The remarkable nucleophilicity of superoxide anion radical. rate constants for reaction of superoxide ion with aliphatic bromides

Highly scalable photoinduced synthesis of silanols via untraversed pathway for chlorine radical (Cl•) generation

The emergence of visible light-mediated synthetic transformations has transpired as a promising approach to redefine traditional organic synthesis in a sustainable way. In this genre, transition metal-mediated photoredox catalysis has led the way and recreated a plethora of organic transformations. However, the use of photochemical energy solely to initiate the reaction is underexplored. With the direct utilization of photochemical energy herein, we have established a general and practical protocol for the synthesis of diversely functionalized organosilanols, silanediols, and polymeric siloxanol engaging a wide spectrum of hydrosilanes under ambient reaction conditions. Streamlined synthesis of bio-active silanols via late-stage functionalization underscores the importance of this sustainable protocol. Interestingly, this work also reveals photoinduced non-classical chlorine radical (Cl•) generation from a readily available chlorinated solvent under aerobic conditions. The intriguing factors of the proposed mechanism involving chlorine and silyl radicals as intermediates were supported by a series of mechanistic investigations.

Photocatalytic Aerobic Coupling of Azaarenes and Alkanes via Nontraditional Cl• Generation

Herein, we demonstrate a nonconventional photocatalytic generation of Cl^• from a common chlorinated solvent, dichloroethane, under aerobic conditions and its successful utilization toward the cross-dehydrogenative coupling of alkanes and azaarenes via hydrogen atom transfer with Cl^•. The process is free from chloride salt, toxic oxidant, and UV light. It is applicable to a broad spectrum of substrates. The proposed mechanism involving Cl^• is supported by a series of mechanistic investigations.

Mechanistic Understanding of Superoxide Radical-Mediated Degradation of Perfluorocarboxylic Acids

Perfluorocarboxylic acids (PFCAs) exhibit strong persistence in sunlit surface waters and in radical-based treatment processes, where superoxide radical (O₂^•-) is an important and abundant reactive oxygen species. Given that the role of O₂^•- during the transformation of PFCAs remains largely unknown, we investigated the kinetics and mechanisms of O₂^•--mediated PFCAs attenuation through complementary experimental and theoretical approaches. The aqueous-phase rate constants between O₂^•- and C3-C8 PFCAs were measured using a newly designed in situ spectroscopic system. Mechanistically, bimolecular nucleophilic substitution (S_N2) is most likely to be thermodynamically feasible, as indicated by density functional theory calculations at the CBS-QB3 level of theory. This pathway was then investigated by ab initio molecular dynamics simulation with free-energy samplings. As O₂^•- approaches PFCA, the C-F bond at the alpha carbon is spontaneously stretched, leading to the bond cleavage. The solvation mechanism for O₂^•--mediated PFCA degradation was also elucidated. Our results indicated that although the less polar solvent enhanced the nucleophilicity of O₂^•-, it also decreased the desolvation process of PFCAs, resulting in reduced kinetics. With these quantitative and mechanistic results, we achieved a defined picture of the O₂^•--initiated abatement of PFCAs in natural and engineered waters.

Nucleophile sensitivity of Drosophila TRPA1 underlies light-induced feeding deterrence

Solar irradiation including ultraviolet (UV) light causes tissue damage by generating reactive free radicals that can be electrophilic or nucleophilic due to unpaired electrons. Little is known about how free radicals induced by natural sunlight are rapidly detected and avoided by animals. We discover that Drosophila Transient Receptor Potential Ankyrin 1 (TRPA1), previously known only as an electrophile receptor, sensitively detects photochemically active sunlight through nucleophile sensitivity. Rapid light-dependent feeding deterrence in Drosophila was mediated only by the TRPA1(A) isoform, despite the TRPA1(A) and TRPA1(B) isoforms having similar electrophile sensitivities. Such isoform dependence re-emerges in the detection of structurally varied nucleophilic compounds and nucleophilicity-accompanying hydrogen peroxide (H₂O₂). Furthermore, these isoform-dependent mechanisms require a common set of TRPA1(A)-specific residues dispensable for electrophile detection. Collectively, TRPA1(A) rapidly responds to natural sunlight intensities through its nucleophile sensitivity as a receptor of photochemically generated radicals, leading to an acute light-induced behavioral shift in Drosophila.

DOI:http://dx.doi.org/10.7554/eLife.18425.001

Atoms are made up of a nucleus that contains protons and neutrons, which is orbited by electrons. The electrons orbit within shells that surround the nucleus and each shell can contain a specific number of electrons. A particle with an outer shell that is missing one or more electrons will be unstable and highly reactive. It will attempt to achieve a full outer shell either by sharing electrons with another particle, or by donating or stealing an electron. Particles that steal electrons are said to be “electrophilic” (electron-loving) while those that donate them are “nucleophilic”.

Electrophilic and nucleophilic particles can damage DNA and proteins. In species from fruit flies to humans, electrophilic substances such as formaldehyde activate a type of ion channel called TRPA1. These ion channels contribute to pain signaling, and their activation triggers unpleasant and painful sensations that deter animals from getting too close to electrophilic substances. However, it is not known if animals have an equivalent mechanism to help them avoid toxic nucleophilic compounds, like carbon monoxide and cyanide.

Du, Ahn, Wen, Seo, Na et al. now show that fruit fly neurons produce two versions of the TRPA1 channel: one that is sensitive to electrophiles, plus a second that is sensitive to nucleophiles in addition to electrophiles. The existence of nucleophile-sensitive TRPA1 helps explain why fruit flies avoid feeding in strong sunlight. Ultraviolet radiation in sunlight triggers the production of reactive forms of oxygen that behave as strong nucleophiles. These reactive oxygen species – which can damage DNA – activate the nucleophile-sensitive TRPA1 and thereby trigger the fly’s avoidance behavior.

Human TRPA1 responds only to electrophiles and not to nucleophiles. By targeting the nucleophile-sensitive version of insect TRPA1, it may thus be possible to develop insect repellants that humans do not find aversive. Furthermore, TRPA1s from some insect species are more sensitive to nucleophiles than others, with a mosquitoes’ being more sensitive than the fruit flies’. This means that insect repellants that target nucleophile-sensitive TRPA1 could potentially repel malaria-transmitting mosquitoes without affecting other insect species.

DOI:http://dx.doi.org/10.7554/eLife.18425.002

Superoxide Ion: Generation and Chemical Implications

Superoxide ion (O2(•-)) is of great significance as a radical species implicated in diverse chemical and biological systems. However, the chemistry knowledge of O2(•-) is rather scarce. In addition, numerous studies on O2(•-) were conducted within the latter half of the 20th century. Therefore, the current advancement in technology and instrumentation will certainly provide better insights into mechanisms and products of O2(•-) reactions and thus will result in new findings. This review emphasizes the state-of-the-art research on O2(•-) so as to enable researchers to venture into future research. It comprises the main characteristics of O2(•-) followed by generation methods. The reaction types of O2(•-) are reviewed, and its potential applications including the destruction of hazardous chemicals, synthesis of organic compounds, and many other applications are highlighted. The O2(•-) environmental chemistry is also discussed. The detection methods of O2(•-) are categorized and elaborated. Special attention is given to the feasibility of using ionic liquids as media for O2(•-), addressing the latest progress of generation and applications. The effect of electrodes on the O2(•-) electrochemical generation is reviewed. Finally, some remarks and future perspectives are concluded.

Reactive oxygen species: Reactions and detection from photosynthetic tissues

Reactive oxygen species (ROS) have long been recognized as compounds with dual roles. They cause cellular damage by reacting with biomolecules but they also function as agents of cellular signaling. Several different oxygen-containing compounds are classified as ROS because they react, at least with certain partners, more rapidly than ground-state molecular oxygen or because they are known to have biological effects. The present review describes the typical reactions of the most important ROS. The reactions are the basis for both the detection methods and for prediction of reactions between ROS and biomolecules. Chemical and physical methods used for detection, visualization and quantification of ROS from plants, algae and cyanobacteria will be reviewed. The main focus will be on photosynthetic tissues, and limitations of the methods will be discussed.

Light-induced oxidation of unsaturated lipids as sensitized by flavins

Triplet-excited riboflavin ((3)RF*) was found by laser flash photolysis to be quenched by polyunsaturated fatty acid methyl esters in tert-butanol/water (7:3, v/v) in a second-order reaction with k approximately 3.0 x 10(5) L mol(-1) s(-1) at 25 degrees C for methyl linoleate and 3.1 x 10(6) L mol(-1) s(-1), with DeltaH(double dagger) = 22.6 kJ mol(-1) and DeltaS(double dagger) = -62.3 J K(-1) mol(-1), for methyl linolenate in acetonitrile/water (8:2, v/v). For methyl oleate, k was <10(4) L mol(-1) s(-1). For comparison, beta-casein was found to have a rate constant k approximately 4.9 x 10(8) L mol(-1) s(-1). Singlet-excited flavin was not quenched by the esters as evidenced by insensitivity of steady-state fluorescence to their presence. Density functional theory (DFT) calculations showed that electron transfer from unsaturated fatty acid esters to triplet-excited flavins is endergonic, while a formal hydrogen atom transfer is exergonic (DeltaG(o)(HAT) = -114.3, -151.2, and -151.2 kJ mol(-1) for oleate, linoleate, and linolenate, respectively, in acetonitrile). The reaction is driven by acidity of the lipid cation radical for which a pK(a) approximately -0.12 was estimated by DFT calculations. Absence of electrochemical activity in acetonitrile during cyclic voltammetry up to 2.0 V versus NHE confirmed that DeltaG(o)(ET) > 0 for electron transfer. Interaction of methyl esters with (3)RF* is considered as initiation of the radical chain, which is subsequently propagated by combination reactions with residual oxygen. In this respect, carbon-centered and alkoxyl radicals were detected using the spin trapping technique in combination with electron paramagnetic resonance spectroscopy. Moreover, quenching of (3)RF* yields, directly or indirectly, radical species which are capable of initiating oxidation in unsaturated fatty acid methyl esters. Still, deactivation of triplet-excited flavins by lipid derivatives was slower than by proteins (factor up to 10(4)), which react preferentially by electron transfer. Depending on the reaction environment in biological systems (including food), protein radicals are expected to interfere in the mechanism of light-induced lipid oxidation.

Initiation of oxidative changes in foods

Initiation of lipid peroxidation in foods may be accomplished by a variety of mechanisms. Two principal initiation reactions involve homolytic scission of preformed peroxides as catalyzed by metal ions and heme proteins and the reaction of activated oxygen species with the lipid substrate to yield peroxides and free radicals. Copper and cytochromes in the milk fat globule membrane may serve as focal points for initiation of lipid peroxidation by catalyzing homolytic scission of peroxides. Activated oxygen species which may be important in initiating oxidative changes in foods include singlet oxygen, hydroxyl radical, ozone, superoxide anion (perhydroxyl radical at low pH), and hydrogen peroxide. Chemical and enzymic reactions in biological materials can generate singlet oxygen, hydroxyl radical, superoxide anion, and hydrogen peroxide. Ozone is primarily a product of photoreactions in polluted air. Reactions involving singlet oxygen, hydroxyl radical, and ozone with food constituents ultimately can yield peroxides which decompose to initiate oxidative chain reactions. Superoxide anion and hydrogen peroxide are relatively inert toward organic molecules but can decompose to produce the more reactive singlet oxygen and hydroxyl radical. Inhibition of reactions initiated by reactive oxygen species in foods should be very important in preserving the oxidative stability of foods. This paper presents a brief review of possible initiation reactions for lipid peroxidation and inhibition of reactions of activated oxygen species that are of importance in food systems.

Activated oxygen species and oxidation of food constituents

Activated oxygen species which may be important in initiating oxidative changes in foods include singlet oxygen, hydroxyl radical, ozone, superoxide anion (perhydroxyl radical at low pH), and hydrogen peroxide. Chemical and enzymic reactions known to occur in biological materials can generate singlet oxygen, hydroxyl radical, superoxide anion, and hydrogen peroxide. Ozone is primarily a product of photoreactions in polluted air. Reactions involving singlet oxygen, hydroxyl radical, and ozone with food constituents can ultimately yield peroxides which decompose to initiate oxidative chain reactions. Superoxide anion and hydrogen peroxide are relatively inert toward organic molecules but can decompose to produce the more reactive singlet oxygen and hydroxyl radical. Inhibition of reactions initiated by reactive oxygen species in foods should be very important in preserving the oxidative stability of foods. The generation, detection, measurement, reaction, and inhibition of reactions of active oxygen species are surveyed in this review.