Was H2O2 generated before oxygenic photosynthesis?

We obviously agree with Wu et al. that H2O2 might accumulate in the Archean land waters devoid of Fe2+. We do disagree on the topic of the half-life of H2O2, as the work cited in support for a longer half-live is not relevant to the conditions in the Archean ocean. While the existence of radicals in quartz is not in doubt, we do question the hypothesis that these radicals oxidize water to HO• and H2O2.

Was hydrogen peroxide present before the arrival of oxygenic photosynthesis? The important role of iron(II) in the Archean ocean.

We address the chemical/biological history of H2O2 back at the times of the Archean eon (2.5-3.9 billion years ago (Gya)). During the Archean eon the pO2 was million-fold lower than the present pO2, starting to increase gradually from 2.3 until 0.6 Gya, when it reached ca. 0.2 bar. The observation that some anaerobic organisms can defend themselves against O2 has led to the view that early organisms could do the same before oxygenic photosynthesis had developed at about 3 Gya. This would require the anaerobic generation of H2O2, and here we examine the various mechanisms which were suggested in the literature for this. Given the concentration of Fe2+ at 20-200 µM in the Archean ocean, the estimated half-life of H2O2 is ca. 0.7 s. The oceanic H2O2 concentration was practically zero. We conclude that early organisms were not exposed to H2O2 before the arrival of oxygenic photosynthesis.

Ferryl for real. The Fenton reaction near neutral pH.

According to the literature, the Fenton reaction yields HO˙ and proceeds with 53 M-1 s-1 at 25 °C and low pH. Above pH 5, the reaction becomes first-order in HO-, and oxygen atom transfer has been detected, which indicates formation of oxidoiron(2+), FeO2+. These observations, and the assumption that the intermediate [FeHOO]+ decays approximately iso-energetically to FeO2+, allow one to estimate an Gibbs energy of formation FeO2+ of +15 ± 10 kJ mol-1, from which follows the one-electron E°'(FeO2+, H2O/[Fe(HO)2]+) = +2.5 ± 0.1 V and the two-electron E°'(FeO2+, 2H+/Fe2+, H2O) = +1.36 ± 0.05 V, both at pH 7. In the presence of HCO3-, formation of FeCO3(aq) occurs which may facilitate formation of the [FeHOO]+ intermediate, and leads to CO3˙-. At pH 7, the product of the Fenton reaction is thus FeO2+, or CO3˙- if HCO3- is present.

Comment on "Theoretical investigations on hydrogen peroxide decomposition in aquo" by T. Tsuneda and T. Taketsugu, Phys. Chem. Chem. Phys., 2018, 20, 24992.

In an ab initio study, Tsuneda and Taketsugu (Phys. Chem. Chem. Phys., 2018, 20, 24992-24999) discuss the Fenton reaction, the reaction of Fe2+ with H2O2. They claim that reaction is endergonic and therefore introduce a new intermediate, a de facto monovalent iron complex. I show here that kinetically and thermodynamically such a monovalent iron complex cannot be formed.

Thinking Outside the Cage: A New Hypothesis That Accounts for Variable Yields of Radicals from the Reaction of CO2 with ONOO.

In biology, the reaction of ONOO- with CO2 is the main sink for ONOO-. This reaction yields CO3•-, NO2•, NO3-, and CO2. There is a long-standing debate with respect to the yield of the radicals relative to ONOO-. The reaction of ONOO- with CO2 results at first in ONOOCO2-. According to one hypothesis, ONOOCO2- is extremely short-lived and devolves into a solvent cage that contains CO3•- and NO2•. Of these solvent cages, approximately two/thirds result in NO3- and CO2, and approximately one/third release CO3•- and NO2• that oxidize the substrate. According to our hypothesis, ONOOCO2- is formed much faster, is relatively long-lived, and may also be an oxidant; the limited yield is the result of ONOOCO2- being scavenged by a second CO2 under conditions of a high CO2 concentration. We disagree with the first hypothesis for three reasons: First, it is based on an estimated K for the reaction of ONOO- with CO2 to form ONOOCO2- of ∼1 M-1, while experiments yield a value of 4.5 × 103 M-1. Second, we argue that the solvent cage as proposed is physically not realistic. Given the less than diffusion-controlled rate constant of CO3•- with NO2•, all radicals would escape from the solvent cage. Third, the reported ∼33% radical is not supported by an experiment where mass balance was established. We propose here a hybrid mechanism. After formation of ONOOCO2-, it undergoes homolysis to yield CO3•- with NO2•, or, depending on [CO2], it is scavenged by a second CO2; CO3•- oxidizes ONOO-, if present. These reactions allow us to successfully simulate the reaction of ONOO- with CO2 over a wide range of ONOO-/CO2 ratios. At lower ratios, fewer radicals are formed, while at higher ratios, radical yields between 30% and 40% are predicted. The differences in radical yields reported may thus be traced to the experimental ONOO-/CO2 ratios. Given a physiological [CO2] of 1.3 mM, the yield of CO3•- and NO2• is 19%, and lower if ONOOCO2- has a significant reactivity of its own.

Reaction of CO2 with ONOO-: One Molecule of CO2 Is Not Enough.

With CO2 present in excess, ONOO- reacts to form an adduct in solution and in the solid state, most likely ONOOCO2-. In solution, the adduct appears within 2 ms and absorbs at 300 with an extinction coefficient, which is either 50% or 100% (preferred) of that of ONOO-, 1.70 × 103 M-1 cm-1, and at 685 nm with an extinction coefficient of 85 M-1 cm-1. When solid [(CH3)4N][ONOO] is treated with CO2, these two maxima are red-shifted by 30-50 nm. The equilibrium constant for adduct formation in solution is (4.5 ± 0.5) × 103 M. The adduct reacts further with another CO2 at a rate of (2.6 ± 0.8) × 104 M-1 s-1 and produces 2 CO2 and NO3-. Thermochemical calculations show that ΟΝΟΟCO2- is a strong two-electron oxidizing agent, E°(ONOOCO2-, H+/NO2-, HCO3-) = +1.28 V at pH 7 and an even stronger one-electron oxidizing agent E°'(ONOOCO2-, H+/NO2•, HCO3-) = +1.51 V at pH 7. The extent of homolysis, that is formation of NO2• and CO3•-, is small, slightly less than 1% relative to ONOO- at the physiological concentration of CO2 of 1.3 mM in plasma. Thus, ONOOCO2- is more relevant than CO3•- under in vivo conditions.

Rust never sleeps: The continuing story of the Iron Bolt.

Since 1981, Gordon Research Conferences have been held on the topic of Oxygen Radicals on a biennial basis, to highlight and discuss the latest cutting edge research in this area. Since the first meeting, one special feature of this conference has been the awarding of the so-called Iron Bolt, an award that started in jest but has gained increasing reputation over the years. Since no written documentation exists for this Iron Bolt award, this perspective serves to overview the history of this unusual award, and highlights various experiences of previous winners of this "prestigious" award and other interesting anecdotes.

Jumpstarting the cytochrome P450 catalytic cycle with a hydrated electron.

Cytochrome P450cam (CYP101Fe3+) regioselectively hydroxylates camphor. Possible hydroxylating intermediates in the catalytic cycle of this well-characterized enzyme have been proposed on the basis of experiments carried out at very low temperatures and shunt reactions, but their presence has not yet been validated at temperatures above 0 °C during a normal catalytic cycle. Here, we demonstrate that it is possible to mimic the natural catalytic cycle of CYP101Fe3+ by using pulse radiolysis to rapidly supply the second electron of the catalytic cycle to camphor-bound CYP101[FeO2]2+ Judging by the appearance of an absorbance maximum at 440 nm, we conclude that CYP101[FeOOH]2+ (compound 0) accumulates within 5 µs and decays rapidly to CYP101Fe3+, with a k440 nm of 9.6 × 104 s-1 All processes are complete within 40 µs at 4 °C. Importantly, no transient absorbance bands could be assigned to CYP101[FeO2+por•+] (compound 1) or CYP101[FeO2+] (compound 2). However, indirect evidence for the involvement of compound 1 was obtained from the kinetics of formation and decay of a tyrosyl radical. 5-Hydroxycamphor was formed quantitatively, and the catalytic activity of the enzyme was not impaired by exposure to radiation during the pulse radiolysis experiment. The rapid decay of compound 0 enabled calculation of the limits for the Gibbs activation energies for the conversions of compound 0 → compound 1 → compound 2 → CYP101Fe3+, yielding a ΔG‡ of 45, 39, and 39 kJ/mol, respectively. At 37 °C, the steps from compound 0 to the iron(III) state would take only 4 µs. Our kinetics studies at 4 °C complement the canonical mechanism by adding the dimension of time.

Low-Temperature Trapping of Intermediates in the Reaction of NO• with O2.

The autoxidation of NO• was studied in glass-like matrices of 2-methylbutane at 110 K and in a 8:3 v/v mixture of 2,2-dimethylbutane and n-pentane (rigisolve) at 80-90 K, by letting gaseous NO• diffuse into these solvents that were saturated with O2. In 2-methyllbutane, we observed a red compound. However, in rigisolve at 85-90 K, a bright yellow color appears that turns red when the sample is warmed by 10-20 K. The new yellow compound is a precursor of the red one and also diamagnetic. The UV-vis spectrum of the yellow compound contains a band which resembles that present in ONOO-. Because the red and yellow intermediates are not paramagnetic, we postulate that O═N-O-O• is in close contact with NO•, or with another O═N-O-O•. Diffusion of gaseous O2 into rigisolve saturated with NO• does not produce a color; however, a weak EPR signal (g = 2.010) is observed. This signal most likely indicates the presence of ONOO•. These findings complement our earlier observation of a red color at low temperatures and the presence of ONOO• in the gas phase (Galliker, B.; Kissner, R.; Nauser, T.; Koppenol, W. H. Chem. Eur. J. 2009, 15, 6161-6168), and they indicate that the termolecular autoxidation of nitrogen monoxide proceeds via the intermediate ONOO• and not via N2O2.

Signaling by sulfur-containing molecules. Quantitative aspects.

There is currently interest in sulfur-containing molecules that may or may not play a role in signaling. We have collected relevant thermodynamic data, namely standard Gibbs energies of formation and electrode potentials at pH 7, and used these to construct a Frost diagram. Thermodynamic data not available in the literature could be estimated with reasonable confidence. At pH 7, the electrode potential of the RSS/RSS- couple is +0.68 V, 0.28 V less than that of the RS, H+/RSH couple. S2- is unstable with respect to HSS- and S2. Generally, polysulfur compounds, with the exception of RSSR, are thermodynamically unstable with respect to disproportionation and ultimately lead to formation of kinetically inert S8(s). About thermoneutral is the formation of RSS- from RSSR and HS-, but formation of HNO from HS- and SNO-, and from HS- and RSNO, is unfavorable. The formation of SSNO-in vivo is kinetically unlikely.

Electrode Potentials of l-Tryptophan, l-Tyrosine, 3-Nitro-l-tyrosine, 2,3-Difluoro-l-tyrosine, and 2,3,5-Trifluoro-l-tyrosine.

Electrode potentials for aromatic amino acid radical/amino acid couples were deduced from cyclic voltammograms and pulse radiolysis experiments. The amino acids investigated were l-tryptophan, l-tyrosine, N-acetyl-l-tyrosine methyl ester, N-acetyl-3-nitro-l-tyrosine ethyl ester, N-acetyl-2,3-difluoro-l-tyrosine methyl ester, and N-acetyl-2,3,5-trifluoro-l-tyrosine methyl ester. Conditional potentials were determined at pH 7.4 for all compounds listed; furthermore, Pourbaix diagrams for l-tryptophan, l-tyrosine, and N-acetyl-3-nitro-l-tyrosine ethyl ester were obtained. Electron transfer accompanied by proton transfer is reversible, as confirmed by detailed analysis of the current waves, and because the slopes of the Pourbaix diagrams obey Nernst's law. E°'(Trp(•),H(+)/TrpH) and E°'(TyrO(•),H(+)/TyrOH) at pH 7 are 0.99 ± 0.01 and 0.97 ± 0.01 V, respectively. Pulse radiolysis studies of two dipeptides that contain both amino acids indicate a difference in E°' of approximately 0.06 V. Thus, in small peptides, we recommend values of 1.00 and 0.96 V for E°'(Trp(•),H(+)/TrpH) and E°'(TyrO(•),H(+)/TyrOH), respectively. The electrode potential of N-acetyl-3-nitro-l-tyrosine ethyl ester is higher, while because of mesomeric stabilization of the radical, those of N-acetyl-2,3-difluoro-l-tyrosine methyl ester and N-acetyl-2,3,5-trifluoro-l-tyrosine methyl ester are lower than that of tyrosine. Given that the electrode potentials at pH 7 of E°'(Trp(•),H(+)/TrpH) and E°'(TyrO(•),H(+)/TyrOH) are nearly equal, they would be, in principle, interchangeable. Proton-coupled electron transfer pathways in proteins that use TrpH and TyrOH are thus nearly thermoneutral.

Hydrogen peroxide, from Wieland to Sies.

A history of the formation of hydrogen peroxide in vivo is presented, starting with the discovery of catalase. The first hypothesis was formulated by Heinrich Wieland, who assumed that dioxygen reacted directly with organic molecules. This view was strongly criticised by Otto Warburg, Helmut Sies' academic grandfather. The involvement of hydrogen peroxide in physiological processes was investigated by Theodor Bücher, the "Doktorvater" of Helmut. Helmut's research made it possible to quantitate hydrogen peroxide in tissues.