yields, determined using steady irradiation methods, are found to be: φ^T_ISO (9- cis ) = 0.06, φ^T_ISO (11- cis ) = 045, φ^T_ISO (13- cis ) = 008, φ^T_ISO (all- trans ) = 0.02-0.05, φ^D_ISO (11- cis , = (4 ± 1) × 10^-3, φ^D_ISO (all- trans ) = (2 ± 1) × 10^-3. The possible role of the triplet state in the isomerization of rhodospin is discussed.     

Photophysical properties of the lowest excited singlet and triplet states of thio- and seleno-psoralens

The decay processes of the lowest excited singlet and triplet states of five heteropsoralens (HPS) were investigated by steady-state and shift-phase fluorometry and by laser-flash photolysis in different solvents. The emission spectra of HPS are detectable only in trifluoroethanol (TFE), where fluorescence lifetimes (τ_F) and quantum yields (φ_F) were measured. The triplet lifetimes (τ_T), triplet (φ_T) and singlet-oxygen production (φ_Δ) quantum yields were determined in benzene, ethanol and TFE by laser-flash photolysis. Semiempirical (INDO/1-CI) calculations allowed the nature of the lowest excited singlet and triplet states and transition probabilities to be obtained. Theoretical and experimental results indicate that the two lowest excited singlet states S₁ and S₂ of HPS are close-lying and different in nature (π,π* and n,π*). The "proximity effect" between these two states controls the photophysical properties of HPS as it does for the other furocoumarins. However, HPS have a peculiar behavior with respect to the related compounds because they are fluorescent and have, in three cases, detectable intersystem crossing only in TFE. This behavior can be tentatively explained by a different energy gap and/or order between the S₁ and S₂ states.     

AN EPR STUDY OF THE SPECIES PRODUCED DURING THE UV PHOTOLYSIS OF HETEROCYCLIC COMPOUNDS IN METHYLTETRAHYDROFURAN AT 77 K

Abstract —The ultraviolet irradiation (290 nm ≤Λ≤ 390 nm) of indole, purine, indazole, acridine and quinoline in 2-methyltetrahydrofuran glass at 77 K produces trapped radicals. Two electron-paramagnetic-resonance (EPR) signals are found at 77 K during illumination, one at high magnetic field (3–25 times 10^-1 T) assigned to the matrix radical and the other at low field (1.3 times 10^-1 to 1–5 times 10^-1 T) attributed to the lowest triplet state of the heterocyclic molecule. Quantum yields for triplet production at 77 K are 0–34 for indole, 0.51 for purine, 0.55 for indazole, 0.15 for acridine, and 0.94 for quinoline. The rate of formation of matrix radicals varies as the n _R^th power of the incident light intensity, I ₀^nR, where 1.6 ≤ n _R=≤ 2. Solvent radical yields, which depend on the light intensity, have been determined. Under the experimental conditions, no signals attributable to trapped electrons or cations have been observed. The dependence of the reciprocal value of the rise lifetime of the low field EPR signal as a function of the intensity of exposure is in accordance with a biphotonic mechanism.     

SPECTRAL PROPERTIES OF THE RHODOPSIN-SYSTEM OF THE CRAYFISH Astacus leptodactylus

Abstract— The absorption spectra of the membrane-bound and of the digitonin-solubilized visual pigment of crayfish Astacus leptodactylus were investigated by conventional spectrophotometry. A method was developed to isolate purified rhabdoms almost entirely free from screening pigments from a single retina. The quantity of isolated and purified rhabdoms from a single retina was sufficient to measure the absorption spectra of the visual pigment.
The absorption spectra of the chromoprotein system (R and M) show that both the membrane-bound and the digitonin-solubilized visual pigment isomers are stable at 0°C and pH 7.0. Rhodopsin and metarhodopsin are photoreversible under these conditions without any light-induced denaturation. The difference spectra for the chromoprotein isomers and those of different photostationary states yield maximal values for ΔE at 570 and 485 nm.
At neutral pH, 0°C, Λ_max of rhodopsin is 530 nm. Irradiation with light of Λ= 630 to 640 nm isomerizes rhodopsin nearly quantitatively to metarhodopsin with Λ_max, of 500 nm. The molar extinction coefficient of metarhodopsin is greater than that of rhodopsin by a factor of ˜ 1.41. each measured at its respective Λ_max Metarhodopsin can be isomerized to rhodopsin by irradiating at Λ > 630 nm. As the absorption spectra of the two chromoprotein isomers overlap, only part of the metarhodopsin can be reversed to rhodopsin. The maximal photoreversion can be achieved by irradiating at 460 nm. The stability of the digitonin-solubilized chromoprotein is remarkably dependent on temperature. Warming the digitonin extract of rhabdoms from 0 to 20 or 30°C caused a shift of the rhodopsin spectrum to shorter wavelengths (Λ_max= 485 nm) accompanied by a decrease of E_Λmax by about 30%.     

FLUORESCENCE QUANTUM YIELDS OF 124-kDa PHYTOCHROME FROM OAT UPON EXCITATION WITHIN DIFFERENT ABSORPTION BANDS

Abstract— A fluorescence quantum yield (emission at650–850 nm) of π= (2.3 ± 0.3)10⁻³ was measured for the red-absorbing form (Pr) of 124-kDa phytochrome from etiolated oat seedlings ( Avena sativa ) upon excitation in the Soret band at Λ^exc= 380 nm. The small difference between this value and the previously determined quantum yield with Λ^exc= 640 nm, π= (3.5 ± 0.4)10⁻³is attributed to a blue-absorbing emitter responsible for the "anomalous" or "blue" emission of the chromoprotein in the region from ca. 400 to 550 nm. The absorption of Pr at 380 nm is consequently somewhat lower than that measured directly from the spectrum. Processes from upper excited states of the Pr phytochromobilin-derived chromophore other than rapid relaxation to the emitting state are not important. A quantum yield of Φ ' 1.2 times 10⁻³ is estimated for the blue fluorescence. The proportion of the blue emitters relative to Pr appears to be relatively high.     

FLUORESCENCE AND THE LOCATION OF TRYPTOPHAN RESIDUES IN PROTEIN MOLECULES

Abstract— Fluorescence spectra of a number of native and denaturated proteins have been analysed, using spectral band width (ΔΛ), spectral maximum position (Λ_m), fluorescence quenching by external ionic quenchers, lifetime (b), and quantum yield ( q ) and its changes upon denaturation. The results enabled a model of fluorescence properties of tryptophan residues in the proteins to be substantiated by considering the existence of three discrete spectral classes, one buried in nonpolar regions of the protein (Λ_m 330–332 nm, ΔΛ= 48–49nm, q 0.11, τ= 2.1 ns) and two on the surface. One of the latter is completely exposed to water (Λ_m# 350–353 nm, ΔΛ= 59–61 nm, q # 0.2, τ= 5.4 ns); the other is in limited contact with water which is probably immobilized by bonding at the macromolecular surface (Λ_m# 340–342 nm, ΔΛ= 53–55 nm, q # 0.3, = 4.4 ns). Some quantitative predictions from the model, for (a) the fraction of fluorescence that is quenched by ionic quenchers, (b) the mean values of quantum yield, and (c) the mean values of fluorescence lifetime for various proteins, show good concordance with independent experimentally determined values.     

PHOTOSENSITIZATION VIA CHARGE TRANSFER OR REVERSIBLE ELECTRON TRANSFER. OXIRANE ISOMERIZATION AND SULFUR DIOXIDE EXTRUSION

Abstract— N, N, N^' N^'-Tetramethylbenzidine (NTMB) photosensitizes the cis-trans isomerization of stilbene oxiranes (SO) and the extrusion of SO₂ from dibenzyl sulfone (DBS). In acetonitrile solution it is found that in the absence of SO or DBS, singlet NTMB undergoes three processes: intersystem crossing to triplet NTMB (φ_ISC= 0.63, k _ISC= 6.3 × 10⁷s^-1), fluorescence (φ_f= 0.30, k _f= 3 × 10⁷s^-1), and formation of a cation by electron ejection (φ_ion= 0.09). Both singlet and triplet sensitization are observed. A charge transfer or reversible electron transfer mechanism is proposed to explain the results.