Oxidation Of Thymine Research Articles

Interaction of Doxorubicin Embedded into Phospholipid Nanoparticles and Targeted Peptide-Modified Phospholipid Nanoparticles with DNA.

The interactions of dsDNA with new targeted drug delivery derivatives of doxorubicin (DOX), such as DOX embedded into phospholipid nanoparticles (NPhs) and DOX with the NGR targeted peptide-modified NPhs were studied electrochemically by differential pulse voltammetry technique. Screen-printed electrodes (SPEs), modified with stable fine dispersions of carbon nanotubes (CNTs), were used for quantitative electrochemical investigations of direct electrochemical oxidation of guanine, adenine, and thymine heterocyclic bases of dsDNA, and their changes in the presence of DOX nanoderivatives. Analysing the shifts of peak potentials of nucleobases in the presence of drug, we have shown that the doxorubicin with NGR targeted peptide changed the mode of interaction in DNA-drug complexes from intercalative to electrostatic. Binding constants (Kb) of DNA-drug complexes were calculated in accordance with adenine, guanine, and thymine oxidation signals. Based on our experiments, we have proven that the surface modification of a drug delivery system with NGR targeted peptide dramatically changed the mechanism of interaction of drug with genetic material. DNA-mediated drug toxicity was calculated based on the concentration-dependent "response" of heterocyclic nucleobases on drug influence. DOX, DOX-loaded phospholipid nanoparticles (NPhs), and DOX with NGR addressed peptide-modified NPhs were moderately toxic in the concentration range of 0.5-290 µM.

Platinum nanoparticles loaded carbon black: reduced graphene oxide hybrid platforms for label-free electrochemical DNA and oxidative DNA damage sensing

• Rapid and effective synthesis of platinum nanoparticles on the carbon black:reduced graphene oxide hybrids were achieved. • Facile and efficient electrode modification was performed with the hybrids. • Characterization of the surfaces proved homogeneous and robust distribution of the structures. • Sensitive fsDNA detection was accomplished with a low detection limit. • DNA damage and protection monitoring were carried out successfully. Carbon black-based hybrid materials have attracted great attention in the design of electrochemical sensors due to their consistent electroanalytical properties. With this in mind, for the first time, we present the preparation of platinum nanoparticles loaded carbon black:reduced graphene oxide (PtNPs/CB:rGO) hybrid platforms for label-free electrochemical DNA and DNA damage sensing. Synthesis of different hybrids was performed via microwave-assisted reduction method for the impregnation of platinum nanoparticles efficiently. These materials were characterized with X-ray diffraction analysis (XRD), Raman spectroscopy, transmission emission spectroscopy (TEM) and subsequently they were used for pencil graphite electrode modification in order to improve the characteristics of the surface. Modified electrodes were firstly applied for fish sperm DNA (fsDNA) analysis. The effect of different ratios (wt.:wt. %) of CB and rGO was investigated and it was shown that the hybrid material with 50:50 ratio had better sensitivity with a low detection limit of 0.14 mg L −1 and a good linearity to fsDNA between 1 and 200 mg L −1 (n = 3) based on guanine (G) oxidation by using differential pulse voltammetry (DPV). In order to expand this comparison, PtNPs/CB and PtNPs/rGO modified electrodes were also incorporated. Furthermore, the electroanalytical response of the hybrid material modified electrode was investigated based on the oxidation of thymine (T). Later on, oxidative DNA damage detection in the presence of Cu(II)/H 2 O 2 reagents and protection studies in the presence of ascorbic acid were performed successfully by using electrochemical impedance spectroscopy (EIS). The study pointed out the remarkable reproducibility and sensitivity of the CB-based modification for nanoparticle decoration.

Reduction of thymine radicals by Tryptophan: A study of CIDNP kinetics

As model for a reaction mimicking a chemical DNA repair process, the reduction of short-lived thymine radicals by tryptophan and N-acetyl tryptophan in a wide pH range in aqueous solution was investigated. To generate the radicals pulsed laser irradiation was implemented to a solution containing the photosensitizer 3,3′,4,4′-tetracarboxy benzophenone (TCBP), thymine, and tryptophan or N-acetyl tryptophan. As a preliminary step, the photoinduced oxidation of thymine by triplet-excited 3,3′,4,4′-tetracarboxy benzophenone was studied and thymine radical intermediates were characterized by means of time-resolved chemically induced dynamic nuclear polarization (CIDNP). It was shown that proton-coupled electron transfer (PCET) from neutral thymine to the triplet excited photosensitizer in neutral aqueous solution (pH 6.3) leads to the formation of a neutral thymine radical; its formation is preceded by the formation of an elusive cationic thymine radical on the geminate stage of the reaction. In contrast, in basic solution (pH 11.7) a thymine anion reacts with the triplet excited photosensitizer by electron transfer resulting in the formation of a TCBP radical anion and a neutral thymine radical. The latter deprotonates rapidly, and a thymine radical anion is formed at these conditions. To determine the rate of the reduction reaction between tryptophan and its N-acetyl derivative and the neutral thymine radical (pH 6.3) or the thymine radical anion (pH 11.7) the CIDNP kinetics obtained in the photoreaction of TCBP and thymine with and without adding a reducing agent were simultaneously analyzed quantitatively. As a result, the second order rate constants of the reduction reaction, kr, were determined: 1.1 × 109 M−1s−1 at pH 6.3 and 1.3 × 107 M−1s−1 at pH 11.7 for N-acetyl tryptophan; 1.1 × 109 M−1s−1 at pH 6.3 and 4.1 × 107 M−1s−1 at pH 11.7 for tryptophan.

Analysis of 5-Hydroxymethyluracil Levels Using Flow Cytometry.

5-hydroxymethyluracil was originally identified as an oxidatively modified DNA base derivative. Recent evidence suggests that its formation may result from the oxidation of thymine in a reaction that is catalyzed by TET proteins. Alternatively, it could be generated through the deamination of 5-hydroxymethylcytosine by activation-induced cytidine deaminase. The standard method for evaluating 5-hydroxymethyluracil content is the highly sensitive and highly specific isotope-dilution automated online two-dimensional ultraperformance liquid chromatography with tandem mass spectrometry (2D-UPLC-MS/MS). Despite many advantages, this method has one great limitation. It is not able to measure compounds at a single-cell level. Our goal was to develop and optimize a method based on flow cytometry that allows the evaluation of 5-hydroxymethyluracil levels at a single cell level in peripheral leukocytes.

Unexpected Thymine Oxidation and Collision-Induced Thymine-Pt-guanine Cross-Linking on 5'-TpG and 5'-GpT by a Photoactivatable Diazido Pt(IV) Anticancer Complex.

The photochemical products of dinucleotides 5'-TpG/5'-GpT with a photoactivatable anticancer Pt(IV) complex (trans,trans,trans-[Pt(N3)2(OH)2(py)2], py = pyridine; 1) were characterized by electrospray ionization mass spectrometry. The primary MS showed the main products were monoplatinated and diplatinated adducts for both the dinucleotides accompanied by the formation of minor triplatinated dinucleotides, indicating that T-N3 and G-N1 may be platination sites additional to the well-known G-N7 site. Surprisingly, a series of minor platinated adducts with oxidation of guanine and/or thymine were observed. Although guanine is more sensitive to oxidation than thymine, thymine can compete with guanine for complex 1-induced oxidation, of which the oxidation adducts were identified as cis- and trans-diastereomers of 5,6-dihydroxy-5,6-dihydrothymidine (cis,trans-ThdGly), 5-formyl-2'-deoxyuridine (5-FormdUrd), and 5-(hydroxymethyl)-2'-deoxyuridine (5-HMdUrd), respectively. While for guanine, apart from 8-hydroxyguanine (8-OH-G) and N-formylamidoiminohydantoin (RedSp), other guanine oxidized adducts such as spiroiminodihydantoin (Sp), dehydroguanidinohydantoin (DGh), and 2,6-diamino-4-hydroxy-5-formamidopyrimidine (FapyG) were also identified. MS/MS analysis showed that unique fragments with a Pt moiety [Pt(N3)(py)] cross-linking the G and T bases were formed during the fragmentation of monoplatinated dinucleotides. Such binding mode to and oxidative damages on DNA bases imposed by the diazido Pt(IV) complex are apparently distinct from those of cisplatin, perhaps accounting for its unique mechanism of action.

A Spontaneous Ring-Opening Reaction Leads to a Repair-Resistant Thymine Oxidation Product in Genomic DNA.

The alphabet of modified DNA bases goes beyond the conventional four letters, with biological roles being found for many such modifications. Herein, we describe the observation of a modified thymine base that arises from spontaneous N1 -C2 ring opening of the oxidation product 5-formyl uracil, after N3 deprotonation. We first observed this phenomenon in silico through ab initio calculations, followed by in vitro experiments to verify its formation at a mononucleoside level and in a synthetic DNA oligonucleotide context. We show that the new base modification (Trex , thymine ring expunged) can form under physiological conditions, and is resistant to the action of common repair machineries. Furthermore, we found cases of the natural existence of Trex while screening a number of human cell types and mESC (E14), thus suggesting potential biological relevance of this modification.

ESI-CID spectral characterization and differentiation of the cross links of thymine formed by one electron oxidation with SO4●-

The analysis of urinary nucleosides is becoming a very good tool in the diagnosis of diseases like AIDS and cancer and consequently the identification of modified nucleosides using mass spectrometry is an area of utmost importance. In this context, there is a high relevance in the understanding of the mechanism of collisionally induced dissociation (CID) of these heterocyclics. The present work characterizes and differentiates the cross-linked products formed by one electron oxidation of thymine (T). Three dimers A, B and C, were analyzed, out of which the dimer A is a C5–C5’ cross link of two T molecules containing a fused tetrahydrofuran ring, while B and C are N–C cross linked products. B and C showed very similar fragmentation pattern but that of A was different. The differentiation between these three were made by monitoring two characteristic peaks, the water loss peak ([M+H]+) and the protonated T fragment ([T + H]+). The probable mechanism of formation of these fragments and their CID mechanism in both positive and negative ionization modes are also explained. The neutral losses of NH3, H2O and NHCO were the prominent mechanism in the positive mode, while in the negative mode, only NHCO and CO losses were observed.

Sequencing 5-Hydroxymethyluracil at Single-Base Resolution.

5‐hydroxymethyluracil (5hmU) is formed through oxidation of thymine both enzymatically and non‐enzymatically in various biological systems. Although 5hmU has been reported to affect biological processes such as protein–DNA interactions, the consequences of 5hmU formation in genomes have not been yet fully explored. Herein, we report a method to sequence 5hmU at single‐base resolution. We employ chemical oxidation to transform 5hmU to 5‐formyluracil (5fU), followed by the polymerase extension to induce T‐to‐C base changes owing to the inherent ability of 5fU to form 5fU:G base pairing. In combination with the Illumina next generation sequencing technology, we developed polymerase chain reaction (PCR) conditions to amplify the T‐to‐C base changes and demonstrate the method in three different synthetic oligonucleotide models as well as part of the genome of a 5hmU‐rich eukaryotic pathogen. Our method has the potential capability to map 5hmU in genomic DNA and thus will contribute to promote the understanding of this modified base.

A biopolymer-based voltammetric sensor for thymine: Elucidation of electrochemical kinetics

In the work accounted here, the electro-oxidation of one of the pyrimidine bases thymine is studied on a glassy carbon electrode modified with the biopolymer, poly (l-glutamic acid), as the probe. On the probe, the overpotential for thymine oxidation in alkaline medium has been significantly decreased attesting the electrocatalytic nature of the biopolymer film. The experimental parameters to obtain the lowest oxidation potential have been optimised and the probe calibrated for the determination of thymine. In the square wave mode, using 0.1 M NaOH as the supporting electrolyte, the oxidation peak current was found to be linear in the range from 30 to 1000 μM, with a detection limit of 9.2 μM. The diffusion coefficient of thymine in 0.1 M sodium hydroxide has been determined using chronoamperometric studies. To study the mechanistic aspects of the electro-oxidation process, variation of the oxidation peak parameters with scan rate has been studied in the linear sweep mode. Electrochemical kinetic parameters, namely charge transfer coefficient α and the standard heterogeneous rate constant ks for the electro-oxidation, have also been determined. The determination of thymine in spiked synthetic blood serum and urine has been conducted to demonstrate the application of the sensor for thymine determination in real samples.

Cyclic mismatch binding ligand CMBL4 binds to the 5'-T-3'/5'-GG-3' site by inducing the flipping out of thymine base.

A newly designed cyclic bis-naphthyridine carbamate dimer CMBL4 with a limited conformational flexibility was synthesized and characterized. Absorption spectra revealed that two naphthyridines in CMBL4 were stacked on each other in aqueous solutions. The most efficient binding of CMBL4 to DNA was observed for the sequence 5′-T-3′/5′-GG-3′ (T/GG) with the formation of a 1:1 complex, which is one of possible structural elements involved in the higher order structures of (TGG)n repeat DNA triggering the genome microdeletion. Surface plasmon resonance assay also showed the binding of CMBL4 with TGG repeat DNA. Potassium permanganate oxidation studies of CMBL4-bound duplex containing the T/GG site showed that the CMBL4-binding accelerated the oxidation of thymine at that site, which suggests the flipping out of the thymine base from a π-stack. Preferential binding was observed for CMBL4 compared with its acyclic variants, which suggests the marked significance of the macrocyclic structure for the recognition of the T/GG site.

Identification of the Glucosyltransferase That Converts Hydroxymethyluracil to Base J in the Trypanosomatid Genome

O-linked glucosylation of thymine in DNA (base J) is an important regulatory epigenetic mark in trypanosomatids. β-d-glucopyranosyloxymethyluracil (base J) synthesis is initiated by the JBP1/2 enzymes that hydroxylate thymine, forming 5-hydroxymethyluracil (hmU). hmU is then glucosylated by a previously unknown glucosyltransferase. A recent computational screen identified a possible candidate for the base J-associated glucosyltransferase (JGT) in trypanosomatid genomes. We demonstrate that recombinant JGT utilizes uridine diphosphoglucose to transfer glucose to hmU in the context of dsDNA. Mutation of conserved residues typically involved in glucosyltransferase catalysis impairs DNA glucosylation in vitro. The deletion of both alleles of JGT from the genome of Trypanosoma brucei generates a cell line that completely lacks base J. Reintroduction of JGT in the JGT KO restores J synthesis. Ablation of JGT mRNA levels by RNAi leads to the sequential reduction in base J and increased levels of hmU that dissipate rapidly. The analysis of JGT function confirms the two-step J synthesis model and demonstrates that JGT is the only glucosyltransferase enzyme required for the second step of the pathway. Similar to the activity of the related Ten-Eleven Translocation (TET) family of dioxygenases on 5mC, our studies also suggest the ability of the base J-binding protein enzymes to catalyze iterative oxidation of thymine in trypanosome DNA. Here we discuss the regulation of hmU and base J formation in the trypanosome genome by JGT and base J-binding protein.

The Electrochemical Approach towards Proton Coupled Electron Transfer Pathways for Oxidation of Thymine in Water

The work describes the use of cyclic voltammetry to investigate the proton coupled electron transfer (PCET) pathways for oxidation of thymine in water. Biologically important compounds like DNA bases, and amino acids showed irreversible behavior when oxidized electrochemically. The reversible redox potentials for such systems (say, irreversible electrochcemical systems) were estimated by analyzing the cyclic voltammograms recorded at different scan rates. The approach presents here is relatively simple and easy for obtaining reversible redox potential of electrochemically irreversible systems. Moreover, for the case of thymine, the obtained reversible oxidation potential also used to establish the PCET-pathways for oxidation of thymine in water.

Are 8-oxoguanine (8-oxoGua) and 5-hydroxymethyluracil (5-hmUra) oxidatively damaged DNA bases or transcription (epigenetic) marks?

The oxidatively modified DNA base 8-oxo-7,8-dihydroguanine (8-oxoGua) is nontoxic and weakly mutagenic. Here we report on new data suggesting a potential for 8-oxoGua to affect the expression of several genes via epigenetic changes resulting in chromatin relaxation. Using pig thymus extract, we analyzed the distribution of 8-oxoGua among different nuclei fractions representative of transcriptionally active and silenced regions. The levels of 8-oxo-7,8-dihydro-2′-deoxyguanosine (8-oxodG) found in transcriptionally active euchromatin (4.37/106 nucleotides) and in the matrix fraction (4.16/106 nucleotides) were about 5 times higher than in transcriptionally silenced heterochromatin (0.91/106 nucleotides). Other experimental data are presented which suggest that 8-oxoGua present in specific DNA sequences may be widely used for transcription regulation.Like 8-oxoGua, 5-hydroxymethyluracil (5-hmUra) is another oxidatively modified DNA base (the derivative is formed by thymine oxidation). Recent experimental evidence supports the notion that 5-hmUra plays an important role in active DNA demethylation. This involves overexpression of activation-induced cytidine deaminase (AID) and ten-eleven translocation 1 (TET1) protein (the key proteins involved in active demethylation), which leads to global accumulation of 5-hmUra. Our preliminary data demonstrate a significant increase of the 5-hmUra levels in pig brain extract when compared with liver extract. The lack of 5-hmUra in Escherichia coli DNA also speaks for a role of this modification in the active demethylation process. It is concluded that 8-oxodG and 5-hmUra in DNA may be considered as epigenetic marks.

Kinetics of the oxidation of thymine and thymidine by triplet 2,2'-dipyridyl in aqueous solutions at different pH values.

The photo-oxidation of the nucleobase, thymine (Thy), and nucleoside, thymidine (dThy), by dipyridyl (DP) has been investigated in aqueous solution using time-resolved laser flash photolysis. The pH dependence of the oxidation rate constants is measured within a large pH scale. As a consequence, the chemical reactivity of the reactants existing in solution at a certain range of pH is predicted. Bimolecular rate constants of the quenching reactions between triplet dipyridyl and thymine and thymidine are, respectively, kq = 2.4 × 107 M–1 s–1 (pH < 5.8) and kq = 1.0 × 107 M–1 s–1 (5.8 < pH < 9.8). Cyclic voltammetry was used to measure the potentials of thymine oxidation and dipyridyl reduction in water at pH < 7. Both results give hints for a proton coupled electron-transfer (PCET) reaction from thymine to triplet dipyridyl.

The Electrochemical Approach towards Proton Coupled Electron Transfer Reaction Pathways for Oxidation of Thymine in Water

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Electrochemical methods for determination of thymine in medical pipefish samples based on HPMαFP/Ppy/GCE-modified electrode

A sensitive electrochemical method was developed for the voltammetric determination of thymine at a composite film-modified electrode 1-phenyl-3-methyl-4-(2-furoyl)-5-pyrazolone (HPMαFP)/polypyrrole (Ppy)/glassy carbon electrode (GCE). The electrochemical parameters of thymine were investigated by cyclic voltammetry and differential pulse voltammetry. In pH = 7.4, one sensitive oxidation peak of thymine with E pa = 0.968 V was observed on the HPMαFP/PPy-modified electrode. The difference of peak potential (∆E pa) was 188 mV lower than that for bare GCE. Compared to the bare GCE and Ppy/GCE, the HPMαFP/Ppy/GCE-modified electrode showed an excellent electrocatalytic effect on the oxidation of thymine and displayed a shift of the oxidation potential in the negative direction with significant increase in the peak current. Under the optimum condition, the concentration calibration range and detection limit are 2 × 10−6–1 × 10−4 and 4.85 × 10−7 M for thymine. This developed method had been applied to the direct determination of thymine in medical pipefish samples with satisfactory results.

Oxidative Thymine Mutation in DNA: Water-Wire-Mediated Proton-Coupled Electron Transfer

One-electron oxidation of A/T-rich DNA leads to mutations at thymine. Experimental investigation of DNA containing methyl-deuterated thymine reveals a large isotope effect establishing that cleavage of this carbon-hydrogen bond is involved in the rate-determining step of the reaction. First-principles quantum calculations reveal that the radical cation (electron hole) generated by DNA oxidation, initially located on adenines, localizes on thymine as the proton is lost from the methyl group, demonstrating the role of proton-coupled electron transfer (PCET) in thymine oxidation. Proton transport by structural diffusion along a segmented "water-wire" culminates in proton solvation in the hydration environment, serving as an entropic reservoir that inhibits reversal of the PCET process. These findings provide insight into mutations in A/T-rich DNA such as replication fork stalling that is implicated in early stage carcinogenesis.

The Human Base Excision Repair Enzyme SMUG1 Directly Interacts with DKC1 and Contributes to RNA Quality Control

Single-strand-selective monofunctional uracil-DNA glycosylase 1 (SMUG1) is a base excision repair enzyme that removes uracil and oxidised pyrimidines from DNA. We show that SMUG1 interacts with the pseudouridine synthase Dyskerin (DKC1) and colocalizes with DKC1 in nucleoli and Cajal bodies. As DKC1 functions in RNA processing, we tested whether SMUG1 excised modified bases in RNA and demonstrated that SMUG1 has activity on single-stranded RNA containing 5-hydroxymethyldeoxyuridine, but not pseudouridine, the nucleoside resulting from isomerization of uridine by DKC1. Moreover, SMUG1 associates with the 47S rRNA precursor processed by DKC1, and depletion of SMUG1 leads to a reduction in the levels of mature rRNA accompanied by an increase in polyadenylated rRNA. Depletion of SMUG1, and, in particular, the combined loss of SMUG1 and DKC1, leads to accumulation of 5-hydroxymethyluridine in rRNA. In conclusion, SMUG1 is a DKC1 interaction partner that contributes to rRNA quality control, partly by regulating 5-hydroxymethyluridine levels.

Simultaneous detection of guanine and adenine in DNA and meat samples using graphitized mesoporous carbon modified electrode

A graphitized mesoporous carbon modified glassy carbon electrode (GCE/GMC) prepared by drop coating method without any pre-anodization of the underlying GCE or external binder/matrix, has been demonstrated for simultaneous electrochemical oxidation of guanine (G) and adenine (A) at oxidation potentials 0.60 and 0.85 V vs. Ag/AgCl, respectively, in the presence of thymine (T) by differential pulse voltammetric method in pH 7 phosphate buffer solution. Control voltammetric experiments with unmodified GCE, graphite nanopowder and multiwalled carbon nanotube modified electrodes yielded either feeble or with high-background current responses. Interestingly, the GCE/GMC showed highly efficient, stable and well-defined voltammetric signals. Thymine oxidation signal noticed discretely at 1.15 V vs. Ag/AgCl on the GCE/GMC was not influenced for the simultaneous determination of G and A. Constructed DPV calibration graphs were linear in the range of 25–200 and 25–150 μM, respectively, for the G and A. Corresponding detection limit (S/N = 3) values are 0.76 and 0.63 μM. Real sample analyses for the detection of G and A concentrations in calf-thymus DNA (detected [G]/[A] ratio = 0.82), beef brain and beef liver were successfully demonstrated with recovery values ~100 %.

Profiling Cytosine Oxidation in DNA by LC-MS/MS

Spontaneous and oxidant-induced damage to cytosine is probably the main cause of CG to TA transition mutations in mammalian genomes. The reaction of hydroxyl radical (·OH) and one-electron oxidants with cytosine derivatives produces numerous oxidation products, which have been identified in large part by model studies with monomers and short oligonucleotides. Here, we developed an analytical method based on LC-MS/MS to detect 10 oxidized bases in DNA, including 5 oxidation products of cytosine. The utility of this method is demonstrated by the measurement of base damage in isolated calf thymus DNA exposed to ionizing radiation in aerated aqueous solutions (0-200 Gy) and to well-known Fenton-like reactions (Fe(2+) or Cu(+) with H(2)O(2) and ascorbate). The following cytosine modifications were quantified as modified 2'-deoxyribonucleosides upon exposure of DNA to ionizing radiation in aqueous aerated solution: 5-hydroxyhydantoin (Hyd-Ura) > 5-hydroxyuracil (5-OHUra) > 5-hydroxycytosine (5-OHCyt) > 5,6-dihydroxy-5,6-dihydrouracil (Ura-Gly) > 1-carbamoyl-4,5-dihydroxy-2-oxoimidazolidine (Imid-Cyt). The total yield of cytosine oxidation products was comparable to that of thymine oxidation products (5,6-dihydroxy-5,6-dihydrothymine (Thy-Gly), 5-hydroxy-5-methylhydantotin (Hyd-Thy), 5-(hydroxymethyl)uracil (5-HmUra), and 5-formyluracil (5-ForUra)) as well as the yield of 8-oxo-7,8-dihydroguanine (8-oxoGua). The major oxidation product of cytosine in DNA was Hyd-Ura. In contrast, the formation of Imid-Cyt was a minor pathway of DNA damage, although it is the major product arising from irradiation of the monomers, cytosine, and 2'-deoxycytidine. The reaction of Fenton-like reagents with DNA gave a different distribution of cytosine derived products compared to ionizing radiation, which likely reflects the reaction of metal ions with intermediate peroxyl radicals or hydroperoxides. The analysis of the main cytosine oxidation products will help elucidate the complex mechanism of oxidative degradation of cytosine in DNA and probe the consequences of these reactions in biology and medicine.