Chlorine substitution promotes phenyl radical loss from C8-phenoxy-2′-deoxyguanosine adducts: implications for biomarker identification from chlorophenol exposure

Abstract

Chlorophenols are persistent organic pollutants, which undergo peroxidase-mediated oxidation to afford phenolic radical intermediates that react at the C8-site of 2'-deoxyguanosine (dG) to generate oxygen-linked C8-dG adducts. Such adducts are expected to contribute to chlorophenol toxicity and serve as effective dose biomarkers for chlorophenol exposure. Electrospray ionization mass spectrometry (ESI-MS) was employed to study collision induced dissociation (CID) for a family of such phenolic O-linked C8-dG adducts. Fragmentation of the deprotonated nucleosides demonstrates that an unexpected homolytic cleavage of the ether linkage to release phenyl radicals and a nucleoside distonic ion with m/z 281 competes effectively with commonly observed breakage of the glycosidic bond to release the deprotonated nucleobase. Increased chlorination of the phenyl ring enhances phenyl radical loss. Density functional theory calculations demonstrate that Cl-substitution decreases phenyl radical stability but promotes homolytic breakage of the C8-phenyl bond in the C8-dG adduct. The calculations suggest that phenyl radical loss is driven by destabilizing steric (electrostatic repulsion) interactions between the ether oxygen atom and ortho-chlorines on the phenyl ring. The distonic ion at m/z 281 represents a unique dissociation product for deprotonated O-linked C8-dG adducts and may prove useful for selective detection of relevant biomarkers for chlorophenol exposure by tandem mass spectrometry using selective reaction monitoring.

C-Linked 8-aryl guanine nucleobase adducts: biological outcomes and utility as fluorescent probes

“…For example, O-linked adducts have been connected with pentachlorophenol (PCP, Fig. 1b ) found in pesticides, disinfectants and wood preservatives, 22 , 23 , 25 , 27 while the food mutagen ochratoxin A (OTA) arising from several species of ( Aspergillus and Penicillium ) fungi has been linked to the formation of a C-linked adduct ( Fig. 1b ).…”

Formation of Bulky DNA Adducts by Non-Enzymatic Production of 1,2-Naphthoquinone-Epoxide from 1,2-Naphthoquinone under Physiological Conditions

“…Loss of water and ammonia from the parent deprotonated molecule was detected as m / z 405, and these results further supported that covalent bonding had not occurred at the N 2 position of dG as described above. The fragment that corresponded to [M – H – dR] − was not detected as is sometimes the case, however, losses of 2′-deoxyribose plus water, m / z 306, 2′-deoxyribose plus water and ammonia, m / z 289, and 2′-deoxyribose plus water and a carbonyl group, m / z 278, were detected. Again, detection of the loss of ammonia supported that covalent bonding had not occurred at the N 2 position of dG.…”