Media Effects on Antioxidant Activities of Phenols and Catechols

Abstract

The H-atom donating activities of 2,6-di-tert-butyl-4-methylphenol (BHT), 2,6-di-tert-butyl-4-methoxyphenol (DBHA), 2,2,5,7,8-pentamethyl-6-hydroxychroman (PMHC), and 3,5-di-tert-butylcatechol (DTBC) toward the nitrogen-centered 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical were measured by stopped flow methods in hexane, 1-propanol, tert-butyl alcohol, and acetone. Decreases in these activities on transferring from hexane to the hydrogen bond accepting (HBA) solvents, the kinetic solvent effect (KSE), are attributed to hydrogen bonding from the phenolic group. Steric hindrance accounts for a lower decrease observed for the highly hindered BHT and DBHA compared to PMHC. The catechol, DTBC, a very active H-atom donor to DPPH in hexane, showed a dramatic loss of activity in HBA solvents, especially acetone. Higher H-atom donating activities of BHT, DBHA, and PMHC were observed toward the oxygen-centered radical of 2,6-di-tert-butyl-4-(4‘-methoxyphenyl)phenoxyl (DBMP), and the decreases in activity in the HBA solvents paralleled those found with DPPH. Thus the KSE was found to be independent of the nature of the abstracting radical for DPPH and DBMP. The inhibition of the oxygen uptake (IOU) method was used to determine the antioxidant activities (k inh) of α-tocopherol, PMHC, catechol, and DTBC during free radical autoxidation of styrene and mixtures of styrene and tert-butyl alcohol. The k inh of α-tocopherol and PMHC dropped to one-tenth of the values with increasing tert-butyl alcohol content due to the HBA activity of the alcohol compared to styrene.

Methods of measurement and evaluation of natural antioxidant capacity/activity (IUPAC Technical Report)

“…Hence, pulse radiolysis and stopped-flow methodologies are used routinely to detect the earliest species in fast reactions, including DPPH • [75,76]. Inexpensive stopped-flow syringe assemblies are available, so antioxidant assays using DPPH should be moved to this technology or at least to autodispensing with instantaneous absorbance recording in microplate readers.…”

“…Natural antioxidant capacity/activity 975 3.5 DPPH assays 3.5.1 Experimental approach DPPH is a stable radical with a deep purple colour whose reaction with other radicals, reducing agents, or compounds capable of HAT leads to loss of colour at 515 nm and loss of its electron paramagnetic resonance (EPR) free radical signal [26,29]. Like ABTS +• , DPPH • reacts with both electron and hydrogen donors [75,76], though more slowly, and steric accessibility to the radical site is a clear issue [27,28]. No antioxidant assay is simpler or less expensive to run than the DPPH assay, which accounts for its popularity and extensive use.…”

Lipoperoxyl Radical Scavenging and Antioxidative Effects of Red Beet Pigments

“…Phenolic hydroxyls have been repeatedly proven as efficient reducers of prooxidant/oxygen radicals under a wide range of conditions (Valgimigli et al 1995;Barclay et al 1999). In accordance, the higher the number of hydroxyl groups, the higher the antioxidant activity of polyphenol phytochemicals such as flavonoids, has been shown (Rice-Evans et al 1996).…”

“…The redox potential of betanin (0.4 V) (Butera et al 2002), would make the molecule an efficient reductant for lipid-derived peroxyl radicals (Buettner 1993). Nevertheless, kinetic solvent effects (Avila et al 1995;Valgimigli et al 1995), in particular polarity and hydrogen bond-accepting ability (HBA) of the solvent, may strongly affect the capacity of phenol antioxidants to transfer the hydroxylic H-atoms to radicals, because of preferential formation of a H-bonded complex between the reducing phenol-OH and a molecule of solvent (Barclay et al 1999). Since methanol has a high HBA (Kamlet and Taft 1976), a strong interference could account for the very modest antioxidant effects of betanin in this solvent.…”

Influence of Tropolone on Poria placenta Wood Degradation

“…The inhibition of oxygen uptake with methyl linoleate (LH) as the substrate was used (3,10). L ⅐ free radicals are generated by the initiator AIBN (2,2Ј-azobis[2-methylpropionitrile]), and oxidation of methyl linoleate is a chain reaction propagated by the processes:…”