Quinones permeate our biotic environment, contributing to both homeostasis and cytotoxicity. All quinones generate reactive oxygen species through redox cycling, while partially substituted quinones also undergo arylation (Michael adduct formation) yielding covalent bonds with nucleophiles such as cysteinyl thiols. In contrast to reactive oxygen species, the role of arylation in quinone cytotoxicity is not well understood. We found that the arylating quinones, including unsubstituted 1,4-benzoquinone (1,4-BzQ) and partially substituted vitamin E congener ␥-tocopherol quinone (␥-TQ), were cytotoxic, with ␥-TQ > 1,4-BzQ, whereas the fully substituted nonarylating vitamin E congener ␣-tocopherol quinone was not. In vitro, both arylating quinones formed Michael adducts with the thiol nucleophile N-acetylcysteine (NAC) at rates where 1,4-BzQ > ␥-TQ. In cultured cells, concurrent addition of NAC eliminated 1,4-BzQ caused toxicity, but preincubation was required for the same NAC detoxification effect on ␥-TQ. These data clearly established the role of arylation in quinone toxicity and revealed that arylating quinone structure affects cytotoxicity by governing detoxification through the rate of adduct formation. Furthermore, arylating quinones induced endoplasmic reticulum (ER) stress by activating the pancreatic ER kinase (PERK) signaling pathway including elF2␣, ATF4, and C͞EBP homologous protein (CHOP). Detoxification by NAC greatly attenuates CHOP induction in arylating quinone-treated cells, suggesting that ER stress is a cellular mechanism for arylating quinone cytotoxicity.quinone adduction ͉ thiol nucleophiles ͉ tocopherols ͉ CHOP ͉ cytotoxicity Q uinones and their phenolic precursors are present throughout the biotic environment and include polyphenols and tocopherols in the diet, drugs in medicine, environmental pollutants such as polycyclic aromatic hydrocarbons, and their metabolic products (1-8). They are involved in a wide variety of biological and chemical processes, including electron transport in animals and plants, photosynthesis, posttranslational modification of proteins, metabolism of cellular signaling molecules such as estrogens and catecholamines, metabolism of antioxidant and signaling tocopherol congeners (vitamin E), and the elimination of polycyclic aromatic hydrocarbons introduced by combustion processes associated with our petroleum-based chemical environment.Quinones are a class of highly reactive compounds. Although all quinones are redox cycling agents that generate reactive oxygen species (ROS), partially substituted quinones also function as arylating agents (1-3, 5, 6). The arylating quinones react with cellular nucleophiles such as thiols on cysteine residues of proteins, glutathione (GSH), and detoxifying agents such as N-acetylcysteine (NAC), forming covalently linked quinonethiol Michael adducts (1-3, 5, 6) that retain the ability to function as redox cycling agents (4, 9). In contrast to well studied ROS generation and consequent oxidative stress in living cells (1-3), the role o...

The rate coefficients for reactions of hydroxyl radical with aromatic hydrocarbons were measured in acetonitrile using a novel laser flash photolysis method. Comparison of kinetic data obtained in acetonitrile with those obtained in aqueous solution demonstrates an unexpected solvent effect on the reactivity of hydroxyl radical. In particular, reactions of hydroxyl radical with benzene were faster in water than in acetonitrile, and by a significant factor of 65. Computational studies, at the B3LYP and CBS-QB3 levels, have confirmed the rate enhancement of hydroxyl radical addition to benzene via calculation of the transition states in the presence of explicit solvent molecules as well as a continuum dielectric field. The origin of the rate enhancement lies entirely in the structures of the transition states and not in the pre-reactive complexes. The calculations reveal that the hydroxyl radical moiety becomes more anionic in the transition state and, therefore, looks more like hydroxide anion. In the transition states, solvation of the incipient hydroxide anion is more effective with water than with acetonitrile and provides the strong energetic advantage for a polar solvent capable of hydrogen bonding. At the same time, the aromatic unit looks more like the radical cation in the transition state. The commonly held view that hydroxyl radical is electrophilic in its reactions with DNA bases is, therefore, strongly dependent on the ability of the organic substrate to stabilize the resulting radical cation.

Three pairs of isomeric, iron-sulfur core dendrimers were prepared. Each isomer pair was distinguished by a 3,5-aromatic substitution pattern (extended) versus 2,6-aromatic substitution pattern (backfolded). Several observations were made that supported the hypothesis that the iron-sulfur cluster cores were encapsulated more effectively in the backfolded isomers as compared to their extended isomeric counterparts. The backfolded isomers were more difficult to reduce electrochemically, consistent with encapsulation in a more hydrophobic microenvironment. Furthermore, heterogeneous electron-transfer rates for the backfolded molecules were attenuated compared to the extended molecules. From diffusion measurements obtained by pulsed field gradient spin-echo NMR and chronoamperometry, the backfolded dendrimers were found to be smaller than the extended dendrimers. Comparison of longitudinal proton relaxation (T(1)) values also indicated a smaller, more compact dendrimer conformation for the backfolded architectures. These findings indicated that the dendrimer size was not the major factor in determining electron-transfer rate attenuation. Instead, the effective electron-transfer distance, as determined by the relative core position and mobility in a dendrimer, is most relevant for encapsulation.

Identification and quantitative estimation of quinone metabolites of gamma-tocopherol (gamma-T) and its derivative gamma-carboxyethyl hydroxychroman (gamma-CEHC) are complicated by their functions as arylating electrophiles. We hypothesize that their biological properties are expressed through arylating quinone electrophile addition (Michael reaction) with thiol nucleophiles in cells and tissues. Glutathione (GSH) reacted with gamma-tocopheryl quinone (gamma-TQ) to form the hydroquinone adduct, which was identified by electrospray time-of-flight MS (ESI-TOF-MS). Tetramethylammonium hydroxide (TMAH) thermochemolysis reduced and methylated quinones and cleaved and methylated thioether adducts. These relatively nonpolar derivatives were readily separated by GC and identified by MS fragmentation patterns. gamma-CEHC was synthesized and oxidized to a product identified as the quinone lactone (gamma-CEHC-QL). TMAH methylated both gamma-CEHC-QL and its GSH adduct without opening the lactone ring, and these products were separated by GC and identified by MS fragmentation patterns. gamma-CEHC-QL reacted with both the cysteinyl enzyme papain and fetal bovine serum, and TMAH thermochemolysis showed that each product mixture contained unreacted precursor and thioether adduct. Cytotoxicities of phenolic precursors, gamma-T and gamma-CEHC, and their quinones, gamma-TQ and gamma-CEHC-QL, respectively, were compared in COS1, NT2, 3T3, and N2a cell lines. Phenolic precursor gamma-T had a small effect only with NT2 and 3T3 cells while gamma-CEHC had no effect in any cell line. Arylating quinones were highly cytotoxic in all cell lines with gamma-TQ showing a significantly greater cytotoxicity than gamma-CEHC-QL. These data are consistent with our arylating electrophile hypothesis as an explanation for some biological activities of Ts through their quinone metabolites.

Thermal treatment of paper leads to the formation of aromatic structures, allowing patterning the substrates and their use as sensors.