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Substituent effects on the reactivity of benzoquinone derivatives with thiols.

Authors
Mbiya-W; Chipinda-I; Siegel-PD; Mhike-M; Simoyi-RH
Source
Chem Res Toxicol 2013 Jan; 26(1):112-123
NIOSHTIC No.
20042204
Abstract
Benzoquinone (BQ) is an extremely potent electrophilic contact allergen that haptenates endogenous proteins through Michael addition (MA). It is also hypothesized that BQ may haptenate proteins via free radical formation. The objective of this study was to assess the inductive effects (activating and deactivating) of substituents on BQ reactivity and the mechanistic pathway of covalent binding to a nucleophilic thiol. The BQ binding of Cys34 on human serum albumin was studied, and for reactivity studies, nitrobenzenethiol (NBT) was used as a surrogate for protein binding of the BQ and benzoquinone derivatives (BQD). Stopped flow techniques were used to determine pseudofirst order rate constants (k) of methyl-, t-butyl-, and chlorine-substituted BQD reactions with NBT, whereas electron pair resonance (EPR) studies were performed to investigate the presence of the free radical mediated binding mechanism of BQD. Characterization of adducts was performed using mass spectrometry and nuclear magnetic resonance spectroscopy (NMR). The rate constant values demonstrated the chlorine-substituted (activated) BQD to be more reactive toward NBT than the methyl and t-butyl-substituted (deactivated) BQD, and this correlated with the respective EPR intensities. The EPR signal, however, was quenched in the presence of NBT suggesting MA as the dominant reaction pathway. MS and NMR results confirmed adduct formation to be a result of MA onto the BQ ring with vinylic substitution also occurring for chlorine-substituted derivatives. The binding positions on BQ and NBT/BQ(D) stoichiometric ratios were affected by whether the inductive effects of the substituents on the ring were positive or negative. The reactivity of BQ and BQD is discussed in terms of the potential relationship to potential allergenic potency.
Keywords
Chemical-reactions; Thiols; Protein-chemistry; Proteins; Free-radical-generation; Chemical-binding; Chemical-analysis; Chemical-hypersensitivity; Contact-dermatitis; Allergic-dermatitis; Allergic-reactions; Skin-irritants; Skin-sensitivity; Solvents; Allergens; Nitro-compounds; Free-radicals; Analytical-chemistry; Analytical-instruments; Mass-spectrometry; Spectrographic-analysis; Spectroscopes; Benzenes; Bioactivation
Contact
Itai Chipinda, Allergy and Clinical Immunology Branch, Health Effects Laboratory Division, National Institute for Occupational Safety and Health, Morgantown, WV 26505-2888, USA
CODEN
CRTOEC
CAS No.
106-51-4; 123-31-9
Publication Date
20130101
Document Type
Journal Article
Email Address
Itai.Chipinda@p66.com
Fiscal Year
2013
NTIS Accession No.
NTIS Price
Identifying No.
B20130306
Issue of Publication
1
ISSN
0893-228X
NIOSH Division
HELD
Priority Area
Healthcare and Social Assistance
Source Name
Chemical Research in Toxicology
State
WV; OR
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