Structure Effect on Antioxidant Activity of Catecholamines toward Singlet Oxygen and Other Reactive Oxygen Species in vitro.

The reactivity of catecholamine neurotransmitters and the related metabolites were precisely investigated toward 1,1-diphenyl-2-picrylhydrazyl (DPPH) radicals and reactive oxygen species. Catecholamines reacted immediately with DPPH radicals, their reactivity being stronger than that of ascorbic acid as a reference. Superoxide scavenging activities of catecholamines determined by WST-1 and electron spin resonance (ESR) spin trapping methods were also high. Whereas tyrosine, the dopamine precursor showed no reactivity toward superoxide. The reactivity toward singlet oxygen was evaluated by observing specific photon emission from singlet oxygen. The results revealed that reactivity of catecholamines was markedly higher than that of sodium azide, and catechin as catechol reference. The reaction of catecholamines and singlet oxygen was further studied by ESR using 55-dimethyl-1-pyrroline N-oxide (DMPO) as a spin trapping reagent and rose bengal as photosensitizer. DMPO-OH signal of epinephrine was significantly small compared to other catecholamines, catechin, and 4-methylcatechol as a reference compound and was as small as that of tyrosine. The signal formation was totally dependent on singlet oxygen, and the presence of catechol compounds. These results indicated that epinephrine is the most potent singlet oxygen quencher than other catecholamines, and the secondary amino group in its alkyl side chain could play a role in unique singlet oxygen quenching property of epinephrine.

Hydrogen abstraction from neurotransmitters by active oxygen species facilitated by intramolecular hydrogen bonding in the radical intermediates.

The reactivity of neurotransmitters toward hydrogen abstraction by an active oxygen species (the cumylperoxyl radical) is comparable to that of a strong antioxidant such as catechin due to the strong intramolecular hydrogen bonding, which has been successfully detected by ESR.

Synthesis and characterization of a series of (hydroperoxo)-, (alkylperoxo)-, and (mu-peroxo)palladium complexes containing the hydrotris(3,5-diisopropylpyrazolyl)borato ligand (Tp(iPr2)): (Tp(iPr2))(py)Pd-OO-X [X = H, t-Bu, Pd(Tp(iPr2))(py)] and (Tp(iPr2))(py)Pd-(mu-kappa(1):kappa(2)-OO)-PdTp(iPr2).

Dehydrative condensation of the hydroxopalladium complex (Tp(iPr2))(py)Pd-OH (1) with hydroperoxides (XOOH: X = H, t-Bu) produces the corresponding (hydroperoxo)-, (Tp(iPr2))(py)Pd-OOH (2a), and (tert-butylperoxo)palladium complexes, (Tp(iPr2))(py)Pd-OOBu(t) (3). Treatment of 2a with PPh(3) results in concomitant ligand displacement giving (Tp(i)(Pr2))(Ph(3)P)Pd-OOH (2b) and oxygenation of PPh(3) giving O=PPh(3). Further condensation between 1 and 2a gives the mu-kappa(1):kappa(1)-peroxo complex (Tp(iPr2))(py)Pd-OO-Pd(Tp(iPr2))(py) (4), while condensation between the bis(mu-hydroxo)dipalladium complex (PdTp(iPr2))(2)(mu-OH)(2) (7) with 2a affords the unsymmetrical mu-kappa(1):kappa(2)-peroxo complex (Tp(iPr2))(py)Pd-OO-PdTp(iPr2) (5). These peroxopalladium complexes 2-5 have been fully characterized by a combination of spectroscopic and crystallographic analyses, which lead to description of the O-O moieties in these complexes as peroxide (O(2)(2-)) with sp(3)-hybridized oxygen atoms. The OOH moiety in 2b is found to interact with the noncoordinated nitrogen atom of the pendant pyrazolyl group through hydrogen bond. The O(2) moieties in 2-5 turn out to be so nucleophilic (basic) as to add across carbon-heteroatom multiple bonds in acetonitrile and acetaldehyde to give the peroxometallacycle Tp(iPr2)Pd[OOC(Me)=N)]Pd(iPr2)(py)(8) (from 2, 4, and 5) and the acetato complex (Tp(iPr2))(py)Pd-OC(=O)CH(3) (9) (from 2-4), respectively. Methyl vinyl ether and styrene, CH(2)=CHY (Y = OMe, Ph), are converted to the corresponding methyl ketones, CH(3)C(=O)Y, upon treatment with 2-4.

Synthesis and Characterization of the Benzoylformato Ferrous Complexes with the Hindered Tris(pyrazolyl)borate Ligand as a Structural Model for Mononuclear Non-Heme Iron Enzymes.

By using a hindered tripodal ligand, hydrotris(3-tert-butyl-5-isopropylpyrazol-1-yl)borate HB(3-tBu-5-iPrpz)(3), a series of monomeric ferrous complexes having acetate, hydroxide, and benzoylformate ligands were synthesized. Reaction of KHB(3-tBu-5-iPrpz)(3) with anhydrous Fe(OAc)(2) yielded acetato complexes Fe(OAc)[HB(3-tBu-5-iPrpz)(3)] (1) and Fe(OAc)[HB(3-tBu-5-iPrpz)(3)](3-iPr-5-tBupzH) (2). A hydroxo complex Fe(OH)[HB(3-tBu-5-iPrpz)(3)] (3) was prepared by the treatment of 1 or 2 with aqueous NaOH. The geometry of Fe(II) in 3 is a slightly distorted tetrahedron as determined by X-ray crystallography. The hydroxo complex 3 reacted with benzoylformic acid to give the benzoylformato complex Fe(O(2)CC(O)Ph)[HB(3-tBu-5-iPrpz)(3)] (4), which showed thermochromism which depended on the coordination geometry of the benzoylformate ligand. The Fe(II) ion in the colorless form of 4 isolated at 4 degrees C is coordinated by a tetrahedral N(3)O(1) ligand donor set including the unidentate benzoylformato ligand. On the other hand, the bluish purple form of 4 isolated at -20 degrees C has a five-coordinate trigonal bipyramidal Fe(II) center. The benzoylformate ligand in this bluish purple form works as a chelate ligand through coordination of the unidentate carboxylate oxygen atom as well as the ketonic oxygen atom. A benzoylformato complex containing an additional pyrazole, Fe(O(2)CC(O)Ph)[HB(3-tBu-5-iPrpz)(3)](3-iPr-5-tBupzH) (5), was obtained by the reaction of 3 with benzoylformic acid in the presence of 3-tert-butyl-5-isopropylpyrazole. The iron atom in 5 is coordinated by an N(4)O(1) ligand donor set with trigonal bipyramidal geometry. A hydrogen-bonding interaction between the carboxylate oxygen atom and the additional pyrazole's NH proton in 5 is suggested from the short distance between O(carboxylate) and N(pyrazole) observed in the X-ray structure and the absence of the nuNH vibration in the IR spectrum.