Effect of Watson−Crick and Hoogsteen Base Pairing on the Conformational Stability of C8-Phenoxyl-2′-deoxyguanosine Adducts

Quantum Chemical Studies of the Structure and Stability of N-Methylated DNA Nucleobase Dimers: Insights into the Mutagenic Base Pairing of Damaged DNA

DNA is constantly under attack from exogenous and endogenous sources that modify the chemical structure of the nucleobases. A common type of nucleobase damage is N-methylation, which can result in mutagenesis. Nevertheless, these lesions are often repaired by the DNA repair enzyme AlkB, albeit at varying rates. Herein we use density functional theory (B3LYP-D3(BJ)/6-311++G(2df,2p)//B3LYP/6-31G(d,p)) to comprehensively examine the structural and energetic properties of base pairs between seven nucleobase lesions resulting from N-methylation on the Watson-Crick (WC) binding face and each canonical nucleobase. By characterizing 105 stable nucleobase dimers, we provide fundamental details regarding the preferred lesion base pairings. Specifically, we reveal that the flexibility of the methylamino group resulting from methylation of an exocyclic amino substituent allows the 2MeG, 4MeC, and 6MeA lesions to maintain a preference for canonical WC base pairing, which correlates with the experimentally reported lack of mutagenicity for these damage products. In contrast, calculated distortions in key structural parameters and altered binding energies for base pairs involving adducts formed upon methylation of a ring nitrogen (namely, 1MeG, 3MeT, 1MeA, and 3MeC) help rationalize the associated mutagenicity and repair efficiencies. Most importantly, our work provides molecular-level information about the interactions between N-methylated and canonical nucleobases that is critical for future large-scale modeling of damaged DNA and enzyme-DNA complexes that strive to further uncover the mutagenicity and repair propensities of these detrimental lesions.

Conformational flexibility and base-pairing tendency of the tobacco carcinogen O6-[4-oxo-4-(3-pyridyl)butyl]guanine

The present work uses DFT calculations to characterize the conformational and hydrogen-bonding properties of O6-[4-oxo-4-(3-pyridyl)butyl]guanine (POB-G), a DNA adduct caused by tobacco. POB-G is found to adopt many isoenergetic conformations that allow for discrete interactions between the bulky moiety and the adducted G and/or pairing base. The calculated structure and stability of the hydrogen-bonded pairs between the Watson-Crick or Hoogsteen face of POB-G and the canonical DNA nucleobases fully rationalize the previously reported mutational spectra. Specifically, the stable, non-distorted pseudo-Watson-Crick POB-G:T pair explains the predominant G➔A mutations, while the stable, yet marginally distorted pairs between the Watson-Crick face of POB-G and A or C clarify the G➔T mutations and non-mutagenic replication. Finally, the stable, yet highly distorted Hoogsteen POB-G:G pair rationalizes the experimentally-observed insertion but lack of persistence of G opposite POB-G in DNA. Overall, these structural insights are critical for guiding future studies that strive to fully understand the adduct mutagenicity, including the accessible conformations and the replication of POB-G-adducted DNA.

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

Aryl radical species derived from enzymatic transformations of aromatic mutagens preferentially react at the 8-site of the guanine (G) nucleobase to afford carbon-linked 8arylG adducts. The resulting lesions possess altered biophysical and genetic coding properties compared to the precursor G nucleoside in B-form DNA. Unlike other adducts, these lesions also possess useful fluorescent properties, since direct attachment of the 8aryl ring extends the purine π-system to afford G mimics with red-shifted excitation maxima and emission that can be sensitive to the microenvironment of the 8arylG base within nucleic acid structures. In B-form DNA, 8arylG adducts are disruptive to duplex formation because they prefer to adopt the syn-conformation about the bond connecting the nucleobase to the deoxyribose backbone, which perturbs Watson-Crick (WC) H-bonding with the opposing cytosine (C). Thus, in a B-form duplex, the emissive properties of 8arylG adducts can be employed as a tool to provide insight into adduct conformation, which can be related to their biological outcomes. However, since Gs preferentially adopt the syn-conformation in left-handed Z-DNA and antiparallel G-quadruplex (GQ) structures, 8arylG lesions can be inserted into syn-G positions without disrupting H-bonding interactions. In fact, 8arylG lesions can serve as ideal fluorescent probes in an antiparallel GQ because their emission is sensitive to GQ folding. This perspective outlines recent developments in the biological implications of 8arylG formation together with their utility as fluorescent G analogs for use in DNA-based diagnostic systems.

Conformational stabilities of iminoallantoin and its base pairs in DNA: implications for mutagenicity

Under acidic conditions, insertion of G opposite Ia may lead to G to C mutations in DNA.

C8-linked bulky guanosine DNA adducts: experimental and computational insights into adduct conformational preferences and resulting mutagenicity

Bulky DNA adducts are formed through the covalent attachment of aryl groups to the DNA nucleobases. Many of these adducts are known to possess conformational heterogeneity, which is responsible for the variety of mutagenic outcomes associated with these lesions. The present contribution reviews several conformational and mutagenic themes that are prevalent among the DNA adducts formed at the C8-site of the guanine nucleobase. The most important conclusions obtained (to date) from experiments are summarized including the anti/syn conformational preference of the adducts, their potential to inflict DNA mutations and mismatch stabilization, and their interactions with DNA polymerases and repair enzymes. Additionally, the unique role that computer calculations can play in understanding the structural properties of these adducts are highlighted.

Application of a fluorescent C-linked phenolic purine adduct for selective N7-metalation of DNA

The C-linked phenolic adduct, C8-(2″-hydroxyphenyl)-2'-deoxyguanosine (o-PhOHdG), has been employed to study the impact of N7-metalation of 2'-deoxyguanosine (dG) within duplex DNA. The phenolic group of o-PhOHdG assists selective metal ion coordination by the N7-site of the attached dG moiety, which is the most important metal binding site in duplex DNA. The biaryl nucleobase probe o-PhOHdG is highly fluorescent in water (Φ(fl) = 0.44), and changes in its absorption and emission were used to determine apparent association constants (K(a)) for binding to Cu(II), Ni(II), and Zn(II). The nucleoside was found to bind Cu(II) (log K(a) = 4.59) and Ni(II) (log K(a) = 3.65) effectively, but it showed relatively poor affinity for Zn(II) (log K(a) = 2.55). The fluorescent nucleobase o-PhOHdG was incorporated into a pyrimidine-rich oligonucleotide substrate (ODN1) and a purine-rich (ODN2) substrate to monitor selective binding of Cu(II) through fluorescence quenching of the enol emission of o-PhOHdG within the DNA substrates. The pyrimidine-rich substrate ODN1 was found to possess greater affinity for Cu(II) than the free nucleobase, while the purine-rich substrate ODN2 exhibited diminished Cu(II) binding affinity. The impact of Cu(II) on duplex stability and structure was determined using UV melting temperature analysis and circular dichroism (CD) measurements. These studies highlight the syn preference for Cu(II)-bound o-PhOHdG within ODN1 duplexes and demonstrate competitive Cu(II) binding by other natural dG nucleobases within ODN2. The metal binding properties of o-PhOHdG are compared to the structurally similar 2-(2'-hydroxyphenyl)benzoxazole (HBO) derivatives and the nucleoside C8-(2-pyridyl)-dG (2PydG) that has also been used to control N7-metal coordination in DNA. Our results show certain advantages to the use of o-PhOHdG that stem from its highly fluorescent nature in aqueous media and provide additional tools for studying the effects of N7-metalation on the structure and stability of duplex DNA.

Conformational flexibility of C8-phenoxylguanine adducts in deoxydinucleoside monophosphates

M06-2X/6-31G(d,p) is used to calculate the structure of all natural deoxydinucleoside monophosphates with G in the 5' or 3' position, the anti or syn conformation, and each natural (A, C, G, T) base in the corresponding flanking position. When the ortho or para C8-phenoxyl-2'-deoxyguanosine (C8-phenoxyl-dG) adduct replaces G in each model, there is little change in the relative base-base orientation or backbone conformation. However, the orientation of the C8-phenoxyl group can be characterized according to the position (5' versus 3'), conformation (anti versus syn), and isomer (ortho versus para) of damage. Although the degree of coplanarity between the phenoxyl ring and G base in the ortho adduct is highly affected by the sequence since the hydroxyl group can interact with neighboring bases, the para adduct generally does not exhibit discrete interactions with flanking bases. For both adducts, steric clashes between the phenoxyl group and the backbone or flanking base destabilize the anti conformation preferred by the natural nucleotide and thereby result in a clear preference for the syn conformation regardless of the sequence or position. This contrasts the conclusions drawn from smaller (nucleoside, nucleotide) models previously used in the literature, which stresses the importance of using models that address the steric constraints present due to the surrounding environment. Since replication errors for other C8-dG bulky adducts have been linked to a preference for the syn conformation, our findings provide insight into the possible mutagenicity of phenolic adducts.

Fluorescent properties and conformational preferences of C-linked phenolic-DNA adducts

Phenolic toxins and mutagenic diazoquinones generate C-linked adducts at the C8 site of 2'-deoxyguanosine (dG) through the intermediacy of radical species. We have previously reported the site-specific incorporation of these adducts into oligonucleotides using a postsynthetic palladium-catalyzed cross-coupling strategy [Omumi (2011 ) J. Am. Chem. Soc. 133 , 42 - 50 ]. We report here the structural impact of these lesions within two decanucleotide sequences containing either 5'- and 3'-flanking pyrimidines or purines. In the complementary strands, the base opposite (N) the C-linked adduct was varied to determine the possibility of mismatch stabilization by the modified nucleobases. The resulting adducted duplex structures were characterized using UV thermal denaturation studies, circular dichroism, fluorescence spectroscopy, and molecular dynamics (MD) simulations. The experimental data showed the C-linked adducts to destabilize the duplex when base paired with its normal partner C but to increase duplex stability within a G:G mismatch. The stabilization within the G:G mismatch was sequence dependent, with flanking purine bases playing a key role in the stabilizing influence of the adduct. MD simulations showed no large structural changes to the B form double helix, regardless of the (anti/syn) adduct preference. Consideration of H-bonding and stacking interactions derived from the MD simulations together with the thermal melting data and changes in fluorescent emission of the adducts upon hybridization to the complementary strands implied that the C-linked phenolic adducts preferentially adopt the syn-conformation within both duplexes regardless of the opposite base N. Given that biological outcome in terms of mutagenicity appears to be strongly correlated to the conformational preference of the corresponding N-linked C8-dG adducts, the potential biological implications of phenolic C-linked adducts are discussed.

An indole-linked C8-deoxyguanosine nucleoside acts as a fluorescent reporter of Watson-Crick versus Hoogsteen base pairing

Pyrrole- and indole-linked C(8)-deoxyguanosine nucleosides act as fluorescent reporters of H-bonding specificity. Their fluorescence is quenched upon Watson-Crick H-bonding to dC, while Hoogsteen H-bonding to G enhances emission intensity. The indole-linked probe is ∼ 10-fold brighter and shows promise as a fluorescent reporter of Hoogsteen base pairing.

Postsynthetic guanine arylation of DNA by Suzuki-Miyaura cross-coupling

Direct radical addition reactions at the C(8)-site of 2'-deoxyguanosine (dG) can afford C(8)-Ar-dG adducts that are produced by carcinogenic arylhydrazines, polycyclic aromatic hydrocarbons, and certain phenolic toxins. Such modified nucleobases are also highly fluorescent for sensing applications and possess useful electron transfer properties. The site-specific synthesis of oligonucleotides containing the C(8)-Ar-G adduct can be problematic. These lesions are sensitive to acids and oxidants that are commonly used in solid-phase DNA synthesis and are too bulky to be accepted as substrates for enzymatic synthesis by DNA polymerases. Using the Suzuki-Miyaura cross-coupling reaction, we have synthesized a number of C(8)-Ar-G-modified oligonucleotides (dimers, trimers, decamers, and a 15-mer) using a range of arylboronic acids. Good to excellent yields were obtained, and the reaction is insensitive to the nature of the bases flanking the convertible 8-Br-G nucleobase, as both pyrimidines and purines are tolerated. The impact of the C(8)-Ar-G lesion was also characterized by electrospray ionization tandem mass spectrometry, UV melting temperature analysis, circular dichroism, and fluorescence spectroscopy. The C(8)-Ar-G-modified oligonucleotides are expected to be useful substrates for diagnostic applications and understanding the biological impact of the C(8)-Ar-G lesion.