Singlet oxygen generation in PUVA therapy studied using electronic structure calculations

The ability of furocoumarins to participate in the PUVA (Psoralen + UV-A) therapy against skin disorders and some types of cancer, is analyzed on quantum chemical grounds. The efficiency of the process relies on its capability to populate its lowest triplet excited state, and then either form adducts with thymine which interfere DNA replication or transfer its energy, generating singlet molecular oxygen damaging the cell membrane in photoactivated tissues. By determining the spin–orbit couplings, shown to be the key property, in the intersystem crossing yielding the triplet state of the furocoumarin, the electronic couplings in the triplet–triplet energy transfer process producing the singlet oxygen, and the reaction rates and lifetimes, the efficiency in the phototherapeutic action of the furocoumarin family is predicted as: khellin < 5-methoxypsoralen (5-MOP) < 8-methoxypsoralen (8-MOP) < psoralen < 4,5′,8-trimethylpsoralen (TMP) < 3-carbethoxypsoralen (3-CPS), the latter being the most efficient photosensitizer and singlet oxygen generator.     

Triplet state properties and triplet-state-oxygen interactions of some linear and angular furocoumarins

Linear and angular furocoumarins with conjugated external carbonyl substituents show higher triplet and singlet oxygen yields than the corresponding unsubstituted molecules. The efficiency of the oxygen quenching process to yield singlet oxygen is also higher for these substituted molecules. These changes are interpreted in terms of the "proximity effect" associated with two nearly degenerate n pi* and pi pi* excited states, and variations in the excess energy following furocoumarin triplet quenching by ground state triplet oxygen to yield singlet oxygen.     

PHOTOBINDING OF PSORALEN AND 8-METHOXYPSORALEN TO CALF THYMUS DNA

Abstract— The photobinding of psoralen and 8-methoxypsoralen to calf thymus DNA induced by 365 nm radiation has been measured for different concentrations of the furocoumarin and nucleotides. The results are consistent with the assumption that dark complexing of the furocoumarin to DNA is a pre-condition for photobinding, but do not exclude the possibility that the free furocoumarin participates in the reactions. The analysis with 'large target' diffusion theory indicates that photobinding should be inefficient for the free excited singlet state and competitive with reactions of the dark-complexed sensitizer for the free triplet state. The analysis indicates also that the diffusive reactions of singlet oxygen generated by the free furocoumarin can compete with photoadduct formation.     

SOME PHOTOPHYSICAL PROPERTIES OF 3-CARBETHOXYPSORALEN, 8-METHOXYPSORALEN and 5-METHOXYPSORALEN TRIPLET STATES

Abstract Triplet absorption spectra, extinction coefficients (ɛ_T), decay rates ( K ₁), oxygen quenching rates (k_q) and intersystem crossing yields (φ_T) for 3-carbethoxypsoralen (3-CPs). 8-methoxypsoralcn (8-MOP) and 5-methoxypsoralen (5-MOP) in methanol are reported. For 8-MOP and 3-CPs corresponding values are also reported with water as the solvent. Some photophysical data are also reported for 5-MOP in water, but ɛ_T and φ_T were not obtained.
The phosphorescence spectra for these furocoumarin derivatives in ethanol at 77 K are reported together with the corresponding lowest triplet energy and lifetime. The values of the various photophysical properties obtained are compared with values reported by previous workers.     

Absolute fluorescence quantum yields for vapour-phase benzene and naphthalene,and comments on the non-radiative processes

Optic—acoustic measurements have been employed in the determination of absolute quantum yields for benzene and naphthalene. Heat yields are measured by a method using oxygen quenching of both triplet and singlet states. For vibrationally relaxed excited singlet states the fluorescence quantum yields, φ^B_f, are 0.16 ± 0.02 and 0.79 ± 0.02 for benzene and naphthalene respectively. For 0.07 torr naphthalene at room temperature with 248 nm excitation, φ_f = 0.35 ± 0.03 and the quantum yield of internal conversion is less than 0.05. The decay of the highly vibrationally excited triplet state is dominated by vibrational relaxation for 0.07 torr naphthalene, but for benzene, even at high pressures, strong competition comes from an indirect coupling process to the ground state.     

TRIPLET EXCITED STATES OF 4'',5''-PHOTOMONOADDUCTS OF 3- CARBETHOXYPSORALEN and 8-METHOXYPSORALEN WITH DNA NUCLEOSIDES

Abstract— Triplet absorption spectra, triplet extinction coefficient and intersystem crossing for 4',5'-monocycloadducts of 3-carbethoxypsoralen (3-CPs) with thymidine (dThd) and uridine (dUrd) in ethanol have been investigated in order to elucidate whether their triplet state properties could be the limitating step for a further photoreaction of 3-CPs monoadducts with DNA nucleosides. The comparison between the triplet characteristics of 4',5'-monoadducts of 3-CPs and those of 8-methoxypsoralen (8-MOP) shows that the quantum yield is much higher in the case of 3-CPs than for 8-MOP. The monofunctionality of 3-CPs cannot therefore be ascribed to the triplet excited states properties of its monoadducts. It is likely that steric hindrance introduced by the bulky carbethoxy group remains a reasonable explanation.     

RELATIVE REACTIVITIES OF THE EXCITED STATES OF FUROCOUMARINS FOR [2+2] PHOTOCYCLOADDITION REACTION WITH TETRAMETHYLETHYLENE

Abstract— The Stern-Volmer constants for fluorescence quenching by tetramethylethylene decrease in the order DMC ≫ DHP > F-2 > 8-MOP. The same order was observed for the quantum yields of [2+2] cycloaddition reaction with tetramethylethylene on direct irradiation. In [2+2] photocycloaddition of F-2 with tetramethylethylene in ethanol, the ratio of quantum yields deduced from singlet and triplet states of F-2; φ₃⁰/φ₁⁰, is about 5. The excited triplet state is the reactive state for the [2+2] photocycloaddition of F-2 with tetramethylethylene in solution but the excited singlet state of F-2 becomes very important in biological conditions.     

Synthesis and photophysical properties of silicon phthalocyanines with axial siloxy ligands bearing alkylamine termini

Eleven silicon phthalocyanines which can be grouped into two homologous series [SiPc[OSi(CH3)2(CH2)(n)N(CH3)2]2, n = 1-6 (series 1), and SiPc[OSi(CH3)2(CH2)3N((CH2)(n)H)2]2, n = 1-6 (series 2)] as well as an analogous phthalocyanine, SiPc[OSi(CH3)2(CH2)3NH2]2, were synthesized. The ground state absorption spectra, the triplet state dynamics, and singlet oxygen quantum yields of 10 of these phthalocyanines were measured. All compounds displayed similar ground state absorption spectral properties in dimethylformamide solution with single Q band maxima at 668 +/- 2 nm and B band maxima at 352 +/- 1 nm. Photoexcitation of all compounds in the B bands generated the optical absorptions of the triplet states which decayed with lifetimes in the hundreds of microseconds region. Oxygen quenching bimolecular rate constants near 2 x 10(9) M(-1) s(-1) were measured, indicating that energy transfer to oxygen was exergonic. Singlet oxygen quantum yields, phi(delta), were measured, and those phthalocyanines in which the axial ligands are terminated by dimethylamine residues at the end of alkyl chains having four or more methylene links exhibited yields near > or = 0.35. Others gave singlet oxygen quantum yields near 0.2, and still others showed singlet oxygen yields of <0.1. The reduced singlet oxygen yields are probably caused by a charge transfer quenching of the 1pi,pi* state of the phthalocyanine by interaction with the lone pair electrons on the nitrogen atoms of the amine termini. In some cases, these can approach and interact with the electronically excited pi-framework, owing to diffusive motions of the flexible oligo-methylene tether.     

FLUORESCENCE AND PHOTOOXIDATION OF 1—ANILINONAPHTHALENE—8—SULFONIC ACID IN REVERSED MICELLAR SOLUTIONS AND THE EFFECT OF CCl4

Abstract— The 1-anilinonaphthalene-8-sulfonic acid solubilized in dodecylammonium propionate reversed micellar cyclohexanic solutions, emitted a strong fluorescence, and was photooxidized under aerobic conditions. Carbon tetrachloride (CCl₄) highly quenched the fluorescence and remarkably enhanced the oxidation reaction. The fluorescence quenching obeyed the Stern-Volmer relation, and the photooxidation was caused by the singlet oxygen generated by the photosensitization of the dye. From the kinetic analysis, it was known that the intersystem crossing rate from the dye excited singlet to triplet was enhanced by CCl₄. Carbon tetrachloride did not quench the triplet state. The ratio of quantum yields for the oxidation in the presence and absence of CCl₄ was independent of the oxygen concentration in the reaction mixture. The fluorescence quenching constant and the intersystem crossing rate were obtained at various solubilized water contents.     

Eosin-sensitized photoreduction of benzil with 1-benzyl-1,4-dihydronicotinamide

The mechanism of eosin-sensitized photoreduction of benzil with 1-benzyl-1,4-dihydronicotinamide — a model compound of NAD(P)H and the behavior of the excited states of eosin have been investigated. The effect of anthracene as a diffusion-controlled quencher of the photoreaction indicates that both excited triplet state and an unquenchable excited singlet state of eosin participated in the sensitized photoreaction. From the Stern-Volmer plot of quantum yield vs. anthracene concentration, the triplet reaction rate constant has been calculated to be 0.78 × 10⁸ L M^?1S^?1 while the singlet reaction rate constant determined from quenching of eosin fluorescence by benzil is equal to 7.2 × 10⁹ L M^?1S^?1. The singlet and triplet quantum yields are also determined to be 0.09 and 0.18 respectively. Since both the singlet and triplet energies of eosin are lower than that of benzil, energy transfer sensitization is not feasible. It is proposed that electron transfer from the excited eosin to benzil is responsible for the initiation.     

TRIPLET EXCITED STATE OF THE 4'5' PHOTOADDUCT OF PSORALEN AND THYMINE

Abstract— The triplet-triplet absorption spectrum of the 4'5' psoralen-thymine mono-adduct has been determined in water and methanol using the technique of laser flash photolysis. The extinction coefficient of the triplet was measured by the energy-transfer method with retinol triplet as standard, and used to determine the singlet → triplet intersystem crossing quantum yield for 353 nm excitation. Reaction rate constants for mono-adduct triplet with thymine and tryptophan were measured in water. Long-lived transient absorptions detected after quenching the mono-adduct triplet with thymine and tryptophan are assigned mainly to the corresponding mono-adduct radical anion, whose spectrum was established in separate pulse radiolysis studies of the mono-adduct in aqueous formate.
The significant singlet → triplet quantum yields found for the mono-adduct might be consistent with the involvement of triplet excited mono-adduct in DNA cross-link formation, as also may be the high reactivity obtained for the triplet with thymine. The initial quenching products observed resulted from a charge-transfer reaction.     

PHOTOPHYSICAL PROPERTIES OF meso-TETRAPHENYLPORPHYRIN and SOME meso-TETRA(HYDROXYPHENYL)PORPHYRINS

Abstract— The o-, m-, and p-isomers of 5, 10, 15, 20- tetra(hydroxyphenyl)-porphyrin have been of recent interest as potential second-generation sensitisers in tumour phototherapy. Fluorescence spectroscopy, nanosecond laser flash photolysis and pulse radiolysis have been used to characterise the singlet and triplet excited states of tetraphenylporphyrin and the o, m-, and p-isomers of tetra(hydroxyphenyl)porphyrin. This has included evaluation of fluorescence yields and lifetimes, triplet spectra, lifetimes, oxygen quenching rate constants, extinction coefficients, and yields and singlet oxygen yields. Whilst the fluorescence quantum yields were low, the triplet yields were all 0.7 ± 10% and the singlet oxygen yields 0.6 ± 10%: all these parameters are in the ranges shown by other efficient porphyrin photosensitisers. The similar photophysical properties found for these compounds suggest that their differing tumour sensitising efficiencies are likely to be due to other factors.     

The effect of DNA binding on initial 8-methoxypsoralen photochemistry.

Abstract—Flash photolysis experiments on the complex of 8-methoxypsoralen with calf thymus DNA have shown binding promotes the anaerobic quenching of the 8-MOP triplet state and inhibits the accessibility to oxygen. The results indicate dark psoralen-DNA interactions suppress the singlet oxygen generation observed in prior work.     

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.     

THE PHOTOSENSITIZERS BENZOPHENOXAZINE and THIAZINES: COMPREHENSIVE INVESTIGATION OF PHOTOPHYSICAL and PHOTOCHEMICAL PROPERTIES

Eight differently substituted title dye compounds have been investigated regarding intersystem crossing, triplet state, fluorescence and singlet excited state pKa properties. In general, non-halogenated oxazines and thiazines as well as a mono bromooxazine show very low triplet quantum yields, phi tau (less than 0.03) and relatively long triplet lifetimes (approximately 40 microseconds) in acidic methanol. The phi tau data correlate well with known singlet oxygen yields. In basic methanol no triplet transient is observed but a significant yield of a ground state transient protonated (base dye) form is produced with a short lifetime, approximately 400 ns. Fluorescence can be seen simultaneously from both the excited base and the protonated base dye forms in basic methanol. For iodinated oxazine or thiazines, the triplet yield increases and can be as high as 0.5 (diiodo case) in acidic methanol. The triplet lifetimes are further shortened to approximately 10 microseconds compared to the non-iodinated derivatives above. The triplet yields of the iodo compounds are higher or equal to known singlet oxygen yields. In basic methanol triplet yields up to 0.2 can be seen, the triplet lifetime are shortened still further to 1 microsecond but no observable protonated form is produced (in distinction to the non-iodinated cases). Consideration is given to the correlation of triplet and singlet oxygen yields, ground and excited pKa properties, spin-orbit coupling and internal conversion properties, solvent effects, and phototherapeutic activity of these dyes.     

Photochemistry of kynurenine, a tryptophan metabolite: properties of the triplet state

Photolysis of aqueous kynurenine (KN) solutions results in the formation of triplet kynurenine TKN. In low pH solutions, triplet formation occurs with almost 100% efficiency, while in neutral solutions the triplet quantum yield is PhiT = 0.018 +/- 0.004. The dissociation constant of TKN, which is attributed to deprotonation of the anilino group, has a pKa value of 4.7. Similar triplet absorption spectra were obtained under direct and acetone-sensitized photolysis. The large difference in quantum yields as a function of pH is attributed to excited-state properties of the first excited singlet state of KN. The rate constant quenching for TKN by oxygen is kq = 2 x 10(9) M(-1) s(-1).     

Aza‐BODIPY Derivatives: Enhanced Quantum Yields of Triplet Excited States and the Generation of Singlet Oxygen and their Role as Facile Sustainable Photooxygenation Catalysts

A new series of aza‐BODIPY derivatives ( 4 a – 4 c , 5 a , c , and 6 b , c ) were synthesized and their excited‐state properties, such as their triplet excited state and the yield of singlet‐oxygen generation, were tuned by substituting with heavy atoms, such as bromine and iodine. The effect of substitution has been studied in detail by varying the position of halogenation. The core‐substituted dyes showed high yields of the triplet excited state and high efficiencies of singlet‐oxygen generation when compared to the peripheral‐substituted systems. The dye 6 c , which was substituted with six iodine atoms on the core and peripheral phenyl ring, showed the highest quantum yields of the triplet excited state (Φ_T=0.86) and of the efficiency of singlet‐oxygen generation (Φ_Δ=0.80). Interestingly, these dyes were highly efficient as photooxygenation catalysts under artificial light, as well as under normal sunlight conditions. The uniqueness of these aza‐BODIPY systems is that they are stable under irradiation conditions, possess strong red‐light absorption (620–680 nm), exhibit high yields of singlet‐oxygen generation, and act as efficient and sustainable catalysts for photooxygenation reactions.     

8-甲氧基呋喃并[3',2':6,7]香豆素与烯烃光环合加成反应的研究I.

The photocycloaddition of 8-methoxypsoralen to cyclothexene and to heptene-1 was found to give 3,4-dihydrobicyclo (4,2,0) octano (7",8":3,4) 8-methoxypsoralen (4) and a mixture of 3, 4-dihydro-(1-n-pentyl) cyclobutano-(2", 3":3,4) 8-methoxypsoralen (5) and the isomer 6 respectively. Stern-Volmer plot and quenching studies indicate that the reaction proceeds via psoralen triplet excited state.     

Excited state behaviour of pentahelicene dinitriles

The excited singlet and triplet states 2,13-dicyano[5]helicene (1) and two para-dicyno[5]helicenes containing one and two methyl groups (2 and 3, respectively) were studied in solvents of different polarity as a function of temperature. Fluorescence quenching by electron donors such as triethyl amine indicated photoinduced electron transfer. In the absence of additives triplet states were observed by flash photolysis. The triplet lifetime at room temperature was rather short (<1 μs) and the decay limited by intramolecular processes, e.g. charge transfer in the cases of 2 or 3. Luminescence of singlet molecular oxygen, O₂(¹Δ_g), was observed with moderate and low quantum yield for 1 and 3, respectively. For 1–3, the triplet lifetime increases by six orders of magnitude on going to −196°C. Two subsequently formed triplet states were observed for 3 at lower temperatures. The effects of temperature and solvent polarity on the quantum yields of fluorescence and phosphorescence and the spectroscopic and kinetic triplet absorption properties were examined. The influence of substituents on the deactivation pathways of excited pentahelicenes are discussed.     

Characterization of new furocoumarin derivatives by their dark and light-mediated action on RNA bacteriophage MS2

The monofunctional and bifunctional furocoumarin derivatives 8-methyl-3-carbethoxypsoralen (8Me3CPs) and 8-methoxypsoralen (8MOP) as well as their thiosubstituted derivatives (2-thio-8Me3CPs and 2-thio-8MOP) were compared in terms of their reactivities towards a ribonucleoprotein, the bacteriophage MS2. The order of their photoreactivities differed from that measured with nuclear DNA and mitochondrial DNA. Besides their widely investigated photoreactivity, their biological activity in the dark and after pre-irradiation was quantified. A parameter was defined which compares the number of molecules acting in the dark for 1 h with the number of absorbed photons which lead to the same degree of inactivation. The parameter for the furocoumarin derivatives examined, including 3-carbethoxypsoralen (3CPs) and 4'-aminomethyl-4,5',8-trimethylpsoralen (AMT), was in the following order: AMT greater than 3CPs greater than 8MOP greater than 8Me3CPs greater than 8Me3CPsS approximately 8MOPS. A similar parameter was also determined for the dark effect of pre-irradiated compounds.