René V. Bensasson

Benzophenone triplet properties in acetonitrile and water. Reduction by lactams

The technique of time resolved laser flash spectroscopy has been used to study the photophysical and photochemical properties of the lowest triplet of benzophenone in water and acetonitrile. New estimates have been obtained for the triplet extinction coefficient, unimolecular and bimolecular rate constant of decay in both solvents, as well as a determination of the quantum yield of ketyl formation in pure water and the rate constants of hydrogen abstraction from various lactams in both solvents. Evidence for hydrogen abstraction from water by a triplet of benzophenone is found. Correlation between rate constants of hydrogen abstraction from lactams by the lowest benzophenone triplet and their ionization potential is discussed. Reversibility of these hydrogen abstractions is considered.

Deactivation of singlet molecular oxygen by thiols and related compounds, possible protectors against skin photosensitivity.

Abstract— From time‐resolved measurements of the decay of singlet molecular oxygen phosphorescence at 1270 nm in D2O, direct estimates have been gained for the rate constants of the singlet oxygen reactions with a group of sulphur compounds in the pD range 5 to 13. In the case of most of the thiols, the results are consistent with singlet oxygen reacting exclusively with the thiolate anions. At the normal physiological pH 7, the apparent rate constants (in units of M‐1 s‐1) were 8.9 times 106 (cysteine), 2.5 times 106 (N‐acetyl cysteine), 2.9 times 106 (glutathione), 3.0 times 105 (2‐mercaptoethanol), 2.3 times 107 (ergothioneine) and 2.7 times 106 (2‐mercaptopropionyl glycine). For methionine the rate constant, 1.4 times 107, was independent of pD in the range studied. These sulphur compounds, in particular N‐acetyl cysteine and ergothioneine, or related compounds, might be considered as possible candidates for protection against skin photosensitivity side effects associated with the photodynamic therapy of solid tumours and as observed in the disease erythropoietic protoporphyria.

Pulse radiolysis study of buckminsterfullerene in benzene solution. Assignment of the C60 triplet-triplet absorption spectrum

Abstract Pulse radiolysis and, to a lesser extent, laser flash photolysis have been used to determine properties of the lowest triplet of C 60 in benzene at room temperature, including its absorption spectrum between 300 and 1600 nm, its molar absorption coefficient (ϵ T = 20200 ± 2000 dm 3 mol −1 cm −1 at 750 nm) and its quantum yield of formation (Φ T = 0.88 ± 0.15). The features of the triplet spectrum are assigned according to the triplet transition energies based on molecular orbital calculations previously reported; the strong band at 750 nm is attributed to the 2 3 G u -1 3 T 2g transition. Implications of the results are discussed, including those related to C 60 as an optical limiting material.

PHOTOPHYSICAL PROPERTIES OF THREE METHANOFULLERENE DERIVATIVES

The three [6,6]-ring bridged methanofullerenes 1, 2, and 3 were synthesized; their ground-state absorption spectra in toluene and cyclohexane solutions are presented and possible assignments discussed. Although these three molecules are very different, they have very similar triplet–triplet spectra and near-unity values for the quantum yield for O2(1Δg) production by energy transfer from the triplet states, implying that the quantum yield of triplet production is approximately 1.

TRIPLET EXCITED STATES OF PYRIMIDINE NUCLEOSIDES AND NUCLEOTIDES

Abstract— The triplet absorption spectra, lifetimes, extinction coefficients, eTT, and intersystem crossing quantum yields to the lowest triplet T1, øT1, of thymidine, thymidine monophosphate, uridine and uridine monophosphate, have been determined in various solvents at 300 K.

NANOSECOND IRRADIATION STUDIES OF BIOLOGICAL MOLECULES-I. COENZYME Q 6 (UBIQUINONE-30)

Abstract— The coenzyme ubiquinone, an isoprenoid benzoquinone present in the electron‐transport chain of mitochondria, has been studied using nanosecond laser flash photolysis and pulse radiolysis. The hitherto undetected triplet excited state of the coenzyme has been identified and some of the physico‐chemical properties determined. These measurements may assist the understanding in molecular terms of the degradative action of light upon biological materials, photophosphorylation and the possible initiation of biological electron transport via quinone light absorption.

Singlet molecular oxygen quenching by saturated and unsaturated fatty-acids and by cholesterol.

The rate constants of molecular singlet oxygen quenching by saturated and unsaturated fatty‐acids and by cholesterol‐membrane critical components ‐ membrane critical components ‐ have been measured by time resolved detection of the 1270 nm phosphorescence of singlet molecular oxygen [O2(1δg)]. We have determined (i) an increment of 5.7 times 102M‐1 s‐1 per ‐CH2‐ in C6D6 and CD3OD for saturated fatty acids between C4 and C20, (ii) an increment of 3 times 104M‐1s‐1 per non‐conjugated cu‐double bond for C18 unsaturated fatty acids, identical in C6D6 and CD3OD, (iii) a lower quenching rate constant by a factor of 2.7 for the trans‐C16 and trans‐C18 as compared to the corresponding cis‐monounsaturated fatty acids, (iv) a rate constant of O2(1δg) quenching by cholesterol of 5.7 times 104M‐l s‐l in benzene. These rate constants are compared to those obtained for other membrane cellular components.

Photoprotection by Carotenoids During Photosynthesis: Motional Dependence of Intramolecular Energy Transfer

A new carotenoporphyrin has been prepared in which a synthetic carotenoid is joined to a tetraarylporphyrin through a flexible trimethylene linkage. This molecule exists primarily in an extended conformation with the carotenoid chromophore far from the porphyrin π-electron system. In benzene solution, where large-amplitude molecular motions are rapid, the molecule can momentarily assume less stable conformations which favor triplet energy transfer, and quenching of the porphyrin triplet by the carotenoid is fast. In a polystyrene matrix or frozen glass such motions are slow, and energy transfer cannot compete with other pathways for depopulating the triplet state. These observations help establish the requirements for biological photoprotection.

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 eaq‐ 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).

SOME FUROCOUMARINS AND ANALOGS: COMPARISON OF TRIPLET PROPERTIES IN SOLUTION WITH PHOTOBIOLOGICAL ACTIVITIES IN YEAST

Abstract— Some photophysical and photochemical properties of two furocoumarins, three furochro‐menes and one difurobenzene have been studied in ethanol by laser flash spectroscopy. Such properties included the triplet absorption spectra. extinction coefficients, quantum yields of formation and reactivities with the nucleic acid bases thymine and uracil, and the amino acid tryptophan. The effects the same compounds have on survival of the yeast, Saccharomyces cerevisiae, after 365 nm photosensitization were also investigated. These effects on survival differed markedly for the different compounds. Possible correlations between the photophysical, photochemical and photobiological data are discussed.