Purine 5′,8-cyclo-2′-deoxynucleoside lesions: formation by radical stress and repair in human breast epithelial cancer cells

Protocol for the simultaneous quantification of oxidative purine lesions in DNA using LC-MS/MS analysis

Most DNA damages induced through oxidative metabolism are single lesions which can accumulate in tissues. Here, we present a protocol for the simultaneous quantification of oxidative purine lesions (cPu and 8-oxo-Pu) in DNA. We describe steps for enzymatic digestion of DNA and sample pre-purification, followed by quantification through liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis. We optimized this protocol in commercially available calf thymus DNA and used genomic and mitochondrial DNA extracted from cell cultures and animal and human tissues.

The Influence of 2'-Deoxyguanosine Lesions on the Electronic Properties of OXOG:::C Base Pairs in Ds-DNA: A Comparative Analysis of Theoretical Studies

DNA is continuously exposed to a variety of harmful factors, which, on the one hand, can force undesirable processes such as ageing, carcinogenesis and mutagenesis, while on the other hand, can accelerate evolutionary changes. Of all the canonical nucleosides, 2'-deoxyguanosine (dG) exhibits the lowest ionization potential, making it particularly prone to the one-electron oxidizing process. The most abundant type of nucleobase damage is constituted by 7,8-dihydro-8-oxo-2'-deoxyguanosine (OXOdG), with an oxidation potential that is 0.56 V lower than that of canonical dG. All this has led to OXOdG, as an isolated lesion, being perceived as a sink for radical cations in the genome. In this paper, a comparative analysis of the electronic properties of an OXOGC base pair within the context of a clustered DNA lesion (CDL) has been conducted. It is based on previous DFT studies that were carried out at the M06-2x/6-31++G** level of theory in non-equilibrated and equilibrated condensed phases. The results of the comparative analysis presented here reveal the following: (A) The ionization potentials of OXOG4C2 were largely unaffected by a second lesion. (B) The positive charge and spin were found predominantly on the OXOG4C2 moiety. (C) The electron-hole transfers A3T3→G4C2 and G4C2←A5T1 were found in the Marcus inverted region and were resistant to the presence of a second DNA lesion in close proximity. It can therefore be reasonably postulated that OXOGC becomes the sink for a radical cation migrating through the double helix, irrespective of the presence of other 2'-deoxyguanosine lesions in the CDL structure.

Radical Reactions in Organic Synthesis: Exploring in-, on-, and with-Water Methods

Radical reactions in water or aqueous media are important for organic synthesis, realizing high-yielding processes under non-toxic and environmentally friendly conditions. This overview includes (i) a general introduction to organic chemistry in water and aqueous media, (ii) synthetic approaches in, on, and with water as well as in heterogeneous phases, (iii) reactions of carbon-centered radicals with water (or deuterium oxide) activated through coordination with various Lewis acids, (iv) photocatalysis in water and aqueous media, and (v) synthetic applications bioinspired by naturally occurring processes. A wide range of chemical processes and synthetic strategies under different experimental conditions have been reviewed that lead to important functional group translocation and transformation reactions, leading to the preparation of complex molecules. These results reveal how water as a solvent/medium/reagent in radical chemistry has matured over the last two decades, with further discoveries anticipated in the near future.

The Influence of 5′,8-Cyclo-2′-Deoxyguanosine on ds-DNA Charge Transfer Depends on Its Diastereomeric Form: A Theoretical Study

The genetic information stored in the nucleobase sequence is continuously exposed to harmful extra- and intra-cellular factors, which can lead to different types of DNA damage, with more than 70 lesion types identified so far. In this article, the influence of a multi-damage site containing (5′R/S) 5′,8-cyclo-2′-deoxyguanosine (cdG) and 7,8-dihydro-8-oxo-2′-deoxyguanosine (OXOdG) on charge transfer through ds-DNA was taken into consideration. The spatial geometries of oligo-RcdG: d[A1(5′R)cG2A3OXOG4A5]*d[T5C4T3C2T1] and oligo-ScdG: d[A1(5′S)cG2A3OXOG4A5]*d[T5C4T3C2T1] were optimized at the M06-2X/6-D95**//M06-2X/sto-3G level of theory in the aqueous phase using ONIOM methodology. For all the electronic property energies under discussion, the M06-2X/6-31++G** level of theory was used. Additionally, the non-equilibrated and equilibrated solvent-solute interactions were into consideration. The obtained results confirm the predisposition of OXOdG to radical cation formation regardless of the presence of other lesions in a ds-DNA structure. In the case of electron transfer, however, the situation is different. An excess electron migration towards (5′S)cdG was found to be preferred in the case of oligo-ScdG, while in the case of oligo-RcdG, OXOdG was favored. The above observation was confirmed by the charge transfer rate constant, vertical/adiabatic ionization potential, and electron affinity energy values, as well as the charge and spin distribution analysis. The obtained results indicate that 5′,8-cyclo-2′-deoxyguanosine, depending on the C5′ atom chirality, can significantly influence the charge migration process through the double helix. The above can be manifested by the slowdown of DNA lesion recognition and removal processes, which can increase the probability of mutagenesis and subsequent pathological processes. With regard to anticancer therapy (radio/chemo), the presence of (5′S)cdG in the structure of formed clustered DNA damage can lead to improvements in cancer treatment.

When UDG and hAPE1 Meet Cyclopurines. How (5'R) and (5'S) 5',8-Cyclo-2'-deoxyadenosine and 5',8-Cyclo-2'-deoxyguanosine Affect UDG and hAPE1 Activity?

Ionizing radiation is a factor that seriously damages cellular mechanisms/macromolecules, e.g., by inducing damage in the human genome, such as 5′,8-cyclo-2′-deoxypurines (cdPus). CdPus may become a component of clustered DNA lesions (CDL), which are notably unfavorable for the base excision repair system (BER). In this study, the influence of 5′S and 5′R diastereomers of 5′,8-cyclo-2′-deoxyadenosine (cdA) and 5′,8-cyclo-2′-deoxyguanosine (cdG) on the uracil-DNA glycosylase (UDG) and human AP site endonuclease 1 (hAPE1) activity has been taken under consideration. Synthetic oligonucleotides containing 2′-deoxyuridine (dU) and cdPu were used as a model of single-stranded CDL. The activity of the UDG and hAPE1 enzymes decreased in the presence of RcdG compared to ScdG. Contrary to the above, ScdA reduced enzyme activity more than RcdA. The presented results show the influence of cdPus lesions located within CDL on the activity of the initial stages of BER dependently on their position toward dU. Numerous studies have shown the biological importance of cdPus (e.g., as a risk of carcinogenesis). Due to that, it is important to understand how to recognize and eliminate this type of DNA damage from the genome.

On the relevance of hydroxyl radical to purine DNA damage

Hydroxyl radical (HO^•) is the most reactive toward DNA among the reactive oxygen species (ROS) generated in aerobic organisms by cellular metabolisms. HO^• is generated also by exogenous sources such as ionizing radiations. In this review we focus on the purine DNA damage by HO^• radicals. In particular, emphasis is given on mechanistic aspects for the various lesion formation and their interconnections. Although the majority of the purine DNA lesions like 8-oxo-purine (8-oxo-Pu) are generated by various ROS (including HO^•), the formation of 5',8-cyclopurine (cPu) lesions in vitro and in vivo relies exclusively on the HO^• attack. Methodologies generally utilized for the purine lesions quantification in biological samples are reported and critically discussed. Recent results on cPu and 8-oxo-Pu lesions quantification in various types of biological specimens associated with the cellular repair efficiency as well as with distinct pathologies are presented, providing some insights on their biological significance.

Oxygen-Dependent Accumulation of Purine DNA Lesions in Cockayne Syndrome Cells

Cockayne Syndrome (CS) is an autosomal recessive neurodegenerative premature aging disorder associated with defects in nucleotide excision repair (NER). Cells from CS patients, with mutations in CSA or CSB genes, present elevated levels of reactive oxygen species (ROS) and are defective in the repair of a variety of oxidatively generated DNA lesions. In this study, six purine lesions were ascertained in wild type (wt) CSA, defective CSA, wtCSB and defective CSB-transformed fibroblasts under different oxygen tensions (hyperoxic 21%, physioxic 5% and hypoxic 1%). In particular, the four 5′,8-cyclopurine (cPu) and the two 8-oxo-purine (8-oxo-Pu) lesions were accurately quantified by LC-MS/MS analysis using isotopomeric internal standards after an enzymatic digestion procedure. cPu levels were found comparable to 8-oxo-Pu in all cases (3–6 lesions/10⁶ nucleotides), slightly increasing on going from hyperoxia to physioxia to hypoxia. Moreover, higher levels of four cPu were observed under hypoxia in both CSA and CSB-defective cells as compared to normal counterparts, along with a significant enhancement of 8-oxo-Pu. These findings revealed that exposure to different oxygen tensions induced oxidative DNA damage in CS cells, repairable by NER or base excision repair (BER) pathways. In NER-defective CS patients, these results support the hypothesis that the clinical neurological features might be connected to the accumulation of cPu. Moreover, the elimination of dysfunctional mitochondria in CS cells is associated with a reduction in the oxidative DNA damage.

Increased levels of 5',8-Cyclopurine DNA lesions in inflammatory bowel diseases

Chronic inflammation is estimated to be a causative factor in a variety of diseases. Under inflammatory conditions reactive oxygen species (ROS) and nitrogen species (RNS) are released leading to DNA damage accumulation and genomic instability. Purine 5′,8-cyclo-2′-deoxynucleosides (cPu) are oxidative DNA lesions, exclusively derived from the attack of HO^• radicals, which are known to have cytotoxic and mutagenic properties. Herein, we have analyzed the presence of cPu in genomic DNA isolated from fresh colon and visceral adipose tissue biopsies collected from inflammatory bowel diseases (IBD)-affected patients and severely obese subjects, respectively, versus what observed in the control specimens. In colon biopsies, characterized by a higher gene expression level of inducible nitric oxide synthase (iNOS), a significant increase of 8-oxo-7,8-dihydro-2′-deoxyadenosine (8-oxo-dA) and 8-oxo-7,8-dihydro-2′-deoxyguanosine (8-oxo-dG) lesions and an accumulation of both diastereomeric cPu have been detected. In contrast, the 8-oxo-dA and 8-oxo-dG levels were extremely lower compared to the colon tissues values and no accumulation of cPu, in the inflamed visceral adipose tissue biopsies isolated from bariatric patients versus the lean counterpart was reported. In addition, in adipose tissue undetectable levels of iNOS have been found. These data suggest a potential involvement of cPu in the colon cancer susceptibility observed in IBD patients.

Oxygen Dependent Purine Lesions in Double-Stranded Oligodeoxynucleotides: Kinetic and Computational Studies Highlight the Mechanism for 5',8-Cyclopurine Formation

The reaction of HO^• radical with DNA is intensively studied both mechanistically and analytically for lesions formation. Several aspects related to the reaction paths of purine moieties with the formation of 5',8-cyclopurines (cPu), 8-oxopurines (8-oxo-Pu), and their relationship are not well understood. In this study, we investigated the reaction of HO^• radical with a 21-mer double-stranded oligodeoxynucleotide (ds-ODNs) in γ-irradiated aqueous solutions under various oxygen concentrations and accurately quantified the six purine lesions (i.e., four cPu and two 8-oxo-Pu) by LC-MS/MS analysis using isotopomeric internal standards. In the absence of oxygen, 8-oxo-Pu lesions are only ∼4 times more than cPu lesions. By increasing oxygen concentration, the 8-oxo-Pu and the cPu gradually increase and decrease, respectively, reaching a gap of ∼130 times at 2.01 × 10^-4 M of O₂. Kinetic treatment of the data allows to estimate the C5' radical competition between cyclization and oxygen trapping in ds-ODNs, and lastly the rate constants of the four cyclization steps. Tailored computational studies by means of dispersion-corrected DFT calculations were performed on the CGC and TAT in their double-strand models for each cPu diastereoisomer along with the complete reaction pathways of the cyclization steps. Our findings reveal unheralded reaction mechanisms that resolve the long-standing issues with C5' radical cyclization in purine moieties of DNA sequences.

DNA Oxidation and Excision Repair Pathways

The physiological impact of the aberrant oxidation products on genomic DNA were demonstrated by embryonic lethality or the cancer susceptibility and/or neurological symptoms of animal impaired in the base excision repair (BER); the major pathway to maintain genomic integrity against non-bulky DNA oxidation. However, growing evidence suggests that other DNA repair pathways or factors that are not primarily associated with the classical BER pathway are also actively involved in the mitigation of oxidative assaults on the genomic DNA, according to the corresponding types of DNA oxidation. Among others, factors dedicated to lesion recognition in the nucleotide excision repair (NER) pathway have been shown to play eminent roles in the process of lesion recognition and stimulation of the enzyme activity of some sets of BER factors. Besides, substantial bulky DNA oxidation can be preferentially removed by a canonical NER mechanism; therefore, loss of function in the NER pathway shares common features arising from BER defects, including cancer predisposition and neurological disorders, although NER defects generally are nonlethal. Here we discuss recent achievements for delineating newly arising roles of NER lesion recognition factors to facilitate the BER process, and cooperative works of BER and NER pathways in response to the genotoxic oxidative stress.

Purine DNA Lesions at Different Oxygen Concentration in DNA Repair-Impaired Human Cells (EUE-siXPA)

Xeroderma Pigmentosum (XP) is a DNA repair disease characterized by nucleotide excision repair (NER) malfunction, leading to photosensitivity and increased incidence of skin malignancies. The role of XP-A in NER pathways has been well studied while discrepancies associated with ROS levels and the role of radical species between normal and deficient XPA cell lines have been observed. Using liquid chromatography tandem mass spectrometry we have determined the four 5’,8-cyclopurines (cPu) lesions (i.e., 5′R-cdG, 5′S-cdG, 5′R-cdA and 5′S-cdA), 8-oxo-dA and 8-oxo-dG in wt (EUE-pBD650) and XPA-deficient (EUE-siXPA) human embryonic epithelial cell lines, under different oxygen tension (hyperoxic 21%, physioxic 5% and hypoxic 1%). The levels of Fe and Cu were also measured. The main findings of our study were: (i) the total amount of cPu (1.82–2.52 lesions/10⁶ nucleotides) is the same order of magnitude as 8-oxo-Pu (3.10–4.11 lesions/10⁶ nucleotides) in both cell types, (ii) the four cPu levels are similar in hyperoxic and physioxic conditions for both wt and deficient cell lines, whereas 8-oxo-Pu increases in all cases, (iii) both wt and deficient cell lines accumulated high levels of cPu under hypoxic compared to physioxic conditions, whereas the 8-oxo-Pu levels show an opposite trend, (iv) the diastereoisomeric ratios 5′R/5′S are independent of oxygen concentration being 0.29 for cdG and 2.69 for cdA for EUE-pBD650 (wt) and 0.32 for cdG and 2.94 for cdA for EUE-siXPA (deficient), (v) in deficient cell lines Fe levels were significantly higher. The data show for the first time the connection of oxygen concentration in cells with different DNA repair ability and the levels of different DNA lesions highlighting the significance of cPu. Membrane lipidomic data at 21% O₂ indicated differences in the fatty acid contents between wild type and deficient cells, envisaging functional effects on membranes associated with the different repair capabilities, to be further investigated.

New Insights into the Reaction Paths of Hydroxyl Radicals with Purine Moieties in DNA and Double-Stranded Oligodeoxynucleotides

The reaction of hydroxyl radical (HO^•) with DNA produces many primary reactive species and many lesions as final products. In this study, we have examined the optical spectra of intermediate species derived from the reaction of HO^• with a variety of single- and double-stranded oligodeoxynucleotides and ct-DNA in the range of 1 μs to 1 ms by pulse radiolysis using an Intensified Charged Coupled Device (ICCD) camera. Moreover, we applied our published analytical protocol based on an LC-MS/MS system with isotopomeric internal standards to enable accurate and precise measurements of purine lesion formation. In particular, the simultaneous measurement of the four purine 5′,8-cyclo-2′-deoxynucleosides (cPu) and two 8-oxo-7,8-dihydro-2′-deoxypurine (8-oxo-Pu) was obtained upon reaction of genetic material with HO^• radicals generated either by γ-radiolysis or Fenton-type reactions. Our results contributed to the debate in the literature regarding absolute level of lesions, method of HO^• radical generation, 5′R/5′S diastereomeric ratio in cPu, and relative abundance between cPu and 8-oxo-Pu.

(5'R)-and (5'S)-purine 5',8-cyclo-2'-deoxyribonucleosides: reality or artifactual measurements? A reply to Chatgilialoglu's comments (this issue)

This rebuttal letter is aimed at refuting the poor and false arguments elaborated by Chatgilialoglu (preceding article) in his response to the position article (Cadet et al. Free Radic Res 2019;53:574-577) that focussed on the putative reliability of the HPLC-MS/MS measurements of five radiation-induced damage to cellular DNA, which included 8-oxo-7,8-dihydro-2'-deoxyadenosine and the (5'R) and (5'S) diastereomers of 5',8-cyclo-2'-deoxyadenosine and 5',8-cyclo-2'-deoxyadenosine (Krokidis et al. Free Radic Res 2017;51:470-482). Unfortunately, none of the main issues we raised on the suitability of the analytical approach and the shortcomings associated with DNA extraction in HPLC based measurement methods of oxidatively generated damage in cells were properly considered in Chatigilialolu's letter. The main questionable issues include the lack of information on the sensitivity of HPLC-MS/MS analysis, the absence of a dose curve that is essential in the formation of damage and the nonconsideration of artifactual oxidation.

5',8-Cyclopurine Lesions in DNA Damage: Chemical, Analytical, Biological, and Diagnostic Significance

Purine 5′,8-cyclo-2′-deoxynucleosides (cPu) are tandem-type lesions observed among the DNA purine modifications and identified in mammalian cellular DNA in vivo. These lesions can be present in two diasteroisomeric forms, 5′R and 5′S, for each 2′-deoxyadenosine and 2′-deoxyguanosine moiety. They are generated exclusively by hydroxyl radical attack to 2′-deoxyribose units generating C5′ radicals, followed by cyclization with the C8 position of the purine base. This review describes the main recent achievements in the preparation of the cPu molecular library for analytical and DNA synthesis applications for the studies of the enzymatic recognition and repair mechanisms, their impact on transcription and genetic instability, quantitative determination of the levels of lesions in various types of cells and animal model systems, and relationships between the levels of lesions and human health, disease, and aging, as well as the defining of the detection limits and quantification protocols.

High levels of oxidatively generated DNA damage 8,5'-cyclo-2'-deoxyadenosine accumulate in the brain tissues of xeroderma pigmentosum group A gene-knockout mice

Xeroderma pigmentosum (XP) is a genetic disorder associated with defects in nucleotide excision repair, a pathway that eliminates a wide variety of helix-distorting DNA lesions, including ultraviolet-induced pyrimidine dimers. In addition to skin diseases in sun-exposed areas, approximately 25% of XP patients develop progressive neurological disease, which has been hypothesized to be associated with the accumulation of an oxidatively generated type of DNA damage called purine 8,5'-cyclo-2'-deoxynucleoside (cyclopurine). However, that hypothesis has not been verified. In this study, we tested that hypothesis by using the XP group A gene-knockout (Xpa^-/-) mouse model. To quantify cyclopurine lesions in this model, we previously established an enzyme-linked immunosorbent assay (ELISA) using a monoclonal antibody (CdA-1) that specifically recognizes 8,5'-cyclo-2'-deoxyadenosine (cyclo-dA). By optimizing conditions, we increased the ELISA sensitivity to a detection limit of ˜one cyclo-dA lesion/10⁶ nucleosides. The improved ELISA revealed that cyclo-dA lesions accumulate with age in the brain tissues of Xpa^-/- and of wild-type (wt) mice, but there were significantly more cyclo-dA lesions in Xpa^-/- mice than in wt mice at 6, 24 and 29 months of age. These findings are consistent with the long-standing hypothesis that the age-dependent accumulation of endogenous cyclopurine lesions in the brain may be critical for XP neurological abnormalities.

Membrane Lipidome Reorganization and Accumulation of Tissue DNA Lesions in Tumor-Bearing Mice: An Exploratory Study

Increased rates of reactive oxygen/nitrogen species (ROS/RNS) are involved in almost all cancer types, associated with tumor development and progression, causing damage to biomolecules such as proteins, nucleic acids and membrane lipids, in different biological compartments. We used a human tumor xenograft mouse model to evaluate for the first time in parallel the remodeling of fatty acid moieties in erythrocyte membrane phospholipids and the level of ROS-induced DNA lesions in liver and kidney tissues. Using liquid chromatography tandem mass spectrometry the 5'R and 5'S diastereoisomers of 5',8-cyclo-2'-deoxyadenosine and 5',8-cyclo-2'-deoxyguanosine, together with 8-oxo-7,8-dihydro-2'-deoxyadenosine, were determined in mice at young (4- and 5-weeks) and old (17-weeks) ages and compared with control SCID mice without tumor implantation. Tumor-bearing mice showed a higher level of ROS-damaged nucleosides in genomic DNA as the age and tumor progress, compared to controls (1.07-1.53-fold in liver and 1.1-1.4-fold in kidney, respectively). The parallel fatty acid profile of erythrocyte membranes showed a profound lipid remodeling during tumor and age progression consisting of PUFA consumption and SFA enrichment (ca 28% and 58%, respectively, in late stage tumor-bearing mice), markers of enhanced oxidative and proliferative processes, respectively. Membrane lipid remodeling and ROS-induced DNA lesions may be combined to afford an integrated scenario of cancer progression and ageing, reinforcing a holistic vision among molecular markers rather than the biomarker identification in a single compartment.

Diastereomeric Recognition of 5',8-cyclo-2'-Deoxyadenosine Lesions by Human Poly(ADP-ribose) Polymerase 1 in a Biomimetic Model

5’,8-Cyclo-2’-deoxyadenosine (cdA), in the 5’R and 5’Sdiastereomeric forms, are typical non strand-break oxidative DNA lesions, induced by hydroxyl radicals, with emerging importance as a molecular marker. These lesions are exclusively repaired by the nucleotide excision repair (NER) mechanism with a low efficiency, thus readily accumulating in the genome. Poly(ADP-ribose) polymerase1 (PARP1) acts as an early responder to DNA damage and plays a key role as a nick sensor in the maintenance of the integrity of the genome by recognizing nicked DNA. So far, it was unknown whether the two diastereomeric cdA lesions could induce specific PARP1 binding. Here, we provide the first evidence of PARP1 to selectively recognize the diastereomeric lesions of 5’S-cdA and 5’R-cdA in vitro as compared to deoxyadenosine in model DNA substrates (23-mers) by using circular dichroism, fluorescence spectroscopy, immunoblotting analysis, and gel mobility shift assay. Several features of the recognition of the damaged and undamaged oligonucleotides by PARP1 were characterized. Remarkably, PARP1 exhibits different affinities in binding to a double strand (ds) oligonucleotide, which incorporates cdA lesions in R and S diastereomeric form. In particular, PARP1 proved to bind oligonucleotides, including a 5’S-cdA, with a higher affinity constant for the 5’S lesion in a model of ds DNA than 5’R-cdA, showing different recognition patterns, also compared with undamaged dA. This new finding highlights the ability of PARP1 to recognize and differentiate the distorted DNA backbone in a biomimetic system caused by different diastereomeric forms of a cdA lesion.

8-Oxo-7,8-dihydro-2'-deoxyguanosine and abasic site tandem lesions are oxidation prone yielding hydantoin products that strongly destabilize duplex DNA

In DNA, 2'-deoxyguanosine (dG) is susceptible to oxidative modification by reactive oxygen species (ROS) yielding many products, one of which is 8-oxo-7,8-dihydro-2'-deoxyguanosine (dOG). Interestingly, dOG is stable but much more labile toward oxidation than dG, furnishing 5-guanidinohydantoin-2'-deoxyribose (dGh) that is favored in the duplex context or spiroiminodihydantoin-2'-deoxyribose (dSp) that is favored in the oxidation of single-stranded contexts. Previously, exposure of DNA to ionizing radiation found ∼50% of the dOG exists as a tandem lesion with an adjacent formamide site. The present work explored oxidation of dOG in a tandem lesion with a THF abasic site analog (F) that models the formamide on either the 5' or 3' side. When dOG was in a tandem lesion, both dGh and dSp were observed as oxidation products. The 5' versus 3' side in which F resided influenced the stereochemistry of the dSp formed. Further, tandem lesions with dOG were found to be up to two orders of magnitude more reactive to oxidation than dOG in an intact duplex. When dOG is in a tandem lesion it is up to fivefold more prone to formation of spermine cross-links during oxidation compared to dOG in an intact duplex. Lastly, dOG, dGh, and each dSp diastereomer were synthesized as part of a tandem lesion in a duplex DNA to establish that dOG tandem lesions decrease the thermal stability by 12-13 °C, while dGh or either dSp diastereomer in a tandem lesion decrease the stability by >20 °C. The biological consequences of these results are discussed.