A procedure based on density functional theory is used for the calculation of the gas-phase bond dissociation enthalpy (BDE) and ionization potential for molecules belonging to the class of phenolic antioxidants. We show that use of locally dense basis sets (LDBS) vs full basis sets gives very similar results for monosubstituted phenols, and that the LDBS procedure gives good agreement with the change in experimental BDE values for highly substituted phenols in benzene solvent. Procedures for estimating the O--H BDE based on group additivity rules are given and tested. Several interesting classes of phenolic antioxidants are studied with these methods, including commercial antioxidants used as food additives, compounds related to Vitamin E, flavonoids in tea, aminophenols, stilbenes related to resveratrol, and sterically hindered phenols. On the basis of these results we are able to interpret relative rates for the reaction of antioxidants with free radicals, including a comparison of both H-atom-transfer and single-electron-transfer mechanisms, and conclude that in most cases H-atom transfer will be dominant.

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Bond dissociation energies, electron affinities, and proton affinities are computed for a variety of molecules containing C−H, N−H, O−H, and S−H bonds using density functional theory with the B3LYP functional. Thermochemistry in which these bonds are broken or ions are formed is particularly relevant to understanding proton transfer (acid−base), electron transfer (redox), and H-atom transfer (free radical) reactions. A series of basis set experiments has led to an optimum compromise between computational speed and accuracy. Several theoretical models are defined and tested, and the medium and higher-level models approach an accuracy of 1 kcal/mol. Use of the above methodology to obtain absolute bond dissociation energies for X−H bonds, isodesmic reaction schemes, substituent effects, redox potentials, and gas-phase acid dissociation constants shows the usefulness of this approach.

Reactive oxygen species (ROS) have been implicated in a growing number of neurological disease states, from acute traumatic injury to neurodegenerative conditions such as Alzheimer’s disease. Considerable evidence suggests that ROS also mediate ototoxicant- and noise-induced cochlear injury, although most of this evidence is indirect. To obtain real-time assessment of noise-induced cochlear ROS production in vivo, we adapted a technique which uses the oxidation of salicylate to 2,3-dihydroxybenzoic acid as a probe for the generation of hydroxyl radical. In a companion paper we described the development and characterization of this method in cochlear ischemia-reperfusion. In the present paper we use this method to demonstrate early elevations in ROS production following acute noise exposure. C57BL/6J mice were exposed for 1 h to intense broad-band noise sufficient to cause permanent threshold shift (PTS), as verified by auditory brainstem responses. Comparison of noise-exposed animals with unexposed controls indicated that ROS levels increase nearly 4-fold in the period 1–2 h following exposure and do not decline over that time. Our ROS measures extend previous results indicating that noise-induced PTS is associated with elevated cochlear ROS production and ROS-mediated injury. Persistent cochlear ROS elevation following noise exposure suggests a sustained process of oxidative stress which might be amenable to intervention with chronic antioxidant therapy.

Calculations on phenol and a large number of phenols substituted with methyl, methoxyl, and amino groups have yielded reliable gas-phase O−H bond dissociation energies, BDE(ArO−H)gas. Geometries for the phenol, ArOH, and aryloxyl radical, ArO, were optimized at the (semiempirical) AM1 level followed by single point density functional theory (DFT) calculations using a 6-31G basis set supplemented with p-functions on the hydrogen atom and the B3LYP density functional. This gave BDE(PhO−H)gas = 86.46 kcal/mol, which is in good agreement with the experimental value of 87.3 ± 1.5 kcal/mol. All but one of the compounds and conformations examined had weaker O−H BDE's than phenol, the exception being o-methoxyphenol with the O−H group pointing toward this substituent (BDE = 87.8 kcal/mol). Where comparison was possible, calculated differences in O−H BDE's were in excellent agreement with experiment (better than 1 kcal/mol). A simple group additivity scheme also gave excellent agreement with calculated BDE (ArO−H)gas values. Some potential new leads to phenolic antioxidants more active than vitamin E have been uncovered.