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

Abstract

Direct addition of aryl radical species to the C(8)-site of 2'-deoxyguanosine (dG) affords C(8)-aryl-dG adducts that are produced by carcinogenic arylhydrazines, polycyclic aromatic hydrocarbons (PAHs), and certain phenolic toxins. A common property of C(8)-arylpurine adduction is the accompaniment of abasic site formation. To determine how the C(8)-aryl moiety contributes to sugar loss, UV-vis spectroscopy has been employed to determine N(7) pK(a1) values and hydrolysis kinetics, while density functional theory (DFT) calculations have been utilized to probe the structural features and stability of the C(8)-aryl-dG adducts bearing different para and ortho substituents. In all cases, the C(8)-aryl-dG adducts adopt a syn conformation containing a strong O(5)'-H...N(3) hydrogen bond with the aryl ring twisted with respect to the nucleobase. The adducts undergo N(7)-protonation with ionization constants and calculated N(7) proton affinity (PA) values similar to those measured for dG. The hydrolysis kinetics shows that C(8)-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 N(7)-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) approximately 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 C(8)-arylpurine adduction within duplex DNA at neutral pH.

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

“… 24 The PhG nucleoside is also fluorescent in H 2 O ( λ em = 395 nm, Φ fl = 0.44 ( ref. 24 )) following excitation at 277 nm (log ε = 4.33 ( ref. 70 )).…”

“…Modification of the 8-site of G with aryl groups also accelerates the rate of acid-catalyzed hydrolysis. 70 Hydrolysis of the unmodified base ( Scheme 1 ) proceeds via a stepwise mechanism, involving initial protonation at N 7 (p K a of the protonated species in 2.34 ( ref. 77 )) followed by rate-limiting unimolecular cleavage of the glycosidic bond ( k 1 ).…”

“…77 )) followed by rate-limiting unimolecular cleavage of the glycosidic bond ( k 1 ). 22 , 70 The oxocarbenium ion subsequently undergoes hydration to produce the 1′-hydroxylated-2′-deoxyribose sugar. Rates of hydrolysis for 8arylG adducts are 5- to 45-fold greater than the unmodified base in 0.1 M HCl, which increases to 9- to 200-fold at pH 4.…”

Structural and biochemical impact of C8-aryl-guanine adducts within the NarI recognition DNA sequence: influence of aryl ring size on targeted and semi-targeted mutagenicity

“…Thus, Kf − and Dpo4 were employed in the present work to investigate how C-linked C8-aryl-dG adducts with different aryl rings impact replication by high-fidelity and translesion polymerases. The results were expected to demonstrate the mutagenic potential of such lesions, establish the impact of aryl ring size and permit comparison to their N-linked C8-dG counterparts.C-Linked C8-aryl-dG adducts are difficult to incorporate into oligonucleotides in a site-specific manner because they are sensitive to acid-catalyzed deglycosylation ( 44 ), thereby limiting the adaptation of standard solid-phase synthesis with 5′- O -DMT protection for this purpose ( 45 ). To overcome this limitation, we developed an efficient solid-phase protocol for inserting representative C8-aryl-dG adducts into the reiterated G 3 -position (X) of Nar I Type II restriction endonuclease recognition sequence (5′-CTCG 1 G 2 CXCCATC) ( 46 ).…”

Synthesis of 8‐Phenoxy‐2′‐deoxyguanosine Nucleoside Analogues

“…[17] Recent studies by the groups of Wetmore and Manderville suggest that under physiological conditions, homolytic cleavage during adduct formation rather than hydrolysis following adduct formation may be responsible for depurination, particularly for purines substituted with small aryl adducts. [18] However, the C 8 - O linkage for adducts 3a-g differs significantly from the C 8 -aryl linkage of the adducts studied by Wetmore and Manderville, which may render their lability relevant under physiological conditions. Upon incorporation of adducts 3 into oligonucleotides, further studies will test this hypothesis.…”