pH-Dependent stability and reactivity of a thiol-quinone methide adduct

Exploiting Alkylquinone Tautomerization: Amine Benzylation

A general protocol for the synthesis of benzylic amines via side-chain amination of alkylquinones is reported. The reactions are initiated by the tautomerization of an alkylquinone to the corresponding quinone methide, which is subsequently trapped in situ by an amine nucleophile. This process is promoted by tertiary amines in protic solvents under mild conditions and is compatible with many functional groups. 1,2- and 1,4-benzoquinones, as well as naphthoquinones, participate in this reaction using a wide range of primary and secondary amines/anilines. The synthetic utility of this transformation is also explored.

A transient product of DNA alkylation can be stabilized by binding localization

A 9-aminoacridine conjugate of a silyl-protected bis(acetoxymethyl)phenol (bisQMP) was synthesized and evaluated as an inducible cross-linking agent of DNA to test our ability to harness the chemistry of reactive quinone methide intermediates (QM). The acridine component was chosen for its ability to delivery an appendage to the major groove of DNA, and the silyl-protected component was chosen for its ability to generate two quinone methide equivalents in tandem upon addition of fluoride. This design created competition between reaction of (1) the 2-amino group of guanine that reacts irreversibly to form a stable QM adduct and (2) the more nucleophilic N7 group of guanine that reacts more efficiently but reversibly to form a labile QM adduct. This lability was apparently compensated by co-localization of the N7 group and QM in the major groove since the N7 adduct appeared to dominate the profile of products formed by duplex DNA. The controlling influence of acridine was also expressed in the sensitivity of the conjugate to ionic strength. High salt concentration inhibited covalent reaction just as it inhibits intercalation of the cationic acridine. As expected for QM formation, the presence of fluoride was indeed necessary for initiating reaction, and no direct benzylic substitution was observed. The conjugate also cross-linked DNA with high efficiency, forming one cross-link for every four alkylation events. Both alkylation and cross-linking products formed by duplex DNA were labile to hot piperidine treatment which led to approximately 40% strand scission and approximately 50% reversion to a material with an electrophoretic mobility equivalent to the parent DNA. All guanines exhibited at least some reactivity including those which were recalcitrant to cross-linking by an oligonucleotide-bisQMP conjugate designed for triplex formation [Zhou, G.; Pande, P.; Johnson, A. E.; Rokita, S. E. Bioorg. Med. Chem. 2001, 9, 2347-2354].

A general strategy for target-promoted alkylation in biological systems

Selective alkylation of a chosen sequence of DNA typically relies on ligand-directed delivery of a compound that expresses an intrinsic reactivity. A significant and biologically relevant enhancement in specificity is theoretically possible if such an intrinsic reactivity could be replaced by a latent activity induced solely by the target of interest, but examples of this are rare and not easily emulated. A simple strategy for target-promoted alkylation is now illustrated by an intramolecular adduct formed by an oligonucleotide-quinone methide conjugate. This adduct persists in the absence of a complementary sequence of DNA for at least 8 days, yet remarkably is able to alkylate target DNA upon duplex hybridization. Neither formation of the intramolecular self-adduct nor transfer of the quinone methide to its target is significantly quenched by 450-fold excess 2-mercaptoethanol. Similarly, noncomplementary DNA is neither subject to alkylation by the self-adduct nor able to effect its consumption. Reversible trapping of the nascent quinone methide through an intramolecular reaction thus appears efficient enough to inhibit competing intermolecular reaction. Only complementary base pairing induces a conformational change necessary to promote intermolecular transfer of the quinone methide. Generalization of this approach based on reversible intramolecular trapping of a reactive intermediate by a ligand with multiple recognition subdomains has the potential for wide-ranging applications in targeting nucleic acids and proteins.

Influence of quinone methide reactivity on the alkylation of thiol and amino groups in proteins: studies utilizing amino acid and peptide models

Quinone methides (QMs) are electrophiles formed in several biological processes including direct oxidations of 4-alkylphenols by cytochromes P450. These species may be responsible for the adverse effects of certain phenolic compounds through protein alkylation, but little information is available concerning specific targets or the resulting mechanisms of cell injury. The present goal was to determine the most likely sites of adduct formation among competing protein nucleophiles utilizing QMs of varying electrophilicity. Reactions of poorly reactive, moderately reactive, and highly reactive QMs, 2,6-di-tert-butyl-4-methylene-2,5-cyclohexadienone (BHT-QM), 6-tert-butyl-2-(2'-hydroxyl-1',1'-dimethylethyl)-4-methylene- 2,5-cyclohexadienone (BHTOH-QM), and 2-tert-butyl-6-methyl-4-methylene-2,5-cyclohexadienone (BDMP-QM), respectively, were investigated in aqueous solutions with nucleophilic amino acids. Each QM rapidly formed a thioether derivative of cysteine with little or no competition from the addition of water (hydration). The alpha-amino groups were the primary sites of alkylation for all other amino acids examined including lysine, histidine, tyrosine, and serine, and the pseudo-first order rates were 5 to 8-fold greater than the rates of hydration. Alkylation of the side chain nitrogens of lysine and histidine occurred at about one-fourth the rate of hydration for BDMP-QM, but no reaction was detectable for BHT-QM and no reactions occurred between QMs and amino acid hydroxyl groups. The results indicate that, based on chemical reactivity, peptide alkylation should occur in the order cysteine thiol > N-terminal amino > N epsilon-lysine = NIm-histidine, with side chain modifications occurring only with the more electrophilic QMs. Reactions of QMs with the tripeptide Gly-His-Lys confirmed the results with amino acids as N alpha-glycine alkylation predominated, but side chain adducts also formed with BHTOH-QM and BDMP-QM. Human hemoglobin was treated with QMs, hydrolyzed, and assayed by HPLC-thermospray mass spectrometry. This work revealed that N epsilon-lysine was the main alkylation site, emphasizing the importance of factors, in addition to chemical reactivity, which influence protein modification by electrophiles.