Hydrogen atom donation

Two parameters are required for the description of hydrogen bonding. One is required to account for the hydrogen atom donating capacity of a substituent and another to account for its hydrogen atom accepting capacity. The simplest approach is the use of riu, the... 

In the case of trivalent transition metal diketonates, ML3, such as manganese, iron or cobalt, photolysis at the LMCT band in hydrogen-atom-donating solvents (511) leads to the formation of a ML2 derivative and /3-diketone, the quantum yield depending on the excitation wavelength and the solvent. The mechanism reported in equations 68a-c has been postulated, where [ML2L] represents the species which formally contains the M(II) cation bound to the /3-diketonato radical [L]. ... 

Hydrogen-atom donation from thiols to carbon-centered radicals was too often assumed to be the only property of thiols that is important in free radical processes in biology. Moreover, it was a common presumption that reaction (1) was the end of the biological pathway in which thiols repair radicals. Equilibria turned out to be much more important in sulfur radical chemistry than was first thought. For instance, the hydrogen-donation reaction was found to be a reversible equilibrium over 30 years after it was first observed. ... 

For a long time, thiyl radicals (RS ) have been considered as rather unreactive species. They were formed as a result of a hydrogen atom donation from thiols (RSH) to carbon-centered radicals, in a process responsible for the repair of damage in biological environment. However, the H-atom donation or repair reaction is now known to involve a reversible equilibrium the RS radicals are able to abstract H-atoms. Hence, the common presumption that H-atom donation by RSH is the end of the biological pathway in which RSH repairs radicals is incorrect. 

The remaining steps that are commonly associated with the non-induced decomposition portion of the reaction in a hydrogen atom donating solvent (R H) are given below, where R is an alkyl group. 

DPPH (l,l-diphenyl-2-picryhydrazyl) is a purple-colored stable free radical that is reduced to the yellow-colored diphenylpicrylhydrazine by free radicals. The DPPH assay measures one electron, such as hydrogen atom donating activity and hence provides a measure of free radical scavenging activity. This assay is suitable for the initial screening of multiple samples, such as plant extracts. Reaction mixtures containing test samples dissolved in DMSO and DPPH in absolute ethanol are incubated at 37°C for 30 min in a 96-well plate and absorbance measured at 515 nm. ... 

However, the conclusions based on these experiments and the calculations may be questioned as a result of recent work in our laboratory [112] carried out to find the source of the hydrogen atoms donated to the cyclopropyl radical (Scheme 38). The results are shown in Table 17. In the D-model the main source of these hydrogen atoms would be... 

Subsequently, Cl was observed in polymerization of MMA with high concentrations of cobaloximes,77 293 phthalocyanines,14 136 294 295 and porphyrins.295 Cl is observed only when the cobaloxime was in the 2+ oxidation state and can catalyze chain transfer.296 Thus, Cl is a phenomenon that is closely connected to CCT. The first explanation involved a hydride (as in eq 31), where P is a polymer obtained from the propagating radical which has been terminated by hydrogen atom donation by Co111—H.161... 

UV/VIS Techniques. The UVAflS method can be ntilized to measnre the hydrogen atom donating abihty of antioxidants to alkoxyl and nitrogen-centered radicals, and some techniqnes are ontlined here ... 

It is well known that substituents have a profound effect on the hydrogen atom donating ability of phenols. Indeed, only those phenols bearing electron donating substituents, particularly at the ortho and/or para positions, are active as antioxidants. In general, this is as expected since such groups are expected to lower the phenolic O—H bond dissociation enthalpy and increase the reaction rates with peroxyl radicals. 

Antioxidant activity 842-845 practical Unlit to 899, 900 Antioxidants—see also Co-antioxidants analysis of 941, 947, 949, 955, 956, 960, 961, 981, 982 as food preservatives 982 biologicaUy active 913 calculations on 895-899 chain-breaking 840, 874 efficiency of 850-895, 900 media effects on 876-895 structural effects on 859-876 future prospects for 899-901 hydrogen atom donating abiUty of 865-867 in aircraft fuel 990 in alcohoUc beverages 973 in drinking water 963 induction period for 843 in foodstuffs 925 inhibition rate constants of 992 in Upid membranes 884-895... 

A communication and full paper tell of the efficient photoreduction of 4-chlorobiphenyl to biphenyl by excitation of 9,10-dihydro-lO-methylacridine (163) or acriflavine (164) in aqueous acetonitrile containing sodium borohydride. A variety of alkyl halides, benzyl halides and chlorobenzenes were also reduced. The reaction proceeds by electron transfer from the excited state of the dihydroacridine to the chloroarene, chloride loss and hydrogen atom donation to the arene radical. Thus photoreduction of the arene is coupled with oxidation of the dihydroacridine to the acridinium salt the latter is reduced back to the dihydroacridine by the borohydride. 

Silverman and Zieske have rationalized how a protein nucleophile other than flavin is involved in MAO inactivation reactions, and why different inactivator compounds specifically react with flavin, protein amino acids, or both (100). Hydrogen atom donation from a cysteine residue to the flavin semiquinone radical would produce a thiyl radical, which could then capture the primary or secondary alkyl radical generated on cyclopropyl ring opening from the amine radical cation of the inactivator. The hydrogen atom abstraction reaction between the flavin and active site amino acid may be an equilibrium process such that either species could be present at any turnover. Hence, a combination of steric constraints and proximity to either the flavin semiquinone radical or the thiol radical will determine the site of adduct formation for a particular inactivator structure. A two-dimensional representation is shown in Scheme 23 (compounds 40-42), which illustrates the proposed equilibrium between the flavin semiquinone radical and amino acid as well as the proposed intermediates for the inactivation of MAO by A-(l-methylcyclopropyl)benzylamine 40 (104), rrradical center relative to the particular protein radical is consistent with proposed site of attachment of inactivator to protein 40 is near the flavin radical, such that exclusive flavin attachment occurs, 41 is positioned closer to the amino... 

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