Reactions In Aqueous Environments Research Articles

A DFT Study of the Amadori Rearrangement above a Phosphatidylethanolamine Surface: Comparison to Reactions in Aqueous Environment

Mechanisms for Amadori rearrangements from Schiff bases from reactions between d-erythrose and ethylamine, glycine, and phosphatidylethanolamine (PE) based on Dmol3/DFT calculations were realized. For the case of PE, calculations were done under periodic boundary conditions (PBCs), in an amine-phospholipid monolayer model with two molecules of PE by cell. In the three cases, the reaction started with a neutral Schiff base, having in the case of PE surface model a positive charged amine group belonging to the adjacent PE molecule. The catalytic role of PE surface components such as amine and phosphate groups is highlighted. All models included water molecules forming hydrogen bond networks, these networks were involved in the reactions by stabilizing reaction intermediates and transition states and performing as proton-transfer carriers, important in all steps of reactions. In all the studied reactions, they take place in two steps, namely, (1) formation of a 1,2-enaminol intermediate and (2) ketonization ...

Activity of water-soluble fullerenes towards OH-radicals and molecular oxygen

bis-functionalized C60 carrying negatively charged carboxyl groups, C60[C(CO−2)2]2, or positively charged quaternary ammonium groups, C60(C4H10N+)2, have been shown to exhibit high water-solubility without cluster formation. This renders them excellent targets for free radical attack and energy transfer reactions in aqueous environment. Absolute rate constants on the order of 8×109 M−1 s−1 and 1.5×109 M−1 s−1 have been measured for the addition of OH radicals to the fullerene core and for the formation of 1O2 through triplet excited states of the fullerenes, respectively, for a variety of bis-functionalized stereoisomers. Comparatively lower rates have been observed for the corresponding energy transfer reactions with γ-cyclodextrin-encapsulated (C60/γ-CD, C60C(CO−2)2/γ-CD, and C60[C(OCH2CH2)3CH3]2/γ-CD), and water-soluble poly-functionalized fullerenes (e,e,e-C60[C(CO−2)2]3, C60[(CH2)4SO3H]6, and C60(OH)18). The latter three also exhibited a lower reactivity towards OH radicals. In case of the γ-CD complexes, OH reacted exclusively with the macromolecular host.

The Elucidation of Peroxyl Radical Reactions in Aqueous Solution with the Help of Radiation‐Chemical Methods

AbstractWhenever free radicals are formed, independent of whether this occurs thermally, is induced by UV or ionizing irradiation, or takes place in redox reactions, they are converted rapidly into the corresponding peroxyl radicals in the presence of oxygen. Peroxyl radical reactions in aqueous environments are observed not only in aquatic systems (e.g., rivers, lakes and oceans) but also in the living cell and to a considerable degree even in the atmosphere (in water droplets). The peroxyl radical chemistry occurring in this medium is often very different from that observed in the gas phase or in organic solvents. In spite of the great importance of these reactions in medicine (for example in anti‐cancer irradiation therapy and ischaemia) there have been comparatively few studies of peroxyl reactions in aqueous media. Radiation‐chemical techniques such as pulse radiolysis offer the best means for carrying out such studies, so that it is not surprising that the majority of the information available in this area has been obtained with the help of radiation‐chemical methods. The radiation chemistry of water can be con trolled in such a manner that the main products are ˙OH radicals (90 % yield), which react with substrate molecules to give substrate radicals and in the presence of oxygen to give substrate peroxyl radicals. The experimental conditions can also be varied in such a way that HO/O radicals can be formed in 100 % yield and caused to react with substrates. We therefore have a simple access to these intermediates, which are extremely important in biological systems. A detailed product analysis, supported by kinetic studies carried out with the help of pulse radiolysis, has been used to clarify the chemistry of a series of peroxyl radicals, so that sufficient material is now available to justify a review of the variety of the peroxyl radical reactions studied by means of radiation‐chemical methods. A more general survey of the physical properties of the peroxyl radicals and their unimolecular and bimolecular reactions will be followed by a discussion of selected examples of various classes of substance. Because of the great biological importance of radical‐induced DNA damage this area will also be treated briefly.

The use of X-ray photoelectron spectroscopy in corrosion science

Most metals in commercial use are protected from reaction with an environment at ambient temperature by an oxide layer of ~5 nm in thickness. This layer may increase in thickness, by solid state diffusion, at high (>300°C) temperature or may dissolve and be replaced by soluble species by electrochemical reactions in aqueous environments. These are the twin processes of corrosion and the problem of preventing such destructive transformations had led to a longstanding interest by corrosion scientists in the thin film: and indeed in sub-monolayers. Prior to the introduction of the techniques of surface derived ion and electron spectroscopy the chemical analyses of films, which had known physical and electronic characteristics, were unavailable. This review attempts to specify the requirements of an analytical technique for thin films and examines the present role of XPS in this context. The areas covered will include elemental analysis, evaluation of valence state, non-destructive information, elemental distribution and finally the use of ion-etch depth profiling. Examples are drawn from corrosion problems in both aqueous and gaseous environments.