Methionine oxidation by reactive oxygen species: reaction mechanisms and relevance to Alzheimer's disease

Reactive oxygen species and biological aging: a mechanistic approach.

Publisher Summary This chapter discusses the formation and essential physicochemical properties of radical species from the three major classes of organic sulfur compounds—namely, thiols, disulfides, and monosulfides. Electron spin resonance (ESR) is often hampered for sulfur-centered radicals owing to the lack of nuclear spin in the main sulfur isotope, 32S. This practically prevents any information on structural aspects. Most of the data available nowadays on sulfur-centered radicals stem from radiation chemical studies, particularly those based on the technique of pulse radiolysis. The latter essentially relies on a time-resolved detection of optical or other physical properties of the radicals. The variety of sulfur-centered radicals seems to exceed that of oxygen-centered radicals. Also, the spectrum of their chemical reactions is at least as manifold as for the oxygen analogs. Among the significant aspects emerging from the available data is the pronounced tendency of sulfur-entered radicals to coordinate with electron-rich centers. Specific electronic structures, such as the 2σ/1σ* three-electron bonds, provide a rationale for many physical and chemical characteristics of the radical species. Another important finding is the frequent involvement of sulfur-centered radicals in thermodynamic equilibria. This appears to be of particular significance for the role of thiyl radicals, because it could explain, for example, some of the insufficiencies in the repair and protection mechanism of thiols. In fact, the equilibria open the possibility that thiyl radicals indirectly could even contribute to biological damage.

[15] Sulfur-centered free radicals

The presence of amino acids in atmospheric precipitation and aerosols has been noted for many years, yet relatively little is known about these or other nitrogen containing organic compounds in the atmosphere. Marine and continental rainwater analyses indicate that atmospheric aerosols, and subsequently atmospheric precipitation, may contain substantial levels of free and combined amino acids. The most likely source of amino N in the remote marine atmosphere appears to be the injection of proteinaceous material through the action of bursting bubbles at the sea-air interface or the long range transport from terrestrial sources. The capacity of these substrates to undergo photooxidation and photodegradation in the atmosphere to simpler species, such as ammonium ions, carboxylic acids, and for the S containing amino acids, oxidized forms of sulfur, has received little attention from atmospheric chemists. The photochemistry of covalently bound amino groups, particularly as found in peptides and amino acids, is discussed here with the purpose of summarizing what is known of their occurrence and their possible importance to atmospheric chemistry.

Amino acid nitrogen in atmospheric aerosols: Occurrence, sources and photochemical modification

The oxidation of diethyl and diphenyl sulfide photosensitized by dicyanoanthracene (DCA), N-methylquinolinium tetrafluoroborate (NMQ(+)), and triphenylpyrylium tetrafluoroborate (TPP(+)) has been explored by steady-state and laser flash photolysis studies in acetonitrile, methanol, and 1,2-dichloroethane. In the Et(2)S/DCA system sulfide-enhanced intersystem crossing leads to generation of (1)O(2), which eventually gives the sulfoxide via a persulfoxide; this mechanism plays no role with Ph(2)S, though enhanced formation of (3)DCA has been demonstrated. In all other cases an electron-transfer (ET) mechanism is involved. Electron-transfer sulfoxidation occurs with efficiency essentially independent of the sulfide structure, is subject to quenching by benzoquinone, and does not lead to Ph(2)SO cooxidation. Formation of the radical cations R(2)S(*+) has been assessed by flash photolysis (medium-dependent yield, dichloroethane>>CH(3)CN>CH(3)OH) and confirmed by quenching with 1,4-dimethoxybenzene. Electron-transfer oxidations occur both when the superoxide anion is generated by the reduced sensitizer (DCA(*-), NMQ(*)) and when this is not the case (TPP(*)). Although it is possible that different mechanisms operate with different ET sensitizers, a plausible unitary mechanism can be proposed. This considers that reaction between R(2)S(*+) and O(2)(*-) mainly involves back electron transfer, whereas sulfoxidation results primarily from the reaction of the sulfide radical cation with molecular oxygen. Calculations indeed show that the initially formed fleeting complex RS(2)(+)...O-O(*) adds to a sulfide molecule and gives strongly stabilized R(2)S-O(*)-(+)O-SR(2) via an accessible transition state. This intermediate gives the sulfoxide, probably via a radical cation chain path. This mechanism explains the larger scope of ET sulfoxidation with respect to the singlet-oxygen process.

Photosensitized Oxidation of Sulfides: Discriminating between the Singlet-Oxygen Mechanism and Electron Transfer Involving Superoxide Anion or Molecular Oxygen

The oxidation of simple aliphatic sulphides [MeSMe, EtSEt, (CH2)4S] by hydroxyl radicals occurs via a complex reaction mechanism. The first step is addition of the OH· to sulphur to form R2ṠOH radicals. At low sulphide concentrations ( 1 ms) positive ions. The stable oxidation product, sulphoxide, has been identified.

K.-Dieter Asmus

Papers

Formation of positive ions and other primary species in the oxidation of sulphides by hydroxyl radicals