Unusual and Persistent Free Radical Intermediate Production from 2-Pyridyl Ketones via UV Irradiation: A Direct ESR Study.

Aryl ketones are often used as photosensitizers and photoinitiators. Free radical intermediates have been suggested, but not confirmed, to be generated after photoirradiation. Here we found, unexpectedly, that a persistent radical was produced from di-2-pyridyl ketone after UV irradiation, which was detected by the direct ESR method.

Mechanistic investigation of the differential synergistic neurotoxicity between pesticide metam sodium and copper or zinc.

Epidemiological studies suggest neurological disorders have been associated with the co-exposure to certain pesticides and transition metals. The present study aims to investigate whether co-exposure to the widely-used pesticide metam sodium and copper (Cu) or zinc ion (Zn) is able to cause synergistic neurotoxicity in neural PC12 cells and its possible mechanism(s). We found that both metam/Cu and metam/Zn synergistically induced apoptosis, intracellular Cu/Zn uptake, reactive oxygen species (ROS) accumulation, double-strand DNA breakage, mitochondrial membrane potential decrease, and nerve function disorder.

The critical role of unique azido-substituted chloro-O-semiquinone radical intermediates in the synergistic toxicity between sodium azide and chlorocatecholic carcinogens.

We have shown previously that exposing bacteria to tetrachlorocatechol (TCC) and sodium azide (NaN) together causes synergistic cytotoxicity in a biphasic mode. However, the underlying chemical mechanism remains unclear. In this study, an unexpected ring-contraction 3(2H)-furanone and two quinoid-compounds were identified as the major and minor reaction products, respectively; and two unusual azido-substituted chloro-O-semiquinone radicals were detected and characterized as the major radical intermediates by complementary applications of direct ESR, HPLC/ESI-Q-TOF and high-resolution MS studies with nitrogen-15 isotope-labeled NaN.

Caffeic Acid Phenyl Ester (CAPE) Protects against Iron-Mediated Cellular DNA Damage through Its Strong Iron-Binding Ability and High Lipophilicity.

Caffeic acid phenethyl ester (CAPE) and its structurally-related caffeic acid (CA), ferulic acid (FA) and ethyl ferulate (EF) are constituents of honeybee propolis that have important pharmacological activities. This study found that CAPE-but not CA, FA, and EF-could effectively prevent cellular DNA damage induced by overloaded iron through decreasing the labile iron pool (LIP) levels in HeLa cells. Interestingly, CAPE was found to be more effective than CA in protecting against plasmid DNA damage induced by Fe(II)-HO or Fe(III)-citrate-ascorbate-HO via the inhibition of hydroxyl radical (•OH) production.

Structure-Activity Relationship Investigation on Reaction Mechanism between Chlorinated Quinoid Carcinogens and Clinically-Used Aldoxime Nerve-Agent Antidote under Physiological Condition.

Pyridinium aldoximes are best-known therapeutic antidotes used for clinical treatment of poisonings by organophosphorus nerve-agents and pesticides. Recently, we found that pralidoxime (2-PAM, a currently clinically used nerve-agent antidote) could also detoxify tetrachloro-1,4-benzoquinone (TCBQ), which is a carcinogenic quinoid metabolite of the widely used wood preservative pentachlorophenol under normal physiological conditions, via an unusually mild and facile Beckmann fragmentation mechanism accompanied by radical homolysis. However, it is not clear whether the less-chlorinated benzoquinones (BQs, ≤ 3) act similarly; if so, what is the structure-activity relationship? In this study, we found that (1) The stability of reaction intermediates produced by different BQs and 2-PAM was dependent not only on the position but also the degree of Cl-substitution on BQs, which can be divided into TCBQ- and DCBQ (dichloro-1,4-benzoquinone)-subgroup; (2) The p value of hydroxlated quinones (BQ-OHs, the hydrolysis products of BQs), determined the stability of corresponding intermediates, that is, the decomposition rate of the intermediates depended on the acidity of BQ-OHs; (3) The p value of the corresponding BQ-OHs could also determine the reaction ratio of Beckmann fragmentation to radical homolysis in BQs/2-PAM.

The critical role of superoxide anion radicals on delaying tetrachlorohydroquinone autooxidation by penicillamine.

We have recently found that penicillamine, a classic copper-chelating thiol-drug for Wilson's disease, can delay tetrachlorohydroquinone (TCHQ) autooxidation via a previously unrecognized redox-activity. However, its underlying molecular mechanism remains not fully understood. In this study, we found, interestingly and unexpectedly, that superoxide dismutase (SOD) can significantly shorten the delay of TCHQ autooxidation by penicillamine, but not by ascorbate; SOD can also markedly increase the yields of the oxidized form of penicillamine.

Unexpected reversible and controllable nuclear uptake and efflux of the DNA "light-switching" Ru(ii)-polypyridyl complex in living cells via ion-pairing with chlorophenolate counter-anions.

An in-depth understanding of the mechanisms of cellular uptake and efflux would facilitate the design of metal complexes with not only better functionality and targeted theranostic efficiency, but also with controlled toxicity. Here we find, unexpectedly, that the DNA "light-switching" Ru(ii)-polypyridyl complex [Ru(phen)(dppz)] already delivered to the nucleus via ion-pairing with chlorophenolate counter-anions can gradually efflux to the cytoplasm when the cells were washed and incubated with fresh culture-medium. Interestingly, [Ru(phen)(dppz)] effluxed to the cytoplasm can be redirected back to the nucleus when the chlorophenolate counter-anions were added again.

Unusual Two-Step Claisen-type Rearrangement Reaction under Physiological Conditions.

-aryl hydroxamic acids, which are best known for their metal-chelating properties in chemical and biomedical research, have been found to markedly detoxify carcinogenic halogenated quinones. However, the exact chemical mechanism underlying such detoxication remains unclear. Here, we show that a very fast reaction took place between -phenylbenzohydroxamic acid (-PhBHA) and 2,5-dichloro-1,4-benzoquinone (DCBQ), forming an unexpected new carbon-carbon bonding phenyl-quinone product with high yield.

Mechanism of synergistic DNA damage induced by caffeic acid phenethyl ester (CAPE) and Cu(II): Competitive binding between CAPE and DNA with Cu(II)/Cu(I).

Caffeic acid phenethyl ester (CAPE) is an active polyphenol of propolis from honeybee hives, and exhibits antioxidant and interesting pharmacological activities. However, in this study, we found that in the presence of Cu(II), CAPE exhibited pro-oxidative rather than antioxidant effect: synergistic DNA damage was induced by the combination of CAPE and Cu(II) together as measured by strand breakage in plasmid DNA and 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) formation, which is dependent on the molar ratio of CAPE:Cu(II). Production of Cu(I) and HO from the redox reaction between CAPE and Cu(II), and subsequent OH formation was found to be responsible for the synergistic DNA damage.

First unequivocal identification of the critical acyl radicals from the anti-tuberculosis drug isoniazid and its hydrazide analogs by complementary applications of ESR spin-trapping and HPLC/MS methods.

The carbon-centered isonicotinic acyl radical of isoniazid (INH), a widely-used frontline anti-tuberculosis drug, has been considered to play a critical role in inhibiting Mycobacterium tuberculosis, but not fully identified. Here we show that this radical intermediate can be unequivocally characterized by complementary applications of ESR spin-trapping and HPLC/MS methods by employing N-tert-butyl-α-phenylnitrone (PBN) as the suitable spin-trapping agent, which can form the most stable radical adduct. More importantly, for the first time, analogous carbon-centered acyl radicals and their respective NAD adducts have also been detected and identified from its two isomers (nicotinic acid hydrazide and 2-pyridinecarbohydrazide) and benzhydrazide which are structurally-related to INH, but not by 2-chloroisonicotinohydrazide.

Chiral Os(II) Polypyridyl Complexes as Enantioselective Nuclear DNA Imaging Agents Especially Suitable for Correlative High-Resolution Light and Electron Microscopy Studies.

The high-resolution technique transmission electron microscopy (TEM), with OsO as the traditional fixative, is an essential tool for cell biology and medicine. Although OsO has been extensively used, it is far from perfect because of its high volatility and toxicity. Os(II) polypyridyl complexes like [Os(phen)(dppz)] (phen = 1,10-phenanthroline; dppz = dipyridophenazine) are not only the well-known molecular DNA "light-switches" but also the potential ideal candidates for TEM studies.

An unexpected antioxidant and redox activity for the classic copper-chelating drug penicillamine.

Penicillamine has been widely-used clinically as a copper-chelating drug for the treatment of copper-overload in Wilson's disease. In this study, we found that penicillamine provided marked protection against cytotoxicity induced by tetrachlorohydroquinone (TCHQ), a major toxic metabolite of the well-known wood preservative pentachlorophenol, while other classic copper-chelating agents do not. We found, unexpectedly, that both TCHQ autooxidation and tetrachlorosemiquinone radical (TCSQ) formation were remarkably delayed by penicillamine.

Unexpected activation of N-alkyl hydroxamic acids to produce reactive N-centered free radicals and DNA damage by carcinogenic chlorinated quinones under normal physiological conditions.

We found recently that benzohydroxamic acid (BHA) could detoxify the chlorinated quinoid carcinogens via an unusual Lossen rearrangement reaction. However, it is not clear what would happen when the nitrogen hydrogen of BHA was substituted with methyl and other alkyl groups. Here we show that N-methyl benzohydroxamic acid (N-MeBHA, a simple model compound for the classic iron-chelator deferoxamine, which is a typical N-alkyl trihydroxamic acid) could react with 2,5-dichloro-1,4-benzoquinone (DCBQ) to form a relatively stable initial carbon-oxygen bonding conjugation intermediate CBQ-O-N-MeBHA.

Targeted live-cell nuclear delivery of the DNA 'light-switching' Ru(II) complex via ion-pairing with chlorophenolate counter-anions: the critical role of binding stability and lipophilicity of the ion-pairing complexes.

We have found recently that nuclear uptake of the cell-impermeable DNA light-switching Ru(II)-polypyridyl cationic complexes such as [Ru(bpy)2(dppz)]Cl2 was remarkably enhanced by pentachlorophenol (PCP), by forming ion-pairing complexes via a passive diffusion mechanism. However, it is not clear whether the enhanced nuclear uptake of [Ru(bpy)2(dppz)]2+ is only limited to PCP, or it is a general phenomenon for other highly chlorinated phenols (HCPs); and if so, what are the major physicochemical factors in determining nuclear uptake? Here, we found that the nuclear uptake of [Ru(bpy)2(dppz)]2+ can also be facilitated by other two groups of HCPs including three tetrachlorophenol (TeCP) and six trichlorophenol (TCP) isomers. Interestingly and unexpectedly, 2,3,4,5-TeCP was found to be the most effective one for nuclear delivery of [Ru(bpy)2(dppz)]2+, which is even better than the most-highly chlorinated PCP, and much better than its two other TeCP isomers.