13 research outputs found
Lipoic Acid and Dihydrolipoic Acid. A Comprehensive Theoretical Study of Their Antioxidant Activity Supported by Available Experimental Kinetic Data
The free radical scavenging activity
of lipoic acid (LA) and dihydrolipoic
acid (DHLA) has been studied in nonpolar and aqueous solutions, using
the density functional theory and several oxygen centered radicals.
It was found that lipoic acid is capable of scavenging only very reactive
radicals, while the dehydrogenated form is an excellent scavenger
via a hydrogen transfer mechanism. The environment plays an important
role in the free radical scavenging activity of DHLA because in water
it is deprotonated, and this enhances its activity. In particular,
the reaction rate constant of DHLA in water with an HOO<sup>•</sup> radical is close to the diffusion limit. This has been explained
on the basis of the strong H-bonding interactions found in the transition
state, which involve the carboxylate moiety, and it might have implications
for other biological systems in which this group is present
ROS Initiated Oxidation of Dopamine under Oxidative Stress Conditions in Aqueous and Lipidic Environments
Dopamine is known to be an efficient antioxidant and to protect neurocytes from oxidative stress by scavenging free radicals. In this work, we have carried out a systematic quantum chemistry and computational kinetics study on the reactivity of dopamine toward hydroxyl (•OH) and hydroperoxyl (•OOH) free radicals in aqueous and lipidic simulated biological environments, within the density functional theory framework. Rate constants and branching ratios for the different paths contributing to the overall reaction, at 298 K, are reported. For the reactivity of dopamine toward hydroxyl radicals, in water at physiological pH, the main mechanism of the reaction is proposed to be the sequential electron proton transfer (SEPT), whereas in the lipidic environment, hydrogen atom transfer (HAT) and radical adduct formation (RAF) pathways contribute almost equally to the total reaction rate. In both environments, dopamine reacts with hydroxyl radicals at a rate that is diffusion-controlled. Reaction with the hydroperoxyl radical is much slower and occurs only by abstraction of any of the phenolic hydrogens. The overall rate coefficients are predicted to be 2.23 × 10<sup>5</sup> and 8.16 × 10<sup>5</sup> M<sup>–1</sup> s<sup>–1</sup>, in aqueous and lipidic environment, respectively, which makes dopamine a very good •OOH, and presumably •OOR, radical scavenger
Molecular Description of Indigo Oxidation Mechanisms Initiated by OH and OOH Radicals
In this work, we report a quantum chemistry mechanistic
study of
the hydroxyl (•OH) and hydroperoxyl (•OOH) radicals
initiated oxidation of indigo, within the density functional theory
framework. All possible hydrogen abstraction and radical addition
reaction pathways have been considered. We find that the reaction
between a free indigo molecule and an •OH radical occurs mainly
through two competing mechanisms: H-abstraction from an NH site and
•OH addition to the central CC double bond. Although
the latter is favored, both channels occur, the indigo chromophore
group structure is modified, and thus the color is changed. This mechanism
adequately accounts for the loss of chromophore in urban air, including
indoor air such as in museums and in urban areas. Regarding the reactivity
of indigo toward •OOH radicals, only •OOH-addition to
the central double bond is thermodynamically feasible. The corresponding
transition state free energy value is about 10 kcal/mol larger than
the one for the •OH initiated oxidation. Therefore, even considering
that the •OOH concentration is considerably larger than the
one of •OH, this reaction is not expected to contribute significantly
to indigo oxidation under atmospheric conditions
Can a Single Water Molecule Really Catalyze the Acetaldehyde + OH Reaction in Tropospheric Conditions?
In recent publications there has been considerable speculation about the possible role of a single water molecule in the catalysis of reactions between organic volatile compounds and OH radicals. In this work we reanalyze the effect of water in the acetaldehyde + OH reaction, using quantum chemistry and computational kinetics calculations in a pseudosecond order mechanism, at average atmospheric water concentrations and temperatures. We show that one water molecule definitely <i>does not</i> accelerate the acetaldehyde + OH reaction under atmospheric conditions. The apparent rate coefficient is considerably smaller than the one in the absence of water, regardless of the method of calculation. According to present results, the possible role of water molecules has been overestimated and new experiments are needed
Acid-Catalyzed Nucleophilic Additions to Carbonyl Groups: Is the Accepted Mechanism the Rule or an Exception?
The
transesterification reaction, and in particular the methanolysis
of ethyl acetate with sulfuric acid as catalyst, is used as a model
reaction to study the acid-catalyzed nucleophilic addition to a carbonyl
group. Continuum solvation methods (SMD and IEF-PCM) and the MPWB1K
functional are used. The reaction mechanism is studied in methanol
and in acetonitrile as solvents. Our results indicate that the acid-catalyzed
addition mechanism is stepwise, and the transition state (TS) is a
contact ion-pair. The counteranion of the acid catalyst remains in
the reaction site playing an important role in the TS of this reaction.
Changes in the reaction kinetics and the ionic/nonionic nature of
the TS with the ionizing ability of the solvent and the strength of
the acid catalyst are explored. Additional calculations at the CBS-Q3
level of theory reinforce the conclusions of this paper. The results
obtained allow the generalization of important ideas regarding the
mechanism of the nucleophilic addition to carbonyl groups