Larry K. Patterson

The kinetics of the reaction of superoxide radical with Fe(III) complexes of EDTA, DETAPAC and HEDTA

To gain an understanding of the mechanism by which the hydroxyl free radical can arise in superoxide generating systems and learn how different chelaters of iron can inhibit this reaction, a pulse radiolysis kinetic study of the reaction of O − 2 with Fe(III)EDTA, Fe(III)HEDTA and Fe(III)DETAPAC (or DTPA) was undertaken. Superoxide reacts readily with Fe(III)EDTA and Fe(III)HEDTA with a pH‐dependent second‐order rate constant having values of 1.9 × 106 M−1.s−1 and 7.6 × 105 M−1.s−1 at pH 7, respectively. However, the rate constant for the reaction of O − 2 with Fe(III)DETAPAC was found to be much slower, the upper limit for the rate constant being 104 M−1.s−1. These results in conjunction with spin‐trapping experiments with Fe(II)EDTA, Fe(II)HEDTA, Fe(II)DETAPAC and H2O2 suggests that DETAPAC inhibits the formation of OH by slowing the reduction of Fe(III) to Fe(II) and not by inhibiting the Fenton reaction.

PULSE RADIOLYSIS STUDIES IN MODEL LIPID SYSTEMS: FORMATION AND BEHAVIOR OF PEROXY RADICALS IN FATTY ACIDS

Abstract— Radiolytic formation and peroxidation of fatty acid radicals have been investigated by pulse radiolysis techniques in oleate, linoleate, linolenate and arachidonate systems. A strong absorption band at 280 nm associated with conjugated radicals, Rconj, formed in polyunsaturated fatty acid moieties has been used as a probe for kinetic processes occurring at doubly allylic sites in the hydrocarbon chain. Formation of Rconj by O‐ has been found to be more efficient than the less selective OH radical. Peroxidation of Rconj is shown to be somewhat slower, (kR+ O2˜ 3 × 108M‐1 s‐1), than O2 reactions with radicals in oleate (kR+ O2= 1 × 109M‐1 s‐1). Peroxy radicals generated in these reactions disappear slowly by essentially second order processes (2kRO1˜ 107M‐1 s‐1). The superoxide radical, O‐2, shows little if any reactivity towards 0.01 M linolenate or arachidonate over periods of 20 s.

RADICAL PROCESSES IN LIPIDS. A LASER PHOTOLYSIS STUDY OF t‐BUTOXY RADICAL REACTIVITY TOWARD FATTY ACIDS

Laser photolysis techniques have been used to measure the reactivity of t‐butoxy radical (t‐BuO) toward various fatty acids. Eight compounds varying both in number and in the configuration of olefinic bonds were examined. It has been found that the rate constant for hydrogen abstraction from these compounds by t‐BuO may be related to the number of secondary, allylic, and doubly allylic hydrogens in each molecule by the equation: kr= [0.072[Hsec] + 0.518[Hallylic] + 2.716[Hdoubly allylic] x 106M‐1 s‐1.

Luminescence behavior as a probe for phase transitions and excimer formation in liquid crystals: dodecylcyanobiphenyl

Abstract The luminescence behavior of a liquid crystal, dodecylcyanobiphenyl, as a function of temperature, is reported. The fluorescence intensity exhibits discontinuities at temperatures which correspond to the phase transitions: crystalline — smectic — isotropic. The results are interpreted in terms of singlet monomer and excimer formation.

Benzophenone-sensitized lipid peroxidation in linoleate micelles

Abstract— Benzophenone‐photosensitized peroxidation in linoleic acid micelles (Type I reaction) has been investigated and quantum yields of both sensitizer photolysis and lipid peroxidation have been obtained. Benzophenone as well as its derivative, benzophenone‐4′‐heptyl‐4‐pentanoic acid (BHPA) have been utilized as sensitizers in this study. A quantum yield of 1.2 for hydroperoxide formation, compared with a benzophenone disappearance quantum yield of 0.41 (in the same linoleate micellar solution) was obtained; yields of 1.3 and 0.6 were found for comparable systems in which BHPA was the sensitizer. Although such data are consistent with the peroxidation mechanism of chain reaction, it is necessary to know the yields of initiating species to arrive at chain reaction lengths in these systems. Such yields have been determined by flash photolysis. The dependencies of sensitizer photolysis and hydroperoxide yields on linoleate concentration have also been determined in order to elucidate the effects of micelle formation or molecular organization on the peroxidative process. Additionally, for comparison in a saturated surfactant (lipid) system hydrogen abstraction was investigated by photolysis techniques in sodium dodecyl sulfate alone using these two sensitizers.

Redox reactions of the urate radical/urate couple with the superoxide radical anion, the tryptophan neutral radical and selected flavonoids in neutral aqueous solutions

The kinetics of several processes involving the potential antioxidant role of urate in physiological systems have been investigated by pulse radiolysis. While the monoanionic urate radical, ·UH-, can be produced directly by oxidation with ·Br-2 or ·OH, it can also be generated by oxidation with the neutral tryptophan radical, ·Trp, with a rate constant of 2 × 107 M-1s-1. This radical, ·UH-, reacts with ·O-2 with a rate constant of 8 × 108 M-1s-1. Also, ·UH- is reduced by flavonoids, quercetin and rutin in CTAB micelles at rate constants of 6 × 106 M-1s-1 and 1 × 106 M-1s-1, respectively. These results can be of value by providing reference data useful in further investigation of the antioxidant character of urate in more complex biological systems.

Theoretical investigation of fluorescence concentration quenching in two‐dimensional disordered systems. Application to chlorophyll a in monolayers of dioleylphosphatidylcholine

A master equation approach is used for investigating energy transfer and trapping in two‐dimensional disordered systems, where the traps are statistical pairs of pigment molecules closer than a critical distance Rc. Fluorescence decay curves are calculated over a range of concentrations as a function of Rc and the Forster transfer radius R0. The concentration dependence of the lifetimes is compared to the fluorescence self‐quenching data that Chauvet et al. obtained from real‐time measurements in monolayers of chlorophyll a and dioleylphosphatidylcholine (DOL). This dependence is found to be close to second order and for a choice of Rc =10 A the experimental data are fit if R0=78±2 A. This value is in close agreement with those found in the literature from depolarization measurements.

RADICAL PROCESSES IN LIPIDS. SELECTIVITY OF HYDROGEN ABSTRACTION FROM LIPIDS BY BENZOPHENONE TRIPLET

Abstract

Monomer–eximer dynamics in spread monolayers. I. Lateral diffusion of pyrene dodecanoic acid at the air–water interface

In this paper, we present a combined experimental and theoretical study of monomer–eximer dynamics in spread monolayers. The system studied here is a spread monolayer of 12‐(1‐pyrene) dodecanoic acid (PDA) and oleic acid at the air–water interface of a neutral aqueous solution. The microscopic kinetic behavior of the PDA probe in the monolayer was monitored through the eximer–monomer, steady‐state photoexcitation of the pyrene moiety. The steady‐state ratio of eximer to monomer intensities, I2/I1, was found to depend linearly on the mole fraction of PDA in oleic acid over the concentration range studied. These data were interpreted theoretically by constructing a reaction‐dynamic model to describe the lateral diffusion and subsequent interaction of the pyrene species. Using analytical results from the theory of random walks on lattices with traps, and from data from Monte Carlo simulations, an estimate is obtained for the two‐dimensional diffusion coefficient of the pyrene probe in the monolayer; this est...

Repair of amino acid radicals of apolipoprotein B100 of low-density lipoproteins by flavonoids. A pulse radiolysis study with quercetin and rutin

Selective oxidative damage to apolipoprotein B in LDL can be effected radiolytically by (*)Br(2)(-) radicals. Twenty-seven Trp residues constitute major primary sites of oxidation, but two-thirds of oxidized Trps ((*)Trp) that are formed are repaired by intramolecular electron transfer from Tyr residues with formation of phenoxyl radicals (TyrO(*)). Analysis of (*)Trp and TyrO(*) transient absorbance changes suggests that other apolipoprotein B residues, probably Cys, are oxidized. LDL-bound quercetin can efficiently repair this damage. Absorption studies show that a LDL particle has the capacity to bind approximately 10 quercetin molecules through interaction with apolipoprotein B. The repair occurs by intramolecular electron transfer characterized by a rate constant of 2 x 10(3) s(-)(1). In contrast, rutin, a related flavonoid which does not bind to LDL, cannot repair oxidized apolipoprotein B. Urate is a hydrophilic plasma antioxidant which displays synergistic antioxidant properties with flavonoids. Urate radicals produced by (*)Br(2)(-) can also be repaired by LDL-bound quercetin. This repair occurs with a reaction rate constant of 6.8 x 10(7) M(-)(1) s(-)(1). Comparison with previous studies conducted with human serum albumin-bound quercetin suggests that quercetin analogues tailored to be carried preferentially by lipoproteins might be more powerful plasma antioxidants than natural quercetin carried by serum albumin.