Spectral and Kinetic Properties of Radical Cations Derived from Oxoisoaporphines: Relevance to Electron-Transfer Processes Involving Phytoalexins

The thermally induced intermolecular electron transfer reaction in acetonitrile between the tetracyanoethylene (TCNE), a π-electron acceptor with a large electron affinity, and six oxoisoaporphines (2,3-dihydro-7<i>H</i>-dibenzo­[<i>de</i>,<i>h</i>]­quinolin-7-one, 5-methoxy-2,3-dihydro-7<i>H</i>-dibenzo­[<i>de</i>,<i>h</i>]­quinolin-7-one, 1-azabenzo­[<i>de</i>]­anthracen-7-one, 5-methoxy-1-azabenzo­[<i>de</i>]­anthracen-7-one, 7<i>H</i>-benzo­[<i>e</i>]­perimidin-7-one, and 2-methyl-7<i>h</i>-benzo­[<i>e</i>]­perimidin-7-one) is reported. Spectral and kinetic characteristics are presented for radical cations derived from these six oxoisoaporphines either generated by a thermal reaction or generated radiolytically in argon-saturated 1,2-dichloroethane, oxygen-saturated acetone, and acetonitrile. The radical cations of oxoisoaporphines are insensitive to oxygen and are mostly characterized by absorption maxima of their most intense bands located at λ_max = 400–410 nm, except of the radical cations derived from 2,3-dihydrooxoisoaporphines. For the latter compounds, the absorption maxima of the most intense absorption bands are located at λ_max = 290–295 nm. Their locations are independent of the presence of functional groups and the solvents used. They are formed in bimolecular processes with pseudo-first-order rate constants ranging from 2.1 × 10⁵ to 1.5 × 10⁶ s^–1 (in solutions containing 10^–4 M of the substrate), depending on the derivative and the solvent used. They are stable either when formed via the electron-transfer reaction with TCNE or when generated in isolation in pulse radiolysis of Ar-saturated 1,2-dichloroethane. In acetone and acetonitrile they decay predominantly by first-order kinetics with the first-order rate constants ranging from 2.3 × 10⁴ to 5.1 × 10⁴ s^–1. Formation of dimeric radical cations for all of the oxoisoaporphines studied was observed in acetonitrile solutions, and for azaoxoisoaporphines also in acetone solutions. The experimental spectra show a reasonably good agreement with the ZINDO/S semiempirical quantum mechanical calculations of radical cation absorptions

Methionine Residue Acts as a Prooxidant in the ^•OH-Induced Oxidation of Enkephalins

Enkephalins are bioactive pentapeptides (Tyr-Gly-Gly-Phe-Leu (Leu-enk) and Tyr-Gly-Gly-Phe-Met (Met-enk)) produced while an organism is under mental and/or physical stress. In the course of their biological action they are exposed to reactive oxygen and nitrogen species. We have reinvestigated the reactions of ^•OH radicals toward these peptides in order to elucidate the oxidation mechanisms and the final products. Nanosecond pulse radiolysis was used to obtain the spectra of the reaction intermediates and their kinetics. Additional insight into details of the oxidation mechanism was gained by identification of main final products by means of UV–vis spectrophotometry, HPLC coupled with fluorescence spectroscopy, and mass spectrometry. The key processes are different in both peptides. In Leu-enk, the first step is an ^•OH radical addition to the aromatic rings of Tyr and Phe residues that leads to hydroxylated residues, dihydroxyphenylalanine (DOPA) from Tyr and tyrosine isomers from Phe, respectively. In Met-enk, these processes are less important, an additional target being the sulfur atom of the methionine residue. Depending on pH either an OH-adduct (hydroxysulfuranyl radical) or a sulfur radical cation undergo intramolecular electron transfer with Tyr residue resulting in a repair of Met and oxidation of Tyr to tyrosyl radicals and a final formation of dityrosine. At low pH, the OH-adducts to Tyr residue are precursors of tyrosyl radicals and dityrosine. Thus, the final products coming from oxidation of the Tyr residue depend strongly on the neighboring residues and the pH

Kinetic Studies on the Formation of Sulfonyl Radicals and Their Addition to Carbon–Carbon Multiple Bonds

The reactions of α-hydroxyl and α-alkoxyl alkyl radicals with methanesulfonyl chloride (MeSO₂Cl) have been studied by pulse radiolysis at room temperature. The alkyl radicals were produced by ionizing radiation of N₂O-saturated aqueous solution containing methanol, ethanol, isopropanol, or tetrahydrofuran. The transient optical absorption spectrum consisted of a broad band in the region 280–380 nm with a maximum at 320 nm typical of the MeSO₂^• radical. The rate constants in the interval of 1.7 × 10⁷–2.2 × 10⁸ M^–1 s^–1 were assigned to an electron-transfer process that leads to MeSO₂Cl^•–, subsequently decaying into MeSO₂^• radical and Cl^–. The rate constants for the addition of CH₃SO₂^• to acrolein and propiolic acid were found to be 4.9 × 10⁹ M^–1 s^–1 and 5.9 × 10⁷ M^–1 s^–1, respectively, in aqueous solutions and reversible. The reactivity of tosyl radical (<i>p</i>-CH₃C₆H₄SO₂^•) toward a series of alkenes bearing various functional groups was also determined by competition kinetics in benzene. The rate constants for the addition of tosyl radical to alkenes vary in a much narrower range than the rate constants for the reverse reaction. The stabilization of the adduct radical substantially contributes to the increase of the rate constant for the addition of tosyl radical to alkenes and, conversely, retards the β-elimination of tosyl radical