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.
This paper reports theoretical gas-phase structures and
energetics using G2(MP2) theory for saturated
oxygen chains of the general formula HO
n
H.
Structural trends are discussed using a simple
hyperconjugation
model which is capable of giving a qualitative explanation for trends
in bond lengths and dihedral angles.
Bond dissociation energies (BDEs) are calculated for chains of
increasing length, giving 49.9, 33.9, and 17.8
kcal/mol for H2O2,
H2O3, and H2O4,
respectively. From an analysis of the radical stabilization energy
of the
fragments remaining after dissociation, it is shown that a minimum
value for the BDE for any hydrogen polyoxide
is 6.4 kcal/mol, which occurs for the center bond in
H2O6, and that longer chains will have a higher
BDE.
Decomposition pathways responsible for the observed instability of
the polyoxides higher than hydrogen peroxide
are discussed and results are given for three low-barrier dissociation
paths: a solvent-assisted path, a base-catalyzed path, and a proton relay mechanism. These mechanisms are
probably general and account for the
instability of polyoxide chains in proton-containing solvents.
Preventing proton transfer, e.g. by perfluoroalkylation, would therefore be expected to increase chain stability, in
agreement with experimental observations.
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