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At least 1,171 records · Page 65Linked to original sources
PubMed2005

Singlet exciplexes between a thioxanthone derivative and substituted aromatic quenchers: role of the resonance integral.

Fluorescence quenching of a thioxanthone derivative by methyl- and methoxy-substituted benzenes (MeB and MeOB, respectively) is performed in solvents of different polarity. Emissive exciplexes are observed even in polar solvents and provide kinetic and spectroscopic data over a large scale of solvent polarity. These data were subsequently analyzed by use of a new theoretical model that leads to a thermodynamic relationship between exciplex and electron-transfer driving forces Delta G(exc) and Delta G(et), respectively. The remarkable agreement found between this model and both kinetic and spectroscopic data supports its validity. Moreover, the difference observed between MeB and MeOB compounds in quenching efficiency is analyzed by this model and provides the main parameters governing exciplex features, especially the resonance integral between locally excited and charge-transfer states.

Journal Article↗
PubMed1976

Steric inhibition of conjugation in lowest excited singlet state of 9-anthramide by hydrogen bond donor solvents: role of solvent in chemical structure.

9-Anthramide has electronic absorption and fluorescence spectra that, in water, are similar to those of anthracene. This result is attributed to steric hindrance of the 9-carboxamido group with the peri-hydrogen atoms in the 1- and 8-positions of the anthracene ring. However, in aprotic solvents, although the absorption spectrum of 9-anthramide is anthracene-like, its fluorescence spectrum is red shifted and structureless. This finding is attributed to excited-state rotation of the 9-carboxamido group into coplanarity with the anthracene ring and indicates that, in water, the hydrogen-bonded solvent cage affects the steric inhibition of conjugation in excited 9-anthramide. These findings suggest that studies of structure and reactivity of drugs in nonaqueous or solid matrixes are probably of only limited value, since in the strongly interacting aqueous media the aqueous solvent cage plays a substantial role in determining molecular structure and reactivity.

Amides↗
PubMed1999

Direct High-Resolution Determination of the Singlet-Triplet Splitting in NH Using Stimulated Emission Pumping.

A completely resolved spectrum of the strongly forbidden NH (a(1)Delta --> X(3)Sigma(-)) transition is observed. The NH radicals in the excited a(1)Delta state are exclusively generated in a Nd:YAG laser photolysis of hydrazoic acid at a wavelength of 266 nm. The NH (a(1)Delta --> X(3)Sigma(-)) intercombination transition around 794 nm is used to produce NH (X(3)Sigma(-)) applying the stimulated emission pumping technique. The ground state radicals are detected by laser-induced fluorescence (LIF). The energy splitting between the NH (X(3)Sigma(-), v = 0, J = 1, N = 0) state and the NH (a(1)Delta, v = 0, J = 2) state is determined with an accuracy of 0.1 cm(-1) to DeltaE = 12 687.8(65) cm(-1). In addition, the radiative lifetime tau of the NH (a(1)Delta --> X(3)Sigma(-)) transition was estimated by a determination of the saturation intensity to be tau approximately 12.5 s. Copyright 1999 Academic Press.

Journal Article↗
PubMed1980

Effects of scavengers of superoxide radicals, hydrogen peroxide, singlet oxygen and hydroxyl radicals on malondialdehyde generation from arachidonic acid by bovine seminal vesicle microsomes.

Superoxide dismutase, catalase and sodium formate did not inhibit the formation of malondialdehyde (MDA) from arachidonic acid, suggesting that O2-., H2O2 and OH. are not involved in the enzymatical oxidation of arachidonic acid. Sodium azide was found to be an inhibitor of MDA production.

Animals↗
PubMed1998

Rate constants for quenching singlet oxygen and activities for inhibiting lipid peroxidation of carotenoids and alpha-tocopherol in liposomes.

The (1)O2 quenching rate constants (kQ) of alpha-tocopherol (alpha-Toc) and carotenoids such as beta-carotene, astaxanthin, canthaxanthin, and lycopene in liposomes were determined in light of the localization of their active sites in membranes and the micropolarity of the membrane regions, and compared with those in ethanol solution. The activities of alpha-Toc and carotenoids in inhibiting (1)O2-dependent lipid peroxidation (reciprocal of the concentration required for 50% inhibition of lipid peroxidation: [IC50](-1)) were also measured in liposomes and ethanol solution and compared with their kQ values. The kQ and [IC50](-1) values were also compared in two photosensitizing systems containing Rose bengal (RB) and pyrenedodecanoic acid (PDA), respectively, which generate (1)O2 at different sites in membranes. The kQ values of alpha-Toc were 2.9 x 10(8) M(-1) s(-1) in ethanol solution and 1.4 x 10(7) M(-1) s(-1) (RB system) or 2.5 x 10(6) M(-1) s(-1) (PDA system) in liposomes. The relative [IC50](-1) value of alpha-Toc in liposomes was also five times higher in the RB system than in the PDA-system. In consideration of the local concentration of the OH-group of alpha-Toc in membranes, the kQ value of alpha-Toc in liposomes was recalculated as 3.3 x 10(6) M(-1) s(-1) in both the RB and PDA systems. The kQ values of all the carotenoids tested in two photosensitizing systems were almost the same. The kQ value of alpha-Toc in liposomes was 88 times less than in ethanol solution, but those of carotenoids in liposomes were 600-1200 times less than those in ethanol solution. The [IC50](-1) value of alpha-Toc in liposomes was 19 times less than that in ethanol solution, whereas those of carotenoids in liposomes were 60-170 times less those in ethanol solution. There were no great differences (less than twice) in the kQ and [IC50](-1) values of any carotenoids. The kQ values of all carotenoids were 40-80 times higher than that of alpha-Toc in ethanol solution but only six times higher that of alpha-Toc in liposomes. The [IC50](-1) values of carotenoid were also higher than that of alpha-Toc in ethanol solution than in liposomes, and these correlated well with the kQ values.

Antioxidants↗
PubMed1981

Single-electron transfer from NADH analogues to singlet oxygen.

Laser flash photolysis techniques have yielded rate constants for physical and reactive quenching modes of O2(1 delta g) by nicotine, nicotinamide adenine dinucleotide (oxidized and reduced forms) and the reduced forms of nicotinamide mononucleotide, nicotinamide adenine dinucleotide phosphate and nicotinamide hypoxanthine dinucleotide. In the case of the last four named compounds, kinetic spectroscopy furnished evidence for one-electron transfers to O2(1 delta g). Specifically, production of O2 was demonstrated unequivocally by reaction with 1,4-benzoquinone. Quantitative determinations revealed the extent of reactive quenching to be near 60% in each case.

Chemical Phenomena↗