Stoichiometric and kinetic studies on Ginkgo biloba extract and related antioxidants

Abstract

Owing to increasing evidence showing the importance of lipid peroxidation in oxidative stress in vivo, the role and evaluation of antioxidants have received much attention. Ginkgo biloba extract (GBE), well-known as an efficient drug against diseases induced by free radicals, has been suggested to exert its effect by antioxidant action. A method was established to determine the activity of GBE as a hydrogen donor by stoichiometric and kinetic studies, and GBE was compared with several other antioxidants such as alpha-tocopherol, propyl gallate, and two kinds of flavonoids which are found in GBE, quercetin, and kaempferol. It was found that there were 6.62 x 10(19) active hydrogens in 1 g of GBE. Stoichiometric studies showed that one molecule of alpha-tocopherol reacted with one molecule of galvinoxyl radical. For quercetin, kaempferol and propyl gallate, the experimental stoichiometric numbers were 4.0, 1.9, and 3.1, respectively. The rates of reaction of antioxidants with galvinoxyl in ethanol were determined spectrophotometrically, using a stopped-flow technique. The second-order rate constant, k2, obtained at 25 degrees C was 0.13 (g/L)(-1)s(-1) for GBE and 5.9 x 10(3), 2.1 x 10(3), 1.2 x 10(4), and 2.4 x 10(3) M(-1)s(-1) for quercetin, kaempferol, propyl gallate, and alpha-tocopherol, respectively. The second-order rate constant, k2', on the molar basis of active hydroxyl groups in the tested substances obtained at 25 degrees C decreased in the order of propyl gallate > alpha-tocopherol > quercetin > GBE approximately kaempferol. This is the first study on GBE as an antioxidant which reports both stoichiometric and kinetic results.

Metal chelation, radical scavenging and inhibition of Aβ42 fibrillation by food constituents in relation to Alzheimer’s disease

“…An empirical biexponential equation (4) values were calculated by dividing the overall bimolecular rate constants (k b ) by the stoichiometric factors (n) (Shi & Niki, 1998),…”

“…Stoichiometric k' b values were calculated by dividing the overall bimolecular rate constants (k b ) by the stoichiometric factors (n) ( Table 1) (Shi & Niki, 1998 The reaction between phenolic antioxidant and radical is known to proceed by at least three mechanisms: hydrogen atom transfer (HAT); proton-coupled electron transfer (PCET), which both involve the transfer of proton and electron in one kinetic step; and sequential proton-loss electron transfer (SPLET), in which the phenol dissociates into a phenoxide anion, followed by electron transfer from the phenoxide anion to the radical that is subsequently protonated (Supplementary Fig. S9) (Di Meo et al, 2013;Litwinienko & Ingold, 2007).…”

Lignicolous fungi from northern Serbia as natural sources of antioxidants

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Kinetic and Stoichiometric Evaluation of Free Radicals Scavengers Activities Based on Diphenyl-Picryl Hydrazyyl (Dpph) Consumption

“…and a number of reviews has been dedicated to discuss the data and the advantages and limitations of the different methodologies [1][2][3][4][5][6] . Among these methodologies, preferential attention has been paid to techniques based on the consumption of stable free radicals: DPPH 7,8 , galvinoxyl 9 , and ABTS derived radical cation 10 . These methodologies have the advantage of their experimental simplicity but suffer from important limitations.…”