Concerning the Hydrolytic Stability of 8-Aryl-2′-deoxyguanosine Nucleoside Adducts: Implications for Abasic Site Formation at Physiological pH

Direct addition of aryl radical species to the C8-site of 2′-deoxyguanosine (dG) affords C8-aryl-dG adducts that are produced by carcinogenic arylhydrazines, polycyclic aromatic hydrocarbons (PAHs), and certain phenolic toxins. A common property of C8-arylpurine adduction is the accompaniment of abasic site formation. To determine how the C8-aryl moiety contributes to sugar loss, UV−vis spectroscopy has been employed to determine N7 pK a1 values and hydrolysis kinetics, while density functional theory (DFT) calculations have been utilized to probe the structural features and stability of the C8-aryl-dG adducts bearing different para and ortho substituents. In all cases, the C8-aryl-dG adducts adopt a syn conformation containing a strong O5′−H···N3 hydrogen bond with the aryl ring twisted with respect to the nucleobase. The adducts undergo N7-protonation with ionization constants and calculated N7 proton affinity (PA) values similar to those measured for dG. The hydrolysis kinetics shows that C8-aryl-dG nucleoside adducts are more prone than dG to acid-catalyzed hydrolysis, with those bearing para substituents having k 1 values that are ca. 90- to 200-fold larger than k 1 for dG, while the effects for the ortho adducts are only ca. 9- to 60-fold larger. Changes in the rate of hydrolysis are further explained by calculations showing that glycosidic bond cleavage in the syn orientation of both neutral and N7-protonated dG has a lower barrier than the anti orientation, and the bulky (phenyl) group further decreases the barrier. Despite adduct reactivity in acidic media, all adducts are relatively stable at physiological pH with t 1/2 ∼25 days, suggesting that they are unlikely intermediates leading to abasic site formation at physiological pH. This information has allowed development of a new rationale for the tendency of abasic site formation to accompany C8-arylpurine adduction within duplex DNA at neutral pH.