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T A Kennedy

Publications and source records attributed to T A Kennedy.

8 recordsLinked to original sources
PubMed2001

Electron and nuclear spin interactions in the optical spectra of single GaAs quantum dots.

Fine and hyperfine splittings arising from electron, hole, and nuclear spin interactions in the magneto-optical spectra of individual localized excitons are studied. We explain the magnetic field dependence of the energy splitting through competition between Zeeman, exchange, and hyperfine interactions. An unexpectedly small hyperfine contribution to the splitting close to zero applied field is described well by the interplay between fluctuations of the hyperfine field experienced by the nuclear spin and nuclear dipole/dipole interactions.

Journal Article↗
PubMed1998

Optical NMR from single quantum dots.

Nuclear magnetic resonance (NMR) from constituent Ga and As nuclei was optically detected on excitonic recombination in single GaAs quantum dots formed by interface fluctuations in GaAs/Al0.3Ga0.7As quantum wells. Orientation of the nuclear spin system by optical pumping causes an Overhauser shift of the excitonic energy levels proportional to the degree of nuclear orientation. NMR was subsequently detected by monitoring changes in the combined Overhauser plus Zeeman splitting of excitons localized in single quantum dots as the RF frequency was swept through a nuclear resonance. The NMR signals originate from approximately 10(5) nuclei in the quantum dot-with dimensions of approximately 4 nm X 10 nm X 100 nm--illustrating the extreme sensitivity and spatial resolution of the technique. NMR from such small structures provides a chemically specific probe of the local environment on the nanometer scale.

Arsenicals↗
PubMed1994

Investigation of the role of cysteines in catalysis by prostaglandin endoperoxide synthase.

The importance of cysteine residues in the cyclooxygenase activity of prostaglandin endoperoxide synthase (PGHS) was investigated using cysteine-specific reagents and site-directed mutagenesis. N-(7-Dimethyl-amino-4-methyl-3-coumarinyl)maleimide (DACM), a hydrophobic maleimide, inactivated both cyclooxygenase and peroxidase activities of apoPGHS in a time-dependent manner but did not affect holoPGHS. Heme titration experiments indicated that modification of apoPGHS with DACM prevented heme binding. Peptide mapping revealed that DACM modified Cys313, Cys512, and Cys540. N-Ethylmaleimide inactivated cyclooxygenase and peroxidase activities of holoPGHS in a time-dependent manner but did not affect apoPGHS. Peptide mapping demonstrated that N-ethylmaleimide reacted primarily with Cys313 in holoPGHS and with Cys540 in apoPGHS. Each of the 3 cysteines was changed to serine by site-directed mutagenesis, and the mutant proteins were expressed in COS-1 cells. The C512S mutant converted arachidonic acid to products to the same extent as wild-type PGHS. In contrast, the C313S and C540S mutants converted arachidonic acid to products to the extent of 10% of wild-type PGHS. These results indicate that Cys313, Cys512, and Cys540 are not essential for cyclooxygenase activity but that alteration of Cys540 or Cys313 dramatically decreases enzyme activity. Both residues are well removed from the cyclooxygenase and peroxidase active sites so our findings reveal that subtle changes, such as substitution of a single oxygen for sulfur atom as far as 30 A from the heme prosthetic group, can significantly alter enzyme activity.

Animals↗
PubMed1992

Spectral analysis of the protein-derived tyrosyl radicals from prostaglandin H synthase.

We have analyzed the low temperature EPR spectra of the protein-derived tyrosyl radicals detected upon addition of arachidonic acid or 5-phenyl-4-pentenyl-1-hydroperoxide (PPHP) to prostaglandin H synthase. With either arachidonic acid or PPHP the initial radical detected is a doublet (peak-to-trough = 35 Gauss) that disappears rapidly and is replaced by a broad singlet (peak-to-trough = 30 Gauss) followed by a narrow singlet (peak-to-trough = 26.5 Gauss). The relative amounts of these signals vary with time and concentration of arachidonic acid. The three tyrosyl radical signals were subjected to computer simulation and power saturation analysis. The data establish that there are only two distinct tyrosyl radical species, the doublet and the narrow singlet. The broad singlet seen at intermediate times and at low arachidonic acid concentrations is a composite of the doublet and the narrow singlet. The composition of the broad singlet in incubations of prostaglandin H synthase with 0.5 mM arachidonic acid is approximately 40% doublet and 60% singlet. The broad singlet signal does not represent a distinct tyrosyl radical species.

Alkenes↗
PubMed1992

Peroxyl radical scavenging by beta-carotene in lipid bilayers. Effect of oxygen partial pressure.

The effect of the partial pressure of oxygen (pO2) on the antioxidant reactions of all-trans-beta, beta-carotene (BC) was investigated in a soybean phosphatidylcholine liposome system. Peroxyl radicals generated by thermolysis of azo-bis(2,4-dimethylvaleronitrile) at 37 degrees C initiated lipid peroxidation. BC inhibited lipid peroxidation, which was monitored by conjugated diene formation, by up to 70% versus control at 160 and 15 torr O2. In contrast, at 760 torr O2 the maximum inhibition was approximately 40% versus control and inhibition was less reproducible. Peroxyl radicals oxidized BC to 5,6-epoxy-beta,beta-carotene and several unidentified polar products. The rates of both product formation and BC consumption were significantly higher at 160 torr than at 15 torr O2. However, at 160 and 760 torr O2, the rates of product formation and BC depletion were similar. In liposomes without azo-bis(2,4-dimethylvaleronitrile), BC depletion at 160 torr was only 64% that at 760 torr O2. These results suggest that both radical trapping and autoxidation reactions consume BC and that the latter are accelerated by high pO2. Autoxidation consumes BC without scavenging peroxyl radicals and may attenuate BC antioxidant activity, especially at high pO2. The similarity in its antioxidant effects at 15 and 160 torr O2 suggests that BC could provide antioxidant protection to any tissue within the normal physiologic range of pO2.

Carotenoids↗
PubMed1989

Redox cycles of vitamin E: hydrolysis and ascorbic acid dependent reduction of 8a-(alkyldioxy)tocopherones.

Oxidation of the biological antioxidant alpha-tocopherol (vitamin E; TH) by peroxyl radicals yields 8a-(alkyldioxy)tocopherones, which either may hydrolyze to alpha-tocopheryl quinone (TQ) or may be reduced by ascorbic acid to regenerate TH. To define the chemistry of this putative two-electron TH redox cycle, we studied the hydrolysis and reduction of 8a-[(2,4-dimethyl-1-nitrilopent-2-yl)dioxy]tocopherone (1) in acetonitrile/buffer mixtures and in phospholipid liposomes. TQ formation in acetonitrile/buffer mixtures, which was monitored spectrophotometrically, declined with increasing pH and could not be detected above pH 4. The rate of TQ formation from 1 first increased with time and then decreased in a first-order terminal phase. Rearrangement of 8a-hydroxy-alpha-tocopherone (2) to TQ displayed first-order kinetics identical with the terminal phase for TQ formation from 1. Both rate constants increased with decreasing pH. Hydrolysis of 1 in acetonitrile/H2(18)O yielded [18O]TQ. These observations suggest that 1 loses the 8a-(alkyldioxy) moiety to produce the tocopherone cation (T+), which hydrolyzes to 2, the TQ-forming intermediate. Incubation of either 1 or 2 with ascorbic acid in acetonitrile/buffer yielded TH. Reduction of both 1 and 2 decreased with increasing pH. In phosphatidylcholine liposomes at pH 7, approximately 10% of the T+ generated from 1 was reduced to TH by 5 mM ascorbic acid. The results collectively demonstrate that T+ is the ascorbic acid reducible intermediate in a two-electron TH redox cycle, a process that probably would require biocatalysis to proceed in biological membranes.

Antioxidants↗
PubMedDate not supplied

Peroxyl radical oxidation of beta-carotene: formation of beta-carotene epoxides.

A chemical model system was used to study peroxyl radical trapping reactions of beta-carotene (1) that may contribute to its antioxidant action in biological systems. Peroxyl radicals generated in hexane by thermolysis of azobis(2,4-dimethylvaleronitrile) (AMVN) at 37 degrees C oxidized 1 to 5,6-epoxy-beta,beta-carotene (2) and a previously unreported product, 15,15'-epoxy-beta,beta-carotene (6), in addition to several unidentified polar products. The epoxide products were purified by high-performance liquid chromatography and characterized by UV-vis spectroscopy, mass spectrometry, and 1H NMR. Epoxides 2 and 6 and the polar products were formed together from the initial stages of the reaction. As the reaction progressed, the epoxides were oxidized further to more polar products. Although epoxides 2 and 6 were formed at similar rates, 2 was oxidized more rapidly than 6. Incubations with [14C]-1 indicate that at their maximum concentrations 2 and 6 account for approximately 20% of the radiolabeled oxidation products. Epoxide formation may result from peroxyl radical addition to the polyene chain to form a resonance-stabilized peroxyl radical adduct. Peroxide bond scission would yield the epoxide and release an alkoxyl radical. Although this two-step sequence produces no net radical trapping, it could produce a kinetically significant inhibition of peroxyl radical propagation and account, in part, for the antioxidant properties of 1. Epoxides 2 and 6 are structurally distinct from retinoids, which are the metabolic products of 1, and therefore may be useful biochemical markers for its antioxidant actions.

Azo Compounds↗