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The bimolecular rate constants k T O2 for oxygen (O 2 ( 3 Σ g - )) quenching and the efficiencies f Δ T with which singlet oxygen (O 2 . ( 1 ΔS g )) is thereby produced are reported for a range of substituted naphthalene triplet states in acetonitrile, benzene, and cyclohexane. The magnitudes of k T O2 and f Δ T are inversely correlated, and both parameters exhibit pronounced sensitivity to the oxidation potential (E M OX ) of the naphthalene derivative and some dependence on the solvent

Nanosecond laser photolysis measurements of sensitized phosphorescence from oxygen have been used to obtain values for singlet oxygen formation efficiencies during oxygen quenching of excited singlet and triplet states of anthracene and naphthalene derivatives. Oxygen quenching of excited singlet states of anthracene and dicyanoanthracene in cyclohexane has been shown to lead to catalysed production of triplet states with unit efficiency in both cases, but concurrent production of singlet oxygen only occurs in the case of 9,10-dicyanoanthracene with efficiency close to unity whereas the efficiency for singlet oxygen production due to direct oxygen quenching of excited singlet anthracene is close to zero. In contrast to these results, oxygen quenching of the triplet states of anthracene and dicyanoanthracene in cyclohexane yields singlet oxygen with unit efficiency whereas the singlet oxygen formation efficiency during oxygen quenching of triplet 1-ethylnaphthalene is only 0.86 in cyclohexane and drops to 0.51 in acetonitrile. This solvent dependence demonstrates the role which charge transfer interactions play in determining singlet oxygen yields. Further information concerning the decay of excited oxygen-aromatic hydrocarbon charge-transfer complexes have been obtained from picosecond laser pump-probe studies where direct excitation is into the charge-transfer bands of oxygenated 1-ethylnaphthalene. Following the excitation of the charge-transfer complex, the triplet state of 1-ethylnaphthalene is rapidly produced with an efficiency which shows a marked solvent dependency, being 0.4 and 0.8 in acetonitrile and cyclohexane, respectively. The measured yields of singlet oxygen formation following excitation into 1-ethylnaphthalene-oxygen charge-transfer complexes are 0.36 and 0.78 in these two solvents which is greater than that expected on the basis of the measured triplet yields. Mechanisms of quenching of excited states by oxygen which explain these results are discussed.

A new method of measuring photoactivity of a major group of inorganic sunscreens (coated titanium dioxide) is presented based on the photobleaching of the radical 1,1-diphenyl-2-picrylhydrazyl.

Using pulse radiolysis and laser flash photolysis, we have investigated the reactions of the deleterious species, e(-)(aq), HOz.rad;, O(2)(*)(-) and O(2)((1)Delta(g)) with 10 water-soluble cyclopropyl-fused C(60) derivatives including a mono-adduct dendro[60]fullerene (d) and C(60) derivatives based on C(60)[C(COOH)(2)](n=2-6), some of which are known to be neuroprotective in vivo. The rate constants for reactions of e(-)(aq) and HOz.rad; lie in the range 0.5-3.3 x 10(10) M(-1) s(-1). The d and bis-adduct monoanion radicals display sharp absorption peaks around 1000 nm (epsilon = 7 000-11 500 M(-1) cm(-1)); the anions of the tris-, tetra-, and penta-adduct derivatives have broader, weaker absorptions. The monohydroxylated radicals have their most intense absorption maxima around 390-440 nm (epsilon = 1000-3000 M(-1) cm(-1)). The anion and hydroxylated radical absorption spectra display a blue-shift as the number of addends increases. The radical anions react with oxygen (k approximately 10(7)-10(9) M(-1) s(-1)). The reaction of O(2)(*)(-) with the C(60) derivatives does not occur via an electron transfer. The rate constants for singlet oxygen reaction with the dendrofullerene and eee-derivative in D(2)O at pH 7.4 are k approximately 7 x 10(7) and approximately 2 x 10(7) M(-1) s(-1) respectively, in contrast to approximately 1.2 x 10(5) M(-1) s(-1) for the reaction with C(60) in C(6)D(6). The large acceleration of the rates for electron reduction and singlet oxygen reactions in water is due to a solvophobic process.

The committed step in the biosynthesis of monoterpenes in mint (Mentha) species is the cyclization of geranyl pyrophosphate to the olefin (-)-4S-limonene catalyzed by limonene synthase (cyclase). Internal amino acid sequences of the purified enzyme from spearmint oil glands were utilized to design three distinct oligonucleotide probes. These probes were subsequently employed to screen a spearmint leaf cDNA library, and four clones were isolated. Three of these cDNA isolates were full-length and were functionally expressed in Escherichia coli, yielding a peptide that is immunologically recognized by polyclonal antibodies raised against the purified limonene synthase from spearmint and that is catalytically active in generating from geranyl pyrophosphate a product distribution identical to that of the native enzyme (principally limonene with small amounts of the coproducts alpha- and beta-pinene and myrcene). The longest open reading frame is 1800 nucleotides and the deduced amino acid sequence contains a putative plastidial transit peptide of approximately 90 amino acids and a mature protein of about 510 residues corresponding to the native enzyme. Several nucleotide differences in the 5'-untranslated region of all three full-length clones suggest the presence of several limonene synthase genes and/or alleles in the allotetraploid spearmint genome. Sequence comparisons with a sesquiterpene cyclase, epi-aristolochene synthase from tobacco, and a diterpene cyclase, casbene synthase from castor bean, demonstrated a significant degree of similarity between these three terpenoid cyclase types, the first three examples of this large family of catalysts to be described from higher plants.

Ethylene-forming enzyme (EFE) catalyzes the final step in biosynthesis of the phytohormone ethylene in higher plants, the oxidation of 1-aminocyclopropane-l-carboxylic acid (ACC). EFE extracted from avocado fruit was partially purified and shown to be a soluble, Fe(II) ascorbate-dependent enzyme. A ripening-related cDNA from avocado, pAVOe3, shows sequence similarity to several Fe(II) and ascorbate-dependent oxidases including EFE from tomato. The AVOe3 antigen was shown to be a soluble protein suggesting its identity with the EFE enzyme. The AVOe3 protein was overexpressed in E. coli with the recovery of EFE activity from the cell extracts. Using residue-specific chemical modification, a histidine critical for catalytic activity was detected. This is likely to be one of three histidine residues highly conserved among sequence-related Fe(II) ascorbate-dependent oxidases.

Biosynthesis of the phytohormone ethylene in higher plants proceeds via the following pathway: S-adenosylmethionine----1-aminocyclopropane-1-carboxylic acid (ACC)----ethylene. Ethylene-forming enzyme (EFE), the enzyme responsible for the oxidation of ACC to ethylene, has been only partially characterized in vitro. We have obtained authentic EFE activity in vitro from extracts of avocado fruit (Persea americana Mill. cv Hass). Ammonium sulfate fractionation revealed the presence of two EFE activities, which we designate as EFE1 and EFE2. EFE1 activity utilizes ACC and O2 as substrates and requires Fe(II) and ascorbate as cofactors. The enzyme has a relatively low Km (32 microM) for ACC, discriminates diastereomers of 1-amino-2-ethyl-cyclopropane-1-carboxylic acid, and is inhibited competitively by 2-aminoisobutyric acid, thus confirming its identity with authentic EFE. Activity is retained in a 100,000 x g supernatant and has a pH optimum of 7.5-8.0, suggesting a cytosolic localization.

Absolute rate constants for the removal of CH2 (ã 1A1) by Ar, NO and H2 have been measured over the temperature range 295-431 K using laser flash kinetic absorption spectroscopy. For NO and H2, no marked temperature dependence is observed. For Ar, the rate constants increase by some 50% over this range and the results are discussed in terms of a theoretical model which relates the removal rate constants to the fractional population of CH2 (ã 1A1) in states perturbed by nearby triplet levels. © 1991.

The efficiencies of singlet oxygen production from the first excited singlet and triplet states of a range of substituted anthracenes in cyclohexane are reported. For all of the anthracene derivatives in this solvent, the efficiency ofsinglet oxygen production from the triplet state, f Δ T , is found to be unity. In contrast, however, the efficiency of singlet oxygen production from the first excited singlet state, f Δ S , varies from zero for anthracene to unity for 9, 10-dicyanoanthracene. It is established that the magnitude of f Δ S is determined by the same factors which govern intersystem crossing yields for anthracene derivatives, i.e., the activation energy for intersystem crossing to higher triplet states