The FeII(citrate) Fenton reaction under physiological conditions

Near UV Photodegradation Mechanisms of Amino Acid Excipients: Formation of the Carbon Dioxide Radical Anion from Aspartate and Fe(III)

Carbon dioxide radical anion (•CO2-) is a powerful reducing agent that can reduce protein disulfide bonds and convert molecular oxygen to superoxide. Therefore, the generation of •CO2- can be detrimental to pharmaceutical formulations. Iron is among the most prevalent impurities in formulations, where Fe(III) chelates of histidine (His) can produce •CO2- upon exposure to near-UV light (Zhang and Schöneich, Eur. J. Pharm. Biopharm. 2023, 190, 231-241). Here, we monitor by spin-trapping in combination with electron paramagnetic resonance spectroscopy and/or high-performance liquid chromatography-mass spectrometry analysis the photochemical formation of •CO2- for a series of common amino acid excipients, including arginine (Arg), methionine (Met), proline (Pro), glutamic acid (Glu), glycine (Gly), aspartic acid (Asp), and lysine (Lys). Our results indicate that in the presence of Fe(III), Asp, and Glu produce significant yields of •CO2- under photoirradiation with near-UV light. Notably, Asp demonstrates the highest efficiency of •CO2- generation compared with that of the other amino acid excipients. Stable isotope labeling indicates that •CO2- exclusively originates from the α-carboxyl group of Asp. Mechanistic studies reveal two possible pathways for •CO2- formation, which involve either a β-carboxyl radical or an amino radical cation intermediate.

Augmenting MEK inhibitor efficacy in BRAF wild-type melanoma: synergistic effects of disulfiram combination therapy

BACKGROUND

MEK inhibitors (MEKi) were shown to be clinically insufficiently effective in patients suffering from BRAF wild-type (BRAF WT) melanoma, even if the MAPK pathway was constitutively activated due to mutations in NRAS or NF-1. Thus, novel combinations are needed to increase the efficacy and duration of response to MEKi in BRAF WT melanoma. Disulfiram and its metabolite diethyldithiocarbamate are known to have antitumor effects related to cellular stress, and induction of endoplasmic reticulum (ER) stress was found to synergize with MEK inhibitors in NRAS-mutated melanoma cells. Therefore, we investigated the combination of both therapeutics to test their effects on BRAF-WT melanoma cells and compared them with monotherapy using the MEKi trametinib.

METHODS

The effects of combined therapy with disulfiram or its metabolite diethyldithiocarbamate and the MEKi trametinib were evaluated in a series of BRAF-WT melanoma cell lines by measuring cell viability and apoptosis induction. Cytotoxicity was additionally assessed in 3D spheroids, ex vivo melanoma slice cultures, and in vivo xenograft mouse models. The response of melanoma cells to treatment was studied at the RNA and protein levels to decipher the mode of action. Intracellular and intratumoral copper measurements were performed to investigate the role of copper ions in the antitumor cytotoxicity of disulfiram and its combination with the MEKi.

RESULTS

Diethyldithiocarbamate enhanced trametinib-induced cytotoxicity and apoptosis induction in 2D and 3D melanoma culture models. Mechanistically, copper-dependent induction of oxidative stress and ER stress led to Janus kinase (JNK)-mediated apoptosis in melanoma cells. This mechanism was also detectable in patient-derived xenograft melanoma models and resulted in a significantly improved therapeutic effect compared to monotherapy with the MEKi trametinib.

CONCLUSIONS

Disulfiram and its metabolite represent an attractive pharmaceutical approach to induce ER stress in melanoma cells that potentiates the antitumor effect of MEK inhibition and may be an interesting candidate for combination therapy of BRAF WT melanoma.

Arbutin alleviates fatty liver by inhibiting ferroptosis via FTO/SLC7A11 pathway

Non-alcoholic fatty liver disease (NAFLD) is a potentially serious disease that affects 30 % of the global population and poses a significant risk to human health. However, to date, no safe, effective and appropriate treatment modalities are available. In recent years, ferroptosis has emerged as a significant mode of cell death and has been found to play a key regulatory role in the development of NAFLD. In this study, we found that arbutin (ARB), a natural antioxidant derived from Arctostaphylos uva-ursi (L.), inhibits the onset of ferroptosis and ameliorates high-fat diet-induced NAFLD in vivo and in vitro. Using reverse docking, we identified the demethylase fat mass and obesity-related protein (FTO) as a potential target of ARB. Subsequent mechanistic studies revealed that ARB plays a role in controlling methylation of the SLC7A11 gene through inhibition of FTO. In addition, we demonstrated that SLC7A11 could alleviate the development of NAFLD in vivo and in vitro. Our findings identify the FTO/SLC7A11 axis as a potential therapeutic target for the treatment of NAFLD. Specifically, we show that ARB alleviates NAFLD by acting on the FTO/SLC7A11 pathway to inhibit ferroptosis.

On The Nature of FeIV =Oaq in Aqueous Media: A DFT analysis

FeIV =Oaq is a key intermediate in many advanced oxidation processes and probably in biological systems. It is usually referred to as FeIV =O2+ . The pKa's of FeIV =Oaq as derived by DFT are: pKa1=2.37 M06 L/6-311++G(d,p) (SDD for Fe) and pKa2=7.79 M06 L/6-311++G(d,p) (SDD for Fe). This means that in neutral solutions, FeIV =Oaq is a mixture of (H2 O)4 (OH)FeIV =O+ and (H2 O)2 (OH)2 FeIV =O. The oxidation potential of FeIV =Oaq in an acidic solution, E0 {(H2 O)5 FeIV =O2+ /FeIII (H2 O)6 3+ , pH 0.0} is calculated with and without a second solvation sphere and the recommended value is between 2.86 V (B3LYP/Def2-TZVP, with a second solvation sphere) and 2.23 V (M06 L/Def2-TZVP without a second solvation sphere). This means that FeIV =Oaq is the strongest oxidizing agent formed in systems involving FeVI O4 2- even in neutral media.

Hydroxyl radical generations form the physiologically relevant Fenton-like reactions

Iron(II) species can participate in the Fenton and Fenton-like reactions to generate the hydroxyl radical that can oxidatively damage biomolecules and induce oxidative stress in biological systems. Many diseases, including neurodegeneration, cardiovascular disease and cancer, are associated with oxidative stress. However, it is proposed recently that hydroxyl radical would not be generated from the Fenton reaction under physiological conditions and thus would not cause oxidative stress in biological systems. This proposal may cause confusion for understanding oxidative stress and can also have impact on therapeutic strategies for the diseases associated with oxidative stress. In this Mini-review, the up-to-date convincing evidences of hydroxyl radical generation from the physiologically relevant Fenton-like reactions of the iron(II) complexes with physiological ligands in human blood plasma, including histidine, citrate and phosphate, are succinctly reviewed. The oxidative damages caused by hydroxyl radical to biomolecules and cells are briefly summarized. These findings strongly challenge the above proposal.

TMT and PRM Based Quantitative Proteomics to Explore the Protective Role and Mechanism of Iristectorin B in Stroke

Stroke is a serious disease caused by the rupture or blockage of the cerebrovascular system. Its pathogenesis is complex and involves multiple mechanisms. Iristectorin B is a natural isoflavone that has certain anti stroke effects. In this study, an in vitro stroke injury model of glyoxylate deprivation was established using PC12 cells, which was used to evaluate the anti-stroke activity of Iristectorin B in ejecta stem. The results showed that Iristectorin B, a natural isoflavone derived from Dried Shoot, significantly reduced the damage to PC12 cells caused by oxygen glucose deprivation/reoxygenation, decreased apoptosis, enhanced cell survival and reduced Ca2+, LDH and ROS levels. The results showed that Iristectorin B had a significant protective effect on Na2S2O4-injured PC12 cells, and the mechanism may be related to the protective effect of neurons in the brain. After protein extraction and various analyses were performed, a series of cutting-edge technologies were organically combined to study the quantitative proteome of each group. Differential proteins were then analyzed. According to the protein screening principle, ferroptosis-related proteins were most closely associated with stroke. The differential proteins associated with ferroptosis screened were SLC3A2, TFR1 and HMOX1, with HMOX1 being the most significantly elevated and reduced via dosing. Iristectorin B may act as a protective agent against stroke by regulating ferroptosis, and SLC3A2, TFR1 and HMOX1 may serve as potential diagnostic biomarkers for stroke, providing additional evidence to support the importance of ferroptosis in stroke.

Iron-Dependent Cell Death: A New Treatment Approach against Pancreatic Ductal Adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is a devastating tumor type where a very high proportion of people diagnosed end up dying from cancer. Surgical resection is an option for only about 20% of patients, where the 5-year survival increase ranges from 10 to 25%. In addition to surgical resection, there are adjuvant chemotherapy schemes, such as FOLFIRINOX (a mix of Irinotecan, oxaliplatin, 5-Fluorouraci and leucovorin) or gemcitabine-based treatment. These last two drugs have been compared in the NAPOLI-3 clinical trial, and the NALIRIFOX arm was found to have a higher overall survival (OS) (11.1 months vs. 9.2 months). Despite these exciting improvements, PDAC still has no effective treatment. An interesting approach would be to drive ferroptosis in PDAC cells. A non-apoptotic reactive oxygen species (ROS)-dependent cell death, ferroptosis was first described by Dixon et al. in 2012. ROS are constantly produced in the tumor cell due to high cell metabolism, which is even higher when exposed to chemotherapy. Tumor cells have detoxifying mechanisms, such as Mn-SOD or the GSH-GPX system. However, when a threshold of ROS is exceeded in the tumor cell, the cell's antioxidant systems are overwhelmed, resulting in lipid peroxidation and, ultimately, ferroptosis. In this review, we point out ferroptosis as an approach to consider in PDAC and propose that altering the cellular ROS balance by combining oxidizing agents or with inhibitors of the main cellular detoxifiers triggers ferroptosis in PDAC.

Near UV light photo-degradation of histidine buffer: Mechanisms and role of Fe(III)

Pharmaceutical formulations are sensitive to light-induced degradation. Recent studies have attributed some of the light sensitivity to the presence of Fe(III), the most prevalent metal leachable from pharmaceutical containers. Histidine (His) can promote Fe(III) leaching from stainless steel, especially at elevated storage temperatures. Since there is the chance that combinations of His and Fe(III) are present in pharmaceutical formulations, we investigated the photo-degradation mechanisms of Fe(III)-containing His buffer during expsoure to near UV light. Our results indicate the formation of carbon dioxide radical anion (•CO2-), a powerful reductant, and other photoproducts such as aldehydes and His-derived radicals. The generation of •CO2- can be promoted by increasing concentrations of Fe(III) and inhibited by the addition of the Fe(III) chelator EDTA. Mechanistically, product formation can be rationalized by photo-induced ligand-to-metal-charge-transfer (LMCT), followed by a series of radical transformations of reaction intermediates.

Reaction of FeaqII with Peroxymonosulfate and Peroxydisulfate in the Presence of Bicarbonate: Formation of FeaqIV and Carbonate Radical Anions

Many advanced oxidation processes (AOPs) use Fenton-like reactions to degrade organic pollutants by activating peroxymonosulfate (HSO₅^-, PMS) or peroxydisulfate (S₂O₈^2-, PDS) with Fe(H₂O)₆²⁺ (Fe_aq^II). This paper presents results on the kinetics and mechanisms of reactions between Fe_aq^II and PMS or PDS in the absence and presence of bicarbonate (HCO₃^-) at different pH. In the absence of HCO₃^-, Fe_aq^IV, rather than the commonly assumed SO₄^•-, is the dominant oxidizing species. Multianalytical methods verified the selective conversion of dimethyl sulfoxide (DMSO) and phenyl methyl sulfoxide (PMSO) to dimethyl sulfone (DMSO₂) and phenyl methyl sulfone (PMSO₂), respectively, confirming the generation of Fe_aq^IV by the Fe_aq^II-PMS/PDS systems without HCO₃^-. Significantly, in the presence of environmentally relevant concentrations of HCO₃^-, a carbonate radical anion (CO₃^•-) becomes the dominant reactive species as confirmed by the electron paramagnetic resonance (EPR) analysis. The new findings suggest that the mechanisms of the persulfate-based Fenton-like reactions in natural environments might differ remarkably from those obtained in ideal conditions. Using sulfonamide antibiotics (sulfamethoxazole (SMX) and sulfadimethoxine (SDM)) as model contaminants, our study further demonstrated the different reactivities of Fe_aq^IV and CO₃^•- in the Fe_aq^II-PMS/PDS systems. The results shed significant light on advancing the persulfate-based AOPs to oxidize pollutants in natural water.

Oxygen for the Newborn: Friend or Foe?

Oxygen supplementation is widely used in neonatal care, however, it can also cause toxic effects if not used properly. Therefore, it appears crucial to find a balance in oxygen administration to avoid damage as a consequence of its insufficient or excessive use. Oxygen toxicity is mainly due to the production of oxygen radicals, molecules normally produced in humans and involved in a myriad of physiological reactions. In the neonatal period, an imbalance between oxidants and antioxidant defenses, the so-called oxidative stress, might occur, causing severe pathological consequences. In this review, we focus on the mechanisms of the production of oxygen radicals and their physiological functions in determining a set of diseases grouped together as "free radical diseases in the neonate". In addition, we describe the evolution of the oxygenation target recommendations during neonatal resuscitation and post-stabilization phases with the aim to define the best oxygen administration according to the newest evidence.

Near-UV and Visible Light Degradation of Iron (III)-Containing Citrate Buffer: Formation of Carbon Dioxide Radical Anion via Fragmentation of a Sterically Hindered Alkoxyl Radical

Citrate is a commonly used buffer in pharmaceutical formulations which forms complexes with adventitious metals such as Fe³⁺. Fe³⁺-citrate complexes can act as potent photosensitizers under near-UV and visible light exposure, and recent studies reported evidence for the photo-production of a powerful reductant, carbon dioxide radical anion (^•CO₂^-), from Fe³⁺-citrate complexes (Subelzu, N.; Schöneich, N., Mol. Pharm. 2020, 17, 4163-4179). The mechanisms of ^•CO₂^- formation are currently unknown but must be established to devise strategies against ^•CO₂^- formation in pharmaceutical formulations which rely on the use of citrate buffer. In this study, we first established complementary evidence for the photolytic generation of ^•CO₂^- from Fe³⁺-citrate through spin trapping and electron paramagnetic resonance (EPR) spectroscopy, and subsequently used spin trapping in conjunction with tandem mass spectrometry (MS/MS) for mechanistic studies on the pathways of ^•CO₂^- formation. Experiments with stable isotope-labeled citrate suggest that the central carboxylate group of citrate is the major source of ^•CO₂^-. Competition studies with various inhibitors (alcohols and dimethyl sulfoxide) reveal two mechanisms of ^•CO₂^- formation, where one pathway involves β-cleavage of a sterically hindered alkoxyl radical generated from the hydroxyl group of citrate.

Role of Reactive Oxygen Species in Aging and Age-Related Diseases: A Review

Research on the role of reactive oxygen species (ROS) in the aging process has advanced significantly over the last two decades. In light of recent findings, ROS takes part in the aging process of cells along with contributing to various physiological signaling pathways. Antioxidants being cells' natural defense mechanism against ROS-mediated alteration, play an imperative role to maintain intracellular ROS homeostasis. Although the complete understanding of the ROS regulated aging process is yet to be fully comprehended, current insights into various sources of cellular ROS and their correlation with the aging process and age-related diseases are portrayed in this review. In addition, results on the effect of antioxidants on ROS homeostasis and the aging process as well as their advances in clinical trials are also discussed in detail. The future perspective in ROS-antioxidant dynamics on antiaging research is also marshaled to provide future directions for ROS-mediated antiaging research fields.

Induction of the macrolide-resistance efflux pump Mega inhibits intoxication of Staphylococcus aureus strains by Streptococcus pneumoniae

Streptococcus pneumoniae (Spn) kills Staphylococcus aureus (Sau) through a contact-dependent mechanism that is catalyzed by cations, including iron, to convert hydrogen peroxide (H₂O₂) to highly toxic hydroxyl radicals (^•OH). There are two well-characterized ABC transporters that contribute to the pool of iron in Spn, named Pia and Piu. Some Spn strains have acquired genes mef(E)/mel encoding another ABC trasporter (Mega) that produces an inducible efflux pump for resistance to macrolides. In macrolide-resistant Spn clinical isolates the insertion of Mega class 1. IV and 2. IVc deleted the locus piaABCD and these strains were attenuated for intoxicating Sau. The goal of this study was to investigate if the disruption of iron acquisition, or the antimicrobial-resistance activity of Mega, contributed to inhibiting the killing mechanism. Neither depletion of iron with 2,2'-dipyridyl-d8 (DP) nor incubating with a double knockout mutant SpnΔpiaAΔpiuA, inhibited killing of Sau. Clinical Spn strains carrying Mega1. IV or Mega2. IVc showed a significant delay for killing Sau. An ex vivo recombination system was used to transfer Mega1. IV or Mega2. IVc to reference Spn strains, which was confirmed by whole genome sequencing, and recombinants TIGR4^{Mega2. IVc}, D39^{Mega2. IVc}, and D39^{Mega1. IV} were delayed for killing Sau. We then compared Sau killing of selected Mega-carrying Spn strains when incubated with sub-inhibitory erythromycin (Mega-induced) or sub-inhibitory cefuroxime. Remarkably, killing of Sau was completely inhibited under the Mega-induced condition whereas incubation with cefuroxime did not interfere with killing. Both mef(E) and mel were upregulated > 400-fold, and spxB (encoding an enzyme responsible for production of most H₂O₂) was upregulated 14.2-fold, whereas transcription of the autolysin (lytA) gene was downregulated when incubated with erythromycin. We demonstrated that erythromycin induction of Mega inhibits the ^•OH-mediated intoxication of Sau and that the inhibition occurred at the post-translational level suggesting that an imbalance of ions in the membrane inhibits these reactions.

What Are the Oxidizing Intermediates in the Fenton and Fenton-like Reactions? A Perspective

The Fenton and Fenton-like reactions are of major importance due to their role as a source of oxidative stress in all living systems and due to their use in advanced oxidation technologies. For many years, there has been a debate whether the reaction of Fe^II(H₂O)₆²⁺ with H₂O₂ yields OH^• radicals or Fe^IV=O_aq. It is now known that this reaction proceeds via the formation of the intermediate complex (H₂O)₅Fe^II(O₂H)⁺/(H₂O)₅Fe^II(O₂H₂)²⁺ that decomposes to form either OH^• radicals or Fe^IV=O_aq, depending on the pH of the medium. The intermediate complex might also directly oxidize a substrate present in the medium. In the presence of Fe^III_aq, the complex Fe^III(OOH)_aq is formed. This complex reacts via Fe^II(H₂O)₆²⁺ + Fe^III(OOH)_aq → Fe^IV=O_aq + Fe^III_aq. In the presence of ligands, the process often observed is L_n(H₂O)_5−nFe^II(O₂H) → L^•+ + L_n−1Fe^III_aq. Thus, in the presence of small concentrations of HCO₃⁻ i.e., in biological systems and in advanced oxidation processes—the oxidizing radical formed is CO₃^•−. It is evident that, in the presence of other transition metal complexes and/or other ligands, other radicals might be formed. In complexes of the type L_n(H₂O)_5−nM^III/II(O₂H⁻), the peroxide might oxidize the ligand L without oxidizing the central cation M. OH^• radicals are evidently not often formed in Fenton or Fenton-like reactions.

Epigenetic Marks, DNA Damage Markers, or Both? The Impact of Desiccation and Accelerated Aging on Nucleobase Modifications in Plant Genomic DNA

Modifications of DNA nucleobases are present in all forms of life. The purpose of these modifications in eukaryotic cells, however, is not always clear. Although the role of 5-methylcytosine (m⁵C) in epigenetic regulation and the maintenance of stability in plant genomes is becoming better understood, knowledge pertaining to the origin and function of oxidized nucleobases is still scarce. The formation of 5-hydroxymetylcytosine (hm⁵C) in plant genomes is especially debatable. DNA modifications, functioning as regulatory factors or serving as DNA injury markers, may have an effect on DNA structure and the interaction of genomic DNA with proteins. Thus, these modifications can influence plant development and adaptation to environmental stress. Here, for the first time, the changes in DNA global levels of m⁵C, hm⁵C, and 8-oxo-7,8-dihydroguanine (8-oxoG) measured by ELISA have been documented in recalcitrant embryonic axes subjected to desiccation and accelerated aging. We demonstrated that tissue desiccation induces a similar trend in changes in the global level of hm⁵C and 8-oxoG, which may suggest that they both originate from the activity of reactive oxygen species (ROS). Our study supports the premise that m⁵C can serve as a marker of plant tissue viability whereas oxidized nucleobases, although indicating a cellular redox state, cannot.

Oxidative damage and DNA repair in desiccated recalcitrant embryonic axes of Acer pseudoplatanus L

BACKGROUND

Most plants encounter water stress at one or more different stages of their life cycle. The maintenance of genetic stability is the integral component of desiccation tolerance that defines the storage ability and long-term survival of seeds. Embryonic axes of desiccation-sensitive recalcitrant seeds of Acer pseudoplatnus L. were used to investigate the genotoxic effect of desiccation. Alkaline single-cell gel electrophoresis (comet assay) methodology was optimized and used to provide unique insights into the onset and repair of DNA strand breaks and 8-oxo-7,8-dihydroguanine (8-oxoG) formation during progressive steps of desiccation and rehydration.

RESULTS

The loss of DNA integrity and impairment of damage repair were significant predictors of the viability of embryonic axes. In contrast to the comet assay, automated electrophoresis failed to detect changes in DNA integrity resulting from desiccation. Notably, no significant correlation was observed between hydroxyl radical (^٠OH) production and 8-oxoG formation, although the former is regarded to play a major role in guanine oxidation.

CONCLUSIONS

The high-throughput comet assay represents a sensitive tool for monitoring discrete changes in DNA integrity and assessing the viability status in plant germplasm processed for long-term storage.

pH Dependence of Hydroxyl Radical, Ferryl, and/or Ferric Peroxo Species Generation in the Heterogeneous Fenton Process

The heterogeneous Fenton process in the presence of Fe-containing minerals is ubiquitous in nature and widely deployed in wastewater treatment. While there have been extensive relevant studies, the dependence on pH of the nature and extent of oxidant generation and key reaction pathways remain unclear. Herein, the adsorption and decomposition of formate and H₂O₂ were quantified in the presence of ferrihydrite within the pH range of 3.0-6.0, and experiments with methyl phenyl sulfoxide were conducted to distinguish between HO^• and weaker oxidant(s) which react via oxygen atom transfer including ferryl ion ([Fe^IVO]²⁺) and/or ferric hydroperoxo intermediates (≡Fe^III(O₂H)). Both HO^• and [Fe^IVO]²⁺/≡Fe^III(O₂H) are concurrently produced on the surface over the acidic to near-neutral pH range. Despite the simultaneous formation of both oxidants, HO^• is the major oxidant responsible for substrate oxidation in the interfacial boundary layer with [Fe^IVO]²⁺/≡Fe^III(O₂H) exhibiting limited exposure to substrates. With an increase of pH, the yield of both oxidants is inhibited by the decreasing availability of surface sites due to ferrihydrite particle aggregation. Increasing pH also favors the nonradical decay of H₂O₂ as evident from the consistent oxidant production rate relative to the surface area (SSA) despite an accelerated H₂O₂ decay rate relative to SSA with pH increase.

Impact of Oxidative DNA Damage and the Role of DNA Glycosylases in Neurological Dysfunction

The human brain requires a high rate of oxygen consumption to perform intense metabolic activities, accounting for 20% of total body oxygen consumption. This high oxygen uptake results in the generation of free radicals, including reactive oxygen species (ROS), which, at physiological levels, are beneficial to the proper functioning of fundamental cellular processes. At supraphysiological levels, however, ROS and associated lesions cause detrimental effects in brain cells, commonly observed in several neurodegenerative disorders. In this review, we focus on the impact of oxidative DNA base lesions and the role of DNA glycosylase enzymes repairing these lesions on brain function and disease. Furthermore, we discuss the role of DNA base oxidation as an epigenetic mechanism involved in brain diseases, as well as potential roles of DNA glycosylases in different epigenetic contexts. We provide a detailed overview of the impact of DNA glycosylases on brain metabolism, cognition, inflammation, tissue loss and regeneration, and age-related neurodegenerative diseases based on evidence collected from animal and human models lacking these enzymes, as well as post-mortem studies on patients with neurological disorders.

Fenton-like reaction of the iron(II)-histidine complex generates hydroxyl radicals: implications for oxidative stress and Alzheimer's disease

The hydroxyl radical (˙OH), generated from Fenton/Fenton-like reactions of iron(II) species in biology, can oxidatively damage biomolecules, inducing oxidative stress and diseases. However, this common understanding has been questioned recently after a carbonate radical was observed from the Fenton-like reaction of the iron(II)-carbonate complex. Herein, we report that the Fenton-like reaction of the iron(II)-histidine complex, one major iron(II) species in blood plasma, can occur at neutral pH to generate ˙OH, not iron(IV). Our findings and critical analyses on relevant studies clarify the above doubt, reveal a new pathway of causing oxidative stress by the iron(II) species, and have implications for Alzheimer's disease.

Hydroxyl radical is a significant player in oxidative DNA damage in vivo

Recent publications have suggested that oxidative DNA damage mediated by hydroxyl radical (˙OH) is unimportant in vivo, and that carbonate anion radical (CO3˙-) plays the key role. We examine these claims and summarize the evidence that ˙OH does play a key role as an important member of the reactive oxygen species (ROS) in vivo.

The Two Faces of the Guanyl Radical: Molecular Context and Behavior

The guanyl radical or neutral guanine radical G(-H)^• results from the loss of a hydrogen atom (H^•) or an electron/proton (e^–/H⁺) couple from the guanine structures (G). The guanyl radical exists in two tautomeric forms. As the modes of formation of the two tautomers, their relationship and reactivity at the nucleoside level are subjects of intense research and are discussed in a holistic manner, including time-resolved spectroscopies, product studies, and relevant theoretical calculations. Particular attention is given to the one-electron oxidation of the GC pair and the complex mechanism of the deprotonation vs. hydration step of GC^•+ pair. The role of the two G(-H)^• tautomers in single- and double-stranded oligonucleotides and the G-quadruplex, the supramolecular arrangement that attracts interest for its biological consequences, are considered. The importance of biomarkers of guanine DNA damage is also addressed.

Pharmaceutical Excipients Enhance Iron-Dependent Photo-Degradation in Pharmaceutical Buffers by near UV and Visible Light: Tyrosine Modification by Reactions of the Antioxidant Methionine in Citrate Buffer

PURPOSE

To evaluate the effect of excipients, including sugars and amino acids, on photo-degradation reactions in pharmaceutical buffers induced by near UV and visible light.

METHODS

Solutions of citrate or acetate buffers, containing 1 or 50 μM Fe³⁺, the model peptides methionine enkephalin (MEn), leucine enkephalin (LEn) or proctolin peptide (ProP), in the presence of commonly used amino acids or sugars, were photo-irradiated with near UV or visible light. The oxidation products were analyzed by reverse-phase HPLC and HPLC-MS/MS.

RESULTS

The sugars mannitol, sucrose and trehalose, and the amino acids Arg, Lys, and His significantly promote the oxidation of peptide Met to peptide Met sulfoxide. These excipients do not increase the yields of hydrogen peroxide, suggesting that other oxidants such as peroxyl radicals are responsible for the oxidation of peptide Met. The addition of free Met reduces the oxidation of peptide Met, but, in citrate buffer, causes the addition of Met oxidation products to Tyr residues of the target peptides.

CONCLUSIONS

Commonly used excipients enhance the light-induced oxidation of amino acids in model peptides.

Iron Fenton oxidation of 2'-deoxyguanosine in physiological bicarbonate buffer yields products consistent with the reactive oxygen species carbonate radical anion not the hydroxyl radical

Product analysis from the iron Fenton oxidation of 2'-deoxyguanosine found reactions in bicarbonate buffer yield 8-oxo-2'-deoxyguanosine and spiroiminodihyantoin consistent with CO3˙-. Reactions in phosphate buffer furnished high yields of sugar oxidation products consistent with HO˙. These observations change the view of DNA oxidation products from the iron-Fenton reaction.

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.

Radicals in 'biologically relevant' concentrations behave differently: Uncovering new radical reactions following the reaction of hydroxyl radicals with DMSO

Methyl radicals play key roles in various chemical and biological processes. Mechanistic studies of methyl radicals with their precursor, Dimethyl Sulfoxide (DMSO), were extensively studied. Though the involved mechanisms seemed to be clarified, essentially none of the studies have been performed at conditions relevant to both biological and catalytic systems, i.e. low steady state radical concentrations. A chain-like reaction, as an inverse function of the radicals concentrations ([^•CH₃]_ss), increases the methyl radical yields. The nature of the additional products obtained differs from those commonly observed. Furthermore it is shown that methyl radicals abstract a methyl group from DMSO to yield ethane. Herein we report a novel mechanism relevant mainly at low steady state radical concentrations, which may change the understanding of certain reaction routes present in both biological systems and catalytic chemical systems. Thus the results point out that mechanistic studies have to be carried out at dose rates forming radicals at analogous concentrations to those present in the process of interest.

The Role of Carbonate in Catalytic Oxidations

CO₂, HCO₃^-, and CO₃^2- are present in all aqueous media at pH > 4 if no major effort is made to remove them. Usually the presence of CO₂/HCO₃^-/CO₃^2- is either forgotten or considered only as a buffer or proton transfer catalyst. Results obtained in the last decades point out that carbonates are key participants in a variety of oxidation processes. This was first attributed to the formation of carbonate anion radicals via the reaction OH^• + CO₃^2- → CO₃^•- + OH^-. However, recent studies point out that the involvement of carbonates in oxidation processes is more fundamental. Thus, the presence of HCO₃^-/CO₃^2- changes the mechanisms of Fenton and Fenton-like reactions to yield CO₃^•- directly even at very low HCO₃^-/CO₃^2- concentrations. CO₃^•- is a considerably weaker oxidizing agent than the hydroxyl radical and therefore a considerably more selective oxidizing agent. This requires reconsideration of the sources of oxidative stress in biological systems and might explain the selective damage induced during oxidative stress. The lower oxidation potential of CO₃^•- probably also explains why not all pollutants are eliminated in many advanced oxidation technologies and requires rethinking of the optimal choice of the technologies applied. The role of percarbonate in Fenton-like processes and in advanced oxidation processes is discussed and has to be re-evaluated. Carbonate as a ligand stabilizes transition metal complexes in uncommon high oxidation states. These high-valent complexes are intermediates in electrochemical water oxidation processes that are of importance in the development of new water splitting technologies. HCO₃^- and CO₃^2- are also very good hole scavengers in photochemical processes of semiconductors and may thus become key participants in the development of new processes for solar energy conversion. In this Account, an attempt to correlate these observations with the properties of carbonates is made. Clearly, further studies are essential to fully uncover the potential of HCO₃^-/CO₃^2- in desired oxidation processes.

On the irrelevancy of hydroxyl radical to DNA damage from oxidative stress and implications for epigenetics

Contrary to frequent reports in the literature, hydroxyl radical is not a key species participating in endogenous oxidative DNA damage. Instead, carbonate radical anion is formed from the Fenton reaction under cellular conditions and from decomposition of nitrosoperoxycarbonate generated during inflammation. Carbonate radical anion is a potent one-electron oxidant capable of generating base radical cations that can migrate over long distances in duplex DNA, ultimately generating 8-oxo-7,8-dihydroguanine at a redox-sensitive sequence such as GGG. Such a mechanism enables G-quadruplex-forming sequences to act as long-range sensors of oxidative stress, impacting gene expression via the DNA repair mechanism that reads and ultimately erases the oxidized base. With a writing, reading and erasing mechanism in place, oxidative 'damage' to DNA might be relabeled as 'epigenetic' modifications.

Fenton aging significantly affects the heavy metal adsorption capacity of polystyrene microplastics

Microplastics released into the environment undergo a variety of aging processes, however, information about the influence of aging on the adsorption behavior of microplastics is limited. In order to better understand the effect of aging polystyrene (PS) on the ability to adsorb heavy metal, H₂O₂ and Fenton reagent were used to investigate the aging properties of PS. Aging PS with these two different aging agents at pH = 4 and room temperature for the same time. Physical and chemical characterization indicated that aging caused oxidation of the surface of PS and the formation of surface micro-cracks. Based on the 2D-COS analysis, the aging process of PS functional groups could occur in the following sequence: 1375 (C-OH) > 1739 (C=O) > 1182 cm^-1 (C-O-C) > 1716 (O-C=O). The adsorption experiments for Cd²⁺ with two different concentrations were carried out by PS with different aging time at room temperature. The adsorption data showed that the adsorption capacity of Cd²⁺ was significantly enhanced after aging compared with pristine PS, and the adsorption capacity of PS after Fenton aging treatment is much stronger than that after H₂O₂ aging treatment. The kinetic analysis of the adsorption data indicates that the adsorption process is more consistent with the second-order kinetics than the first-order kinetics, and it is further concluded that the adsorption of Cd²⁺ by PS is a relatively complicated process. According to the fitting results of adsorption isotherms, the adsorption process of pristine PS mainly occurs on the surface, but with the continuous aging, more adsorption sites may be exposed on the surface of PS, so it can be concluded that the adsorption mechanism of Cd²⁺ by PS is the coexistence of physics and chemistry. This study indicates aging microplastics may have a significant impact on the destination and migration of metal contaminants, which deserves to be further concerned.