The relative contributions of vitamin E, urate, ascorbate and proteins to the total peroxyl radical-trapping antioxidant activity of human blood plasma Wayner, D. D. M.; Burton, G.W.; Ingold, K. U.; Barclay, L. R. C.; Locke, S. J.

[reaction: see structure] The antioxidant activity of curcumin (1, 7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione) was determined by inhibition of controlled initiation of styrene oxidation. Synthetic nonphenolic curcuminoids exhibited no antioxidant activity; therefore, curcumin is a classical phenolic chain-breaking antioxidant, donating H atoms from the phenolic groups not the CH(2) group as has been suggested (Jovanovic et al. J. Am. Chem. Soc. 1999, 121, 9677). The antioxidant activities of o-methoxyphenols are decreased in hydrogen bond accepting media.

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Tlris lecrure is dedicated ro Keith Irzgolcl L. R. C . BARCLAY. Can J. Chem. 71, 1 (1993). This review outlines a decade of research that employs quantitative kinetic methods of autooxidation to phospholipids aggregated into membranes. The classical rate law for autoxidation: was found to apply to heterogeneous systems of bilayers and micelles, where k, and 2k, are the rate constants for chain propagation and termination, respectively, and R,, the rate of chain initiation, is controlled b thermal initiators. The oridizobility of a typical lipid chain, linoleate 18:2, at 30°C was similar (0.02-0.04 M -" S -'~) in different media (solution, micelles, bilayers) and for different procedures using water-soluble or lipid-soluble initiators and inhibitors. A reduction in the absolute rate constant for termination, 2k,, by a hundredfold in bilayers of dilinoleoylphosphatidyl choline (DLPC) compared to that in rerr-butyl alcohol solution, and a drop in the k, by a factor of five are interpreted in terms of diffusion of polarized peroxyl radicals from the hydrophobic bilayer phase to the aqueous surface, where peroxyls are strongly solvated by water. Such phenomena may also account for significant changes as observed by "P NMR spectra in bilayer lamellar structure accompanying extensive peroxidation. Analysis of the hydroperoxides formed by peroxidation of mixed bilayers of DLPC + DPPC (16:O) initiated by a water-soluble initiator, azobis(2-amidinopropane.Hc1) (ABAP), showed a linear trend between the ratio of cis,trarzs to trnrls,frarzs geometrical isomeric hydroperoxides and [DPLC] consistent with a peroxidation mechanism proposed in homogeneous solution.The antioxidant activities, k,,,, of three classes of antioxidants: (a) polyalkyl-6-hydroxychromans (e.g., vitamin E), (0) polyalkyl-4-methoxyphenols, and (c) trialkylphenols, were measured in DLPC membranes. The results show an overall leveling and depression of k,,,', values in DLPC membranes in the series ( a ) (by several orders of magnitude), (b), and (c) compared to known values in solution in chlorobenzene. In aqueous bilayers it is proposed that kin,, values are attenuated by hydrogen bonding by water at both the p a r a ether oxygen and phenolic groups-. Restricted diffusion (e.g., of a-tocopherol) may also reduce antioxidant activities in membranes. A synergistic effect between ascorbic acid and atocopherol was discovered under conditions of inhibited peroxidation of linoleate in SDS micelles. The natural peptide glutathione, GSH, however, acts as a co-nrztio.ri~111rt with vitamin E by trapping peroxyls in the aqueous phase.Solid cholesterol was found to partition directly into PC lipsomes by shaking, above or below the phase transition temperature, and membrane-bound cholesterol, unlike the solid, undergoes facile peroxidation. A water-soluble form of c--tocopherol complexed with bovine serum albumin (a-toc:BSA) is an effective antioxidant for autoxidations of linoleate in SDS micelles. In contrast, a-toc:BSA required a long equilibration time (e.g., 12 h) wi...

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.