This article is a review of the fundamental chemistry of the tocopherols and tocotrienols relevant to their antioxidant action. Despite the general agreement that α-tocopherol is the most efficient antioxidant and vitamin E homologuein vivo, there was always a considerable discrepancy in its “absolute” and “relative” antioxidant effectivenessin vitro, especially when compared to γ-tocopherol. Many chemical, physical, biochemical, physicochemical, and other factors seem responsible for the observed discrepancy between the relative antioxidant potencies of the tocopherolsin vivo andin vitro. This paper aims at highlighting some possible reasons for the observed differences between the tocopherols (α-, β-, γ-, and δ-) in relation to their interactions with the important chemical species involved in lipid peroxidation, specifically trace metal ions, singlet oxygen, nitrogen oxides, and antioxidant synergists. Although literature reports related to the chemistry of the tocotrienols are quite meager, they also were included in the discussion in virtue of their structural and functional resemblance to the tocopherols.

The chemistry and antioxidant properties of tocopherols and tocotrienols

Reversible activation of covalent molecules by transition-metal complexes. The role of the covalent molecule

Rational design strategies based on practical fluorescence modulation mechanisms would enable us to rapidly develop novel fluorescence probes for target molecules. Here, we present a practical and general principle for modulating the fluorescence properties of fluorescein. We hypothesized that (a) the fluorescein molecule can be divided into two moieties, i.e., the xanthene moiety as a fluorophore and the benzene moiety as a fluorescence-controlling moiety, even though there is no obvious linker structure between them, and (b) the fluorescence properties can be modulated via a photoinduced electron transfer (PeT) process from the excited fluorophore to a reducible benzene moiety (donor-excited PeT; d-PeT). To evaluate the relationship between the reduction potential of the benzene moiety and the fluorescence properties, we designed and synthesized various derivatives in which the reduction potential of the benzene moiety was fine tuned by introducing electron-withdrawing groups onto the benzene moiety. Our results clearly show that the fluorescence properties of fluorescein derivatives were indeed finely modulated depending upon the reduction potential of the benzene moiety. This information provides a basis for a practical strategy for rational design of novel functional fluorescence probes.

Rational Principles for Modulating Fluorescence Properties of Fluorescein

Publisher Summary The liquid phase oxidation of hydrocarbons by molecular oxygen forms the basis for a wide variety of petrochemical processes including the manufacture of phenol and acetone from cumene, adipic acid from cyclohexane, terephthalic acid from p -xylene, acetaldehyde and vinyl acetate from ethylene, and propylene oxide from propylene. The majority of these processes employ catalysis by transition metal complexes to attain maximum selectivity and efficiency. This chapter describes the subject of metal-catalyzed oxidations of organic compounds in the liquid phase, largely within a mechanistic framework. A better understanding of the catalytic action of metal complexes is essential from the point of view of increasing selectivity and efficiency. Metal-catalyzed oxidations may be conveniently divided into two types—homolytic and heterolytic. The homolytic catalysis involves soluble transition metal salts (homogeneous), such as the acetates or naphthenates of Co, Mn, and Cu, or the metal oxides (heterogeneous). Heterolytic catalysis involves reactions of organic substrates coordinated to transition metals.

Metal-Catalyzed Oxidations of Organic Compounds in the Liquid Phase: A Mechanistic Approach

Halogen oxides: Radicals, sources and reservoirs in the laboratory and in the atmosphere

Publisher Summary This chapter describes different types of peroxide. The solid hydrated peroxides of elements such as copper, praseodymium, or cerium may well contain O2–ions: the only roughly stoichiametric peroxides of thorium and plutonium represent a slightly different class of compound, but may still be essentially ionic. The peroxides of transition metals either in complexes or in high oxidation states (or in both), however, contain peroxide linked to the metal by directed bonds, and are closely related to the oxygenated O2-carrying complexes. The characteristic and interesting properties of transition metal complexes are largely associated with compounds of the last two classes. Most of the known transition metal peroxides have been obtained by the action of H2O2 on some derivative of the metal; in aqueous solution, the proportion of peroxide in the product increases with increasing pH, though this may not be true in solvents other than water. Peroxides of rhenium and cobalt, and of course the oxygenated O2-carrying species containing iron, iridium, or copper, have been made from O2 instead of H2O2; there is evidence that peroxy derivatives of uranium have been obtained by heating compounds of uranium in air, while peroxides are often postulated as unstable intermediates in the oxidation by air of such species as chromium.

E. A. V. Ebsworth

Papers

Peroxy Compounds of Transition Metals

The chemistry of oxygen