The first purification and unequivocal characterization of the radical form of the carbon-centered quinone ketoxy radical adduct

Abstract

We found, unexpectedly, that the radical form of the carboncentered quinone ketoxy radical adduct with a recently developed spin-trapping agent BMPO can not only be directly detected and identified using HPLC/high resolution MS, but can also be isolated and purified using semi-preparative HPLC, enabling direct observation of its clean 6-line ESR signal.Halogenated quinones are a class of toxicological intermediates which can cause acute hepatoxicity, nephrotoxicity, and carcinogenesis. 1,2 They have also been observed as reactive oxidation intermediates or products in processes used to oxidize or destroy polychlorinated persistent organic pollutants in various chemical and enzymatic systems. [1][2][3][4] More recently, several polyhalogenated quinones, which are suspected bladder carcinogens, were identified as new chlorination disinfection byproducts in drinking water. 4 We have recently shown that halogenated quinones could enhance the decomposition of hydroperoxides and formation of alkoxyl/hydroxyl radicals independent of transition metal ions. [5][6][7][8] A new carbon-centered quinone ketoxy radical ( CBQ-OH, MW: 157) adduct with the spin-trapping agent DMPO (5,5-dimethyl-1-pyrroline N-oxide) (simply referred to here as DMPO-157) was observed during the reaction between 2,5-dichloro-1,4-benzoquinone (DCBQ) and t-butylhydroperoxide (t-BuOOH). 7 However, although we can observe the ESR signal for DMPO-157, we cannot directly detect its nitroxide radical form, but only its corresponding more stable non-radical nitrone form, by mass spectrometry (MS). Similarly, we can isolate only the ESR-silent DMPO-157 nitrone adduct by HPLC, but not its corresponding ESR-active nitroxide form. 7 In other words, we could not obtain the pure DMPO-157 nitroxide radical adduct so that we can observe its clean 6-line ESR signal without any interference of other concurrently formed radicals such as t-butoxyl and methyl radicals. The reason might be that either the DMPO-157 nitroxide radical adduct was not stable enough to pass through the HPLC column and decayed to its ESR-silent nitrone form during the separation process or the concentration of the nitroxide radical adduct in the collected fraction was just too low to be detected by ESR.It should be more convincing if we can isolate and purify this unusual quinone ketoxy radical with a suitable spin-trapping agent and observe its clean ESR signal without any interference of other concurrently formed radicals, because this will provide more direct and stronger experimental evidence for the existence of such a new kind of quinone radical, which will further support our previously proposed molecular mechanism for metal-independent decomposition of hydroperoxides by halogenated quinones (see Scheme 2 in ref. 7). Therefore, in this study, we plan to address the following questions: can we find a spin-trapping agent which can form a much more stable nitroxide radical adduct with the quinone ketoxy radical? if so, can we directly detect the radical form by MS? and can we isolate and p...

Observation of superoxide production during catalysis of Bacillus subtilis oxalate decarboxylase at pH 4

“…Previous studies have utilized spin trapping in combination with mass spectrometry to confirm the identity of spin trapped radical adducts with both DMPO and BMPO spin traps [57–62]. Here, we report for the first time on the detection and identification of a hydroperoxyl-adduct of BMPO when OxDC reacts with oxalate under turnover conditions.…”

Why Does 2,3,5,6-Tetrachlorophenol Generate the Strongest Intrinsic Chemiluminescence among All Nineteen Chlorophenolic Persistent Organic Pollutants during Environmentally-friendly Advanced Oxidation Process?

“…During our study of metal-independent hydroxyl/alkoxyl/ketoxy radical production by halogenated quinones and H 2 O 2 (or organic hydroperoxides) 20 21 22 23 24 25 , and their potential biological effects 26 , we found that unprecedented hydroxyl radical-dependent two-step chemiluminescence could be generated by polyhalogenated quinones and H 2 O 2 23 . Recently, we also observed an intrinsic CL generation by all 19 chlorophenols (the parent compounds of chlorinated quinones) and the classic • OH-generating Fenton system, even in the absence of fluorescent agents 27 .…”

Detecting and Quantifying Polyhaloaromatic Environmental Pollutants by Chemiluminescence-Based Analytical Method

“…The most well-known pathway for • OH generation is through the classic Fenton or Fenton-like reactions mediated by reactive transition metal ions [37,38]. We recently found an unprecedented metal-independent • OH-generating system: polyhaloquinones and H 2 O 2 , and the molecular mechanism of typical nucleophilic substitution coupling with homolytic decomposition for • OH generation was proposed [3,[39][40][41][42][43][44][45][46][47]. More interestingly, an unexpected intrinsic CL emission can also be produced in this novel • OH-generating system, which was found to be specifically dependent on • OH production [44,[48][49][50][51].…”