Oxidation of 8-oxo-7,8-dihydro-2'-deoxyguanosine by oxyl radicals produced by photolysis of azo compounds

An Exploration of the Transformation of the 8-Oxo-7,8-Dihydroguanine Radical Cation to Protonated 2-Amino-5-Hydroxy-7,9-Dihydropurine-6,8-Dione in a Base Pair

A theoretical investigation was performed to disclose the transformation mechanism of 8-oxo-7,8-dihydroguanine radical cation (8-oxoG⋅⁺ ) to protonated 2-amino-5-hydroxy-7,9-dihydropurine-6,8-dione (5-OH-8-oxoG) in base pair. The energy profiles for three possible pathways of the events were mapped. It is shown that direct loss of H7 from base paired 8-oxoG⋅⁺ is the only energetically favorable pathway to generate neutral radical, 8-oxoG(-H7)⋅. Further oxidation of 8-oxoG(-H7)⋅ : C to 8-oxoG(-H7)⁺  : C is exothermic. However, the 8-oxoG(-H7)⁺  : C deprotonation from all possible active sites is infeasible, indicating the inaccessible second proton loss and the lack of essential intermediate 2-amino-7,9-dihydropurine-6,8-dione (8-oxoG^OX ). This makes 8-oxoG(-H7)⁺ act as the precursor of hydration leading to the generation of protonated 5-HO-8-oxoG by stepwise fashion in base pair, which would initiate the step down guanidinohydantoin (Gh) pathway. These results clearly specify the structure-dependent transformation for 8-oxoG⋅⁺ and verify the emergence of protonated 5-HO-8-oxoG in base pair.

Singlet Oxygen Oxidation of the Radical Cations of 8-Oxo-2'-deoxyguanosine and Its 9-Methyl Analogue: Dynamics, Potential Energy Surface, and Products Mediated by C5-O2 -Addition

8-Oxo-2'-deoxyguanosine (OG) is the most common DNA lesion. Notably, OG becomes more susceptible to oxidative damage than the undamaged nucleoside, forming mutagenic products in vivo. Herein the reactions of singlet O₂ with the radical cations of 8-oxo-2'-deoxyguanosine (OG^.+ ) and 9-methyl-8-oxoguanine (9MOG^.+ ) were investigated using ion-molecule scattering mass spectrometry, from which barrierless, exothermic O₂ -addition products were detected for both reaction systems. Corroborated by static reaction potential energy surface constructed using multi-reference CASPT2 theory and molecular dynamics simulated in the presence of the reactants' kinetic and internal energies, the C5-terminal O₂ -addition was pinpointed as the most probable reaction pathway. By elucidating the reaction mechanism, kinetics and dynamics, and reaction products and energetics, this work constitutes the first report unraveling the synergetic damage of OG by ionizing radiation and singlet O₂ .

Excision of Oxidatively Generated Guanine Lesions by Competitive DNA Repair Pathways

The base and nucleotide excision repair pathways (BER and NER, respectively) are two major mechanisms that remove DNA lesions formed by the reactions of genotoxic intermediates with cellular DNA. It is generally believed that small non-bulky oxidatively generated DNA base modifications are removed by BER pathways, whereas DNA helix-distorting bulky lesions derived from the attack of chemical carcinogens or UV irradiation are repaired by the NER machinery. However, existing and growing experimental evidence indicates that oxidatively generated DNA lesions can be repaired by competitive BER and NER pathways in human cell extracts and intact human cells. Here, we focus on the interplay and competition of BER and NER pathways in excising oxidatively generated guanine lesions site-specifically positioned in plasmid DNA templates constructed by a gapped-vector technology. These experiments demonstrate a significant enhancement of the NER yields in covalently closed circular DNA plasmids (relative to the same, but linearized form of the same plasmid) harboring certain oxidatively generated guanine lesions. The interplay between the BER and NER pathways that remove oxidatively generated guanine lesions are reviewed and discussed in terms of competitive binding of the BER proteins and the DNA damage-sensing NER factor XPC-RAD23B to these lesions.

Azocompounds as generators of defined radical species: Contributions and challenges for free radical research

Peroxyl radicals participate in multiple processes involved in critical changes to cells, tissues, pharmacueticals and foods. Some of these reactions explain their association with degenerative pathologies, including cardiovascular and neurological diseases, as well as cancer development. Azocompounds, and particularly AAPH (2,2'-Azobis(2-methylpropionamidine) dihydrochloride), a cationic water-soluble derivative, have been employed extensively as sources of model peroxyl radicals. A considerable number of studies have reported mechanistic data on the oxidation of biologically-relevant targets, the scavenging activity of foods and natural products, and the reactions with, and responses of, cultured cells. However, despite the (supposed) experimental simplicity of using azocompounds, the chemistry of peroxyl radical production and subsequent reactions is complicated, and not always considered in sufficient depth when analyzing experimental data. The present work discusses the chemical aspects of azocompounds as generators of peroxyl (and other) radicals, together with their contribution to our understanding of biochemistry, pharmaceutical and food chemistry research. The evidence supporting a role for the formation of alkoxyl (RO•) and other radicals during thermal and photochemical decomposition of azocompounds is assessed, together with the potential influence of such species on the reactions under study.

Aggregation of α- and β- caseins induced by peroxyl radicals involves secondary reactions of carbonyl compounds as well as di-tyrosine and di-tryptophan formation

The present work examined the role of Tyr and Trp in oxidative modifications of caseins, the most abundant milk proteins, induced by peroxyl radicals (ROO^•). We hypothesized that the selectivity of ROO^• and the high flexibility of caseins (implying a high exposure of Tyr and Trp residues) would favor radical-radical reactions, and di-tyrosine (di-Tyr) and di-tryptophan (di-Trp) formation. Solutions of α- and β-caseins were exposed to ROO^• from thermolysis and photolysis of AAPH (2,2'-azobis(2-methylpropionamidine)dihydrochloride). Oxidative modifications were examined using electrophoresis, western blotting, fluorescence, and chromatographic methodologies with diode array, fluorescence and mass detection. Exposure of caseins to AAPH at 37 °C gave fragmentation, cross-linking and protein aggregation. Amino acid analysis showed consumption of Trp, Tyr, Met, His and Lys residues. Quantification of Trp and Tyr products, showed low levels of di-Tyr and di-Trp, together with an accumulation of carbonyls indicating that casein aggregation is, at least partly, associated with secondary reactions between carbonyls and Lys and His residues. AAPH photolysis, which generates a high flux of free radicals increased the extent of formation of di-Tyr in both model peptides and α- and β- caseins; di-Trp was only detected in peptides and α-casein. Thus, in spite of the high flexibility of caseins, which would be expected to favor radical-radical reactions, the low flux of ROO^• generated during AAPH thermolysis disfavours the formation of dimeric radical-radical cross-links such as di-Tyr and di-Trp, instead favoring other O₂-dependent crosslinking pathways such as those involving secondary reactions of initial carbonyl products.

Generation of 8-oxo-7,8-dihydroguanine in G-Quadruplexes Models of Human Telomere Sequences by One-electron Oxidation

The mechanistic aspects of one-electron oxidation of G-quadruplexes in the basket (Na⁺ ions) and hybrid (K⁺ ions) conformations were investigated by transient absorption laser kinetic spectroscopy and HPLC detection of the 8-oxo-7,8-dihydroguanine (8-oxoG) oxidation product. The photo-induced one-electron abstraction from G-quadruplexes was initiated by sulfate radical anions (SO₄ ˙^- ) derived from the photolysis of persulfate ions by 308 nm excimer laser pulses. In neutral aqueous solutions (pH 7.0), the transient absorbance of neutral guanine radicals, G(-H)˙, is observed following the complete decay of SO₄ ˙^- radicals (~10 μs after the actinic laser flash). In both basket and hybrid conformations, the G(-H)˙ decay is biphasic with one component decaying with a lifetime of ~0.1 ms, and the other with a lifetime of 20-30 ms. The fast decay component (~0.1 ms) in G-quadruplexes is correlated with the formation of 8-oxoG lesions. We propose that in G-quadruplexes, G(-H)˙ radicals retain radical cation character by sharing the N1-proton with the O⁶ -atom of G in the [G˙⁺ : G] Hoogsteen base pair; this [G(-H)˙: H⁺ G <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>⇄</mml:mo></mml:math> G˙⁺ : G] leads to the hydration of G˙⁺ radical cation within the millisecond time domain, and is followed by the formation of the 8-oxoG lesions.

Recent Advances in the Synthesis of Hydantoins: The State of the Art of a Valuable Scaffold

The review highlights the hydantoin syntheses presented from the point of view of the preparation methods. Novel synthetic routes to various hydantoin structures, the advances brought to the classical methods in the aim of producing more sustainable and environmentally friendly procedures for the preparation of these biomolecules, and a critical comparison of the different synthetic approaches developed in the last twelve years are also described. The review is composed of 95 schemes, 8 figures and 528 references for the last 12 years and includes the description of the hydantoin-based marketed drugs and clinical candidates.

Removal of oxidatively generated DNA damage by overlapping repair pathways

It is generally believed that the mammalian nucleotide excision repair pathway removes DNA helix-distorting bulky DNA lesions, while small non-bulky lesions are repaired by base excision repair (BER). However, recent work demonstrates that the oxidativly generated guanine oxidation products, spiroimininodihydantoin (Sp), 5-guanidinohydantoin (Gh), and certain intrastrand cross-linked lesions, are good substrates of NER and BER pathways that compete with one another in human cell extracts. The oxidation of guanine by peroxynitrite is known to generate 5-guanidino-4-nitroimidazole (NIm) which is structurally similar to Gh, except that the 4-nitro group in NIm is replaced by a keto group in Gh. However, unlike Gh, NIm is an excellent substrate of BER, but not of NER. These and other related results are reviewed and discussed in this article.

Oxidatively generated DNA damage after Cu(II) catalysis of dopamine and related catecholamine neurotransmitters and neurotoxins: Role of reactive oxygen species

There is increasing evidence supporting a causal role for oxidatively damaged DNA in neurodegeneration during the natural aging process and in neurodegenerative diseases such as Parkinson and Alzheimer. The presence of redox-active catecholamine neurotransmitters coupled with the localization of catalytic copper to DNA suggests a plausible role for these agents in the induction of oxidatively generated DNA damage. In this study we have investigated the role of Cu(II)-catalyzed oxidation of several catecholamine neurotransmitters and related neurotoxins in inducing oxidatively generated DNA damage. Autoxidation of all catechol neurotransmitters and related congeners tested resulted in the formation of nearly a dozen oxidation DNA products resulting in a decomposition pattern that was essentially identical for all agents tested. The presence of Cu(II), and to a lesser extent Fe(III), had no effect on the decomposition pattern but substantially enhanced the DNA product levels by up to 75-fold, with dopamine producing the highest levels of unidentified oxidation DNA products (383±46 adducts/10(6) nucleotides), nearly 3-fold greater than 8-oxo-7,8-dihydro-2'-deoxyguanosine (122±19 adducts/10(6) nucleotides) under the same conditions. The addition of sodium azide, 2,2,6,6-tetramethyl-4-piperidone, tiron, catalase, bathocuproine, or methional to the dopamine/Cu(II) reaction mixture resulted in a substantial decrease (>90%) in oxidation DNA product levels, indicating a role for singlet oxygen, superoxide, H(2)O(2), Cu(I), and Cu(I)OOH in their formation. Whereas the addition of N-tert-butyl-α-phenylnitrone significantly decreased (67%) dopamine-mediated oxidatively damaged DNA, three other hydroxyl radical scavengers, ascorbic acid, sodium benzoate, and mannitol, had little to no effect on these oxidation DNA product levels, suggesting that free hydroxyl radicals may have limited involvement in this dopamine/Cu(II)-mediated oxidatively generated DNA damage. These studies suggest a possible contributory role of oxidatively generated DNA damage by dopamine and related catechol neurotransmitters/neurotoxins in neurodegeneration and cell death. We also found that a naturally occurring broad-spectrum antioxidant, ellagic acid, was substantially effective (nearly 50% inhibition) at low doses (1μM) at preventing this dopamine/Cu(II)-mediated oxidatively generated DNA damage. Because dietary ellagic acid has been found to reduce oxidative stress in rat brains, a neuroprotective role of this polyphenol is plausible.