Radical mechanisms for 1,5-dihydroflavin reduction of carbonyl compounds.

Abstract

A discussion of the expected mechanism of radical reduction of carbonyl compounds by dihydroflavin as it relates to the structure of the carbonyl compound is provided. Factors which must be taken into account are the free energies of formation of the radical anions (.CR209), the acidity of the conjugate acid of the carbonyl function (R2C=OH), and the stability of the car-

The structure–activity relationship of flavonoids as scavengers of peroxynitrite

“…The higher scavenging potency of galangin compared with galangin 3-O-methyl ether may suggest that C-3 methoxylation reduced the scavenging effect of flavonols. From chemical and kinetic viewpoints, it would be reasonable to assume that the free hydroxyl group at the C-3 position would enhance the free-radical scavenging ability; since according to the reduction mechanism of free radicals, a scavenger should be capable of providing electrons or hydrogens, or of receiving electron pairs, while the hydrogen bond between the 3-hydroxyl group and the 4-keto group would increase this ability (Williams et al, 1975). On the other hand, galangin and kaempferol, which possess only one hydroxyl group at C-4' on the B ring, showed a rather weak activity.…”

Kinetics and mechanisms of the 1,5-dihydroflavin reduction of carbonyl compounds and the flavin oxidation of alcohols. 4. Interconversion of formaldehyde and methanol

“…Due to the necessary presence of methanol as a stabilizer in the formaldehyde solutions, the large concentration of formaldehyde over flavin present, and a lack of an adequate means to quantitatively determine methanol at ~10-5 M, it was not possible to do a complete product analysis. However, based on electrochemical potential calculations (see ref 10), it is apparent that the reduction of Flox by methanol will occur in basic solution at high methanol concentration due to the shift of the equilibrium, F1H2t/CH20 <=t Flox/CH30H (K^) to the left. At pH 10.9 and methanol concentrations varying between 1.5 and 9.2 M, the reduction of Flox by methanol does occur.…”

“…Figure10. Employing the initial state of FIH2T (1 M) and CH2O (1 M) the free energies (AG°) of formation of plausible radical intermediates and products (at 1 M) are plotted vs. pH.…”

“…0 the standard free energy of formation of B from A (AG°\ -b) exceeds the standard free energy of formation of B' from A ( (7° --b') by ~13 kcal M'1 and AC*eXpti by12-13 kcal M'1 (for calculations of AG°values see ref10). For these reasons states B and C cannot occur along the reaction coordinate in the reduction of H2CO by FIH2 or F1H'.…”

The kinetics and mechanisms of 1,5-dihydroflavin reduction of carbonyl compounds and flavin oxidation of alcohols. 3. Oxidation of benzoin by flavin and reduction of benzil by 1,5-dihydroflavin

“…Base-catalyzed racemization of (+)-benzoin was studied (aerobically) in the same solvent, with the same carbonate buffers and in the same pH range employed in the kinetic investigation of the reduction of Flox by benzoin. The racemization was found to follow first-order kinetics to 4 to 611 /2. The determined rate constants are included in Figure 2 Oxidation of benzoin with 302 was followed spectrophotometrically by observation of the time dependence for benzil formation at 375 nm.…”