Synthesis and characterization of multinuclear manganese-containing tungstosilicates.

The five manganese-containing, Keggin-based tungstosilicates [Mn(II)3(OH)3(H2O)3(A-α-SiW9O34)](7-) (1), [Mn(III)3(OH)3(H2O)3(A-α-SiW9O34)](4-) (2), [Mn(III)3(OH)3(H2O)3(A-ß-SiW9O34)](4-) (3), [Mn(III)3Mn(IV)O3(CH3COO)3(A-α-SiW9O34)](6-) (4), and [Mn(III)3Mn(IV)O3(CH3COO)3(A-ß-SiW9O34)](6-) (5) were synthesized in aqueous medium by interaction of [A-α-SiW9O34](10-) or [A-ß-SiW9O34H](9-) with either MnCl2 (1) or [Mn(III)8Mn(IV)4O12(CH3COO)16(H2O)4] (2-5) under carefully adjusted reaction conditions. The obtained salts of these polyanions were analyzed in the solid state by single-crystal X-ray diffraction, IR spectroscopy, and thermogravimetric analysis. The salts of polyanions 1, 2, and 4 were further characterized in the solid state by magnetic studies, as well as in solution by electrochemistry.

Caffeic acid inhibits the formation of 7-carboxyheptyl radicals from oleic acid under flavin mononucleotide photosensitization by scavenging singlet oxygen and quenching the excited state of flavin mononucleotide.

We examined the effects of caffeic acid (CA) and related compounds on 7-carboxyheptyl radical formation. This analysis was performed using a standard D2O reaction mixture containing 4.3 mM oleic acid, 25 µM flavin mononucleotide (FMN), 160 mM phosphate buffer (pH 7.4), 10 mM cholic acid, 100 mM α-(4-pyridyl-1-oxide)-N-tert-butylnitrone, and 1 mM Fe(SO4)2(NH4)2 during irradiation with 7.8 J/cm2 at 436 nm. 7-Carboxyheptyl radical formation was inhibited by CA, catechol, gallic acid, chlorogenic acid, ferulic acid, noradrenalin, 2-hydroxybenzoic acid, 3-hydroxybenzoic acid, and 4-hydroxybenzoic acid. Quinic acid, benzoic acid, and p-anisic acid had no effect on radical formation. These results suggest that a phenol moiety is essential for these inhibitory effects. The fluorescence intensity of FMN decreased by 69%±2% after CA addition, suggesting that CA quenches the singlet excited state of FMN. When 1 mM CA was added to a standard reaction mixture containing 25 µM FMN, 140 mM phosphate buffer (pH 7.4), and 10 mM 4-oxo-2,2,6,6-tetramethylpiperidine, the electron spin resonance signal of 4-oxo-2,2,6,6-tetramethylpiperidinooxy disappeared. This finding suggests that singlet oxygen was scavenged completely by CA. Therefore, CA appears to inhibit 7-carboxyheptyl radical formation by scavenging singlet oxygen and quenching the excited state of FMN.

Determination of the structures of radicals formed in the reaction of antimalarial drug artemisinin with ferrous ions.

While artemisinin 1 has been widely used to treat malaria in traditional Chinese medicine, its exact antimalarial mechanism remains unclear. To elucidate the mechanisms of the antimalarial action by artemisinin, the reactions of artemisinin, artemether 2 and artesunate 3 with Fe2+ were analyzed using an electron spin resonance (ESR), high performance liquid chromatography-electron spin resonance (HPLC-ESR) and high performance liquid chromatography-electron spin resonance-mass spectrometer (HPLC-ESR-MS). α-(4-Pyridyl-1-oxide)-N-tert-butylnitrone (4-POBN) was used as a spin trap reagent in the reactions. Radicals were detected by ESR and structures of the radicals were determined by HPLC-ESR-MS. Based on the ions, m/z 478, m/z 418 and m/z 238 which were determined by HPLC-ESR-MS, we identified following radicals: artemisinin-derived primary and secondary radicals, 6 and 7; acetyl radical, 8; a radical formed through elimination of acetyl group from 6, 10 in the reaction of artemisinin with Fe2+. Radicals, 7 and 8 were also detected in the reaction of artemether and artesunate.