Trapping a Labile Adduct Formed between an ortho-Quinone Methide and 2′-Deoxycytidine

Discovery of Novel N-(4-Hydroxybenzyl)valine Hemoglobin Adducts in Human Blood

Humans are exposed to a wide range of electrophilic compounds present in our diet and environment or formed endogenously as part of normal physiological processes. These electrophiles can modify nucleophilic sites of proteins and DNA to form covalent adducts. Recently, powerful untargeted adductomic approaches have been developed for systematic screening of these adducts in human blood. Our earlier untargeted adductomics study detected 19 unknown adducts to N-terminal valine in hemoglobin (Hb) in human blood. We now describe a full characterization of one of these adducts, which corresponds to the addition of a 4-hydroxybenzyl (4-OHBn) group to N-terminal valine in Hb to form N(4-hydroxybenzyl)valine (4-OHBn-Val). The adduct structure was determined by comparison of its accurate mass, HPLC retention time, and MS/MS fragmentation to that of authentic standards prepared by chemical synthesis. Average 4-OHBn-Val adduct concentrations in 12 human blood samples were estimated to 380 ± 160 pmol/g Hb. Two possible routes of 4-OHBnVal adduct formation are proposed using two different precursor electrophiles: 4-quinone methide (4-QM) and 4-hydroxybenzaldehyde (4-OHBA). We found that 4-QM reacts rapidly with valine to form the 4-OHBn-Val adduct; however, the quinone methide is unstable under physiological conditions due to hydrolysis. It was shown that 4-OHBA forms reversible Schiff base adducts with valine, which can be stabilized via reduction in blood generating the 4-OHBn-Val adduct. In addition, trace amounts of isomeric 2-hydroxybenzyl-valine (2-OHBn-Val) adducts were detected in 12 human blood samples (estimated mean adduct level, 5.0 ± 1.4 pmol/g Hb). Further studies are needed to quantify the contributions from identified possible precursor electrophiles to the observed hydroxybenzyl adducts in humans.

Influence of Water in the Photogeneration and Properties of a Bifunctional Quinone Methide

Quinone methides (QM) are crucial reactive species in molecular biology and organic chemistry, with little known regarding the mechanism(s) for the generation of short-lived reactive QM intermediates from relevant precursors in aqueous solutions. In this study, several time-resolved spectroscopy methods were used to directly examine the photophysics and photochemical pathways of 1,1'-(2,2'-dihydroxy-1,1'-binaphthyl-6,6'-diyl)bis(N,N,N-trimethylmethanaminium) bromide (BQMP-b) from initial photoexcitation to the generation of the key reactive binol QM intermediate (BQM) in aqueous solution. The fluorescence of BQMP-b is effectively quenched with a small amount of water, which suggests an excited state intramolecular proton transfer (ESIPT) occurs. The kinetics isotope effects observed in femtosecond and nanosecond time-resolved transient absorption experiments provide evidence for the participation of water molecules in the BQMP-b singlet excited state ESIPT process and in the subsequent -HNMe₃⁺ group release and ground state intramolecular proton transfer that give rise to production of the reactive BQM intermediate. Nanosecond time-resolved resonance Raman (ns-TR³) measurements were also employed to investigate the structure and properties of several intermediates, including the key reactive BQM in aqueous solution. The ns-TR³ and density functional theory (DFT) computational results were compared, and this indicates the binol moiety and water molecules both have important roles in the characteristics and structure of the key reactive BQM intermediate produced from BQMP-b. The results presented here also provide new benchmark characterization of bifunctional quinone methide intermediates that can be utilized to guide direct time-resolved spectroscopic study of the alkylation and interstrand cross-linking reactions of quinone methides with DNA in the future.

Oxidative quenching of quinone methide adducts reveals transient products of reversible alkylation in duplex DNA

ortho-Quinone methides (ortho-QM) and para-quinone methides are generated by xenobiotic metabolism of numerous compounds including environmental toxins and therapeutic agents. These intermediates are highly electrophilic and have the potential to alkylate DNA. Assessing their genotoxicity can be difficult when all or some of their resulting adducts form reversibly. Stable adducts are most easily detected but are not necessarily the most prevalent products formed initially as DNA repair commences. Selective oxidation of ortho-QM-DNA adducts by bis[(trifluoroacetoxy)iodo]benzene (BTI) rapidly quenches their reversibility to prevent QM regeneration and allows for observation of the kinetic products. The resulting derivatives persist through standard enzymatic digestion, chromatography, and mass spectral analysis. The structural standards required for this approach have been synthesized and confirmed by two-dimensional NMR spectroscopy. The adducts of dA N(6), dG N1, dG N(2), and guanine N7 are converted to the expected para-quinol derivatives within 5 min after addition of BTI under aqueous conditions (pH 7). Concurrently, the adduct of dA N1 forms a spiro derivative comparable to that characterized previously after oxidation of the corresponding dC N3 adduct. By application of this oxidative quenching strategy, the dC N3 and dA N1 adducts have been identified as the dominant products formed by both single- and double-stranded DNA under initial conditions. As expected, however, these labile adducts dissipate within 24 h if not quenched with BTI. Still, the products favored by kinetics are responsible for inducing the first response to ortho-QM exposure in cells, and hence, they are also key to establishing the relationship between biological activity and molecular structure.

ortho-Quinone methide (o-QM): a highly reactive, ephemeral and versatile intermediate in organic synthesis

In this critical review, we provide a comprehensive view of the chemistry of ortho-quinone methides as versatile reactive intermediates in organic synthesis.