Bimolecular Fashion Research Articles

Selective Cobalt-Mediated Formation of Hydrogen Peroxide from Water under Mild Conditions via Ligand Redox Non-Innocence.

Despite the broad importance of hydrogen peroxide (H2O2) in oxidative transformations, there are comparatively few viable routes for its production. The majority of commercial H2O2 is currently produced by the stepwise reduction of dioxygen (O2) via the anthraquinone process, but direct electrochemical formation from water (H2O) would have several advantages─namely, avoiding flammable gases or stepwise separations. However, the selective oxidation of H2O to form H2O2 over the thermodynamically favored product of O2 is a difficult synthetic challenge. Here, we present a molecular H2O oxidation system with excellent selectivity for H2O2 that functions both stoichiometrically and catalytically. We observe high efficiency for electrocatalytic H2O2 production at low overpotential with no O2 observed under any conditions. Mechanistic studies with both calculations and kinetic analyses from isolated intermediates suggest that H2O2 formation occurs in a bimolecular fashion via a dinuclear H2O2-bridged intermediate with an important role for a redox non-innocent ligand. This system showcases the ability of metal-ligand cooperativity and strategic design of the secondary coordination sphere to promote kinetically and thermodynamically challenging selectivity in oxidative catalysis.

Electrophile‐Induced Nucleophilic Substitution of the nido‐Dicarbaundecaborate Anion nido‐7,8‐C2B9H12− by Conjugated Heterodienes

Substitution of the dicarbaundecaborate anion nido-7,8-C2B9H12(-) (1) by precise hydride abstraction followed by nucleophilic attack usually leads to symmetric products 10-R-nido-7,8-C2B9H11. However, thioacetamide (MeC(S)NH2) as nucleophile and acetone/AlCl3 as hydride abstractor gave asymmetric 9-[MeC(NHiPr)S]-nido-7,8-C2B9H11 (2), whereas N,N-dimethylthioacetamide (MeC(S)NMe2) gave the expected symmetric 10-[MeC(NMe2)S]-nido-7,8-C2B9H11 (4). For the formation of 2, acetone and thioacetamide are assumed to give the intermediate MeC(S)N(CMe2) (3), which then attacks 1 with formation of 2. Similarly, reaction of acetyliminium chloride [MeC(O)NH(CPh2)]Cl (5) with 1 in THF gave a mixture of 9- and 10-substituted [MeC(NHCHPh2)O]-nido-7,8-C2B9H11 (6 and 7, respectively). These reactions are the first examples in which compounds (here heterodienes) that unite the functionalities of both hydride acceptor and nucleophilic site react with 1 in a bimolecular fashion. Furthermore, the analogous reaction of 1 and 5 (in an equilibrium mixture with acetyl chloride and benzophenone imine) in MeCN afforded 10-[MeC(NCPh2)NH]-nido-7,8-C2B9H11 (8) and MeC(O)NHCHPh2 (9).

The rapid decomposition of water-soluble p-nitrophenyl disulfides induced by aqueous sodium hydroxide

A pair of p-nitrophenyl disulfide carboxylic acids are shown to react rapidly in aqueous base to give p-nitrothiophenol-derived anions. Experimental and PM3 computational results support the view that hydroxide ions attack, in a bimolecular fashion, at the appropriate sulfenyl sulfur to produce p-nitrophenyl mercaptide anions.

Selection of ribozymes that catalyse multiple-turnover Diels–Alder cycloadditions by using in vitro compartmentalization

In vitro compartmentalization (IVC) has previously been used to evolve protein enzymes. Here, we demonstrate how IVC can be applied to select RNA enzymes (ribozymes) for a property that has previously been unselectable: true intermolecular catalysis. Libraries containing 10(11) ribozyme genes are compartmentalized in the aqueous droplets of a water-in-oil emulsion, such that most droplets contain no more than one gene, and transcribed in situ. By coencapsulating the gene, RNA, and the substrates/products of the catalyzed reaction, ribozymes can be selected for all enzymatic properties: substrate recognition, product formation, rate acceleration, and turnover. Here we exploit the complementarity of IVC with systematic evolution of ligands by exponential enrichment (SELEX), which allows selection of larger libraries (>/=10(15)) and for very small rate accelerations (k(cat)/k(uncat)) but only selects for intramolecular single-turnover reactions. We selected approximately 10(14) random RNAs for Diels-Alderase activity with five rounds of SELEX, then six to nine rounds with IVC. All selected ribozymes catalyzed the Diels-Alder reaction in a truly bimolecular fashion and with multiple turnover. Nearly all ribozymes selected by using eleven rounds of SELEX alone contain a common catalytic motif. Selecting with SELEX then IVC gave ribozymes with significant sequence variations in this catalytic motif and ribozymes with completely novel motifs. Interestingly, the catalytic properties of all of the selected ribozymes were quite similar. The ribozymes are strongly product inhibited, consistent with the Diels-Alder transition state closely resembling the product. More efficient Diels-Alderases may need to catalyze a second reaction that transforms the product and prevents product inhibition.

Gas-Phase Reactions of Carbon Dioxide with Atomic Transition-Metal and Main-Group Cations: Room-Temperature Kinetics and Periodicities in Reactivity

The chemistry of carbon dioxide has been surveyed systematically with 46 atomic cations at room temperature using an inductively-coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. The atomic cations were produced at ca. 5500 K in an ICP source and allowed to cool radiatively and to thermalize by collisions with Ar and He atoms prior to reaction downstream in a flow tube in helium buffer gas at 0.35 +/- 0.01 Torr and 295 +/- 2 K. Rate coefficients and products were measured for the reactions of first-row atomic ions from K(+) to Se(+), of second-row atomic ions from Rb(+) to Te(+) (excluding Tc(+)), and of third-row atomic ions from Cs(+) to Bi(+). CO(2) was found to react in a bimolecular fashion by O atom transfer only with 9 early transition-metal cations: the group 3 cations Sc(+), Y(+), and La(+), the group 4 cations Ti(+), Zr(+), and Hf(+), the group 5 cations Nb(+) and Ta(+), and the group 6 cation W(+). Electron spin conservation was observed to control the kinetics of O atom transfer. Addition of CO(2) was observed for the remaining 37 cations. While the rate of addition was not measurable some insight was obtained into the standard free energy change, DeltaG(o), for CO(2) ligation from equilibrium constant measurements. A periodic variation in DeltaG(o) was observed for first row cations that is consistent with previous calculations of bond energies D(0)(M(+)-CO(2)). The observed trends in D(0) and DeltaG(o) are expected from the variation in electrostatic attraction between M(+) and CO(2) which follows the trend in atomic-ion size and the trend in repulsion between the orbitals of the atomic cations and the occupied orbitals of CO(2). Higher-order CO(2) cluster ions with up to four CO(2) ligands also were observed for 24 of the atomic cations while MO(2)(+) dioxide formation by sequential O atom transfer was seen only with Hf(+), Nb(+), Ta(+), and W(+).

Mechanism of cysteine desulfurase Slr0387 from Synechocystis sp. PCC 6803: kinetic analysis of cleavage of the persulfide intermediate by chemical reductants.

Cysteine desulfurases (CDs) are pyridoxal-5'-phosphate (PLP)-dependent enzymes that cleave sulfur from cysteine via an enzyme cysteinyl persulfide intermediate. In vitro studies of these enzymes have generally employed dithiothreitol as a cosubstrate to reductively cleave the persulfide intermediate, and it has been suggested that persulfide cleavage is the rate-limiting step for catalysis. In this study, the kinetics and mechanisms of cleavage of the persulfide intermediate in Slr0387 (CD-0387), a sequence group I (NifS/IscS-like) cysteine desulfurase from Synechocystis sp. PCC 6803, by physiological and nonphysiological reductants have been examined, and the extent to which this step is rate-limiting for catalysis has been determined. The observations that dithiols such as dithiothreitol (DTT) cleave the persulfide with approximately 100-fold greater efficiency than structurally similar monothiols such as 2-mercaptoethanol (2-ME), that cleavage by DTT exhibits saturation kinetics, and that the dependence of the observed first-order rate constant for persulfide cleavage by DTT on the concentration of the dithiol corresponds precisely with that for formation of a complex between DTT and the PLP cofactor of the resting enzyme suggest that persulfide cleavage by dithiols occurs by prior formation of a complex, in which addition of one thiol to the cofactor positions the second thiol for attack. This conclusion and the observation that a second molecule of L-cysteine can bind to the cofactor in the persulfide form of CD-0387 explain why several CDs are subject to potent inhibition by L-cysteine during turnover with DTT: binding of L-cysteine prevents formation of the PLP-DTT adduct and renders the dithiol no better than a monothiol, which must react with the persulfide in bimolecular fashion. Consistent with this rationale, catalysis by CD-0387 with 2-ME as cosubstrate, while less efficient, is not subject to potent inhibition by L-cysteine. The similarity of the maximum rate constant for persulfide cleavage by DTT to k(cat) suggests that persulfide cleavage is, in fact, primarily rate-determining, and this conclusion is confirmed by the observation that k(cat) is approximately 10-fold greater when tris-(2-carboxyethyl)phosphine (TCEP), the most efficient persulfide cleaver identified, is used as the reducing cosubstrate. The faster turnover with TCEP provides a chemical model for activation of CD-0387 and other CDs by the presence of accessory factors that serve as efficient acceptors of the persulfide sulfur.

Reaction of Ar2+ with C60 to produce C603+: first observation of double electron-transfer ionization?

The gas-phase reaction of translationally cold Ar2+ (1S0) ions with C60 has been studied using a selected-ion flow tube (SIFT) apparatus. Amongst several product channels detected, which arose from competing reactions of contaminants (N2, O2, H2O) and from the He buffer gas itself as well as from the target process Ar2++C60, we observed an apparently minor but unambiguous signal due to a C603+ product ion. We surmise that this product channel arises from Ar2+(1S0)+C60 in a bimolecular fashion, most probably by an initial (and highly exothermic) direct double electron transfer leading to the generation of either highly excited (C602+)∗ or of electronically excited (Ar)∗. Trication formation from either of these possible high-energy intermediates would then follow according to either an autoionization or a Penning ionization mechanism.

Effect of zero- to one-dimensional transformation on multiparticle Auger recombination in semiconductor quantum rods.

We study the effect of the zero- to one-dimensional (1D) transformation on multiparticle Auger recombination using a series of elongated semiconductor nanocrystals (quantum rods). We observe a transition from the three- to two-particle recombination process as the nanocrystal aspect ratio is increased. This transition indicates that in the 1D confinement limit, Auger decay is dominated by Coulomb interactions between 1D excitons that recombine in a bimolecular fashion. One consequence of this effect is strongly reduced decay rates of higher multiparticle states that lead to increased optical-gain lifetimes and efficient light amplification due to involvement of excited electronic states.

Model for the Binding of the Inactivation N−Terminal to the Ion Pore of Shaker Potassium Channel: Both Electrostatic Attraction and Covalent Linkage Are Required for Rapid Inactivation

A model is presented for relating the binding of the inactivation N-terminal to the ion pore of the Shaker potassium channel (ShB) to the bimolecular binding of the N-terminal peptide with the deletion mutant ShB¢646. The binding site is modeled as a small patch on the surface of the channel protein, to which the N-terminal “inactivation ball” is tethered by a flexible linker. The potential energy due to electrostatic interactions between the channel and the N-terminal is ‚U(r) )- Qexp[-(r - a)/I]/(1 + a/I)r, where a is the closest approach distance and I is the screening length determined by the ionic strength. The probability density for the endto-end vector of the flexible linker (with L residues) is taken from a previous study [Zhou, J. Phys. Chem. B 2001, 105, 6763] as p(r) ) (3/4﷿lpbL) 3/2 exp(-3r 2 /4lpbL)(1-5lp/4bL + ...). The intramolecular binding rate constant kon in of the intact ShB is related to the bimolecular binding rate constant kon bi via kon in ) kon bi p(a)/ sexp[-‚U(r)]p(r)dV. The model rationalizes a number of important experimental observations. (1) The weaker ionic strength dependence of kon in is quantitatively reproduced by the relation between kon in and kon bi . (2) The linker length dependence of kon in (observed when the linker length is reduced by deletion and extended by insertion) is qualitatively predicted by the L dependence of p(r). (3) The fact that koff in ) koff bi and both are insensitive to the change in ionic strength is due to the stereospecificity of the binding site. If the binding of the activation N-terminal were to occur in a bimolecular fashion, the millisecond inactivation time would have required the presence of the N-terminal at a concentration of 0.2 mM, even after considering the binding rate enhancement by the electrostatic attraction of the channel pore. The difficult task of maintaining such a high concentration underscores the importance of covalently linking the inactivation peptide to the ion channel.

A point mutation in the maxi-K clone dSlo forms a high affinity site for charybdotoxin

This work investigated the interaction of CTX with two cloned analogues of the maxi-K channel, dSlo and hSlo. dSlo has been reported to be CTX insensitive. Single channel analysis revealed that dSlo was weakly blocked by the toxin, with a very high K D of 5.8 μM. The hSlo channels bound wild-type, recombinant CTX with high affinity and in a bimolecular fashion, and displayed a half-blocking concentration ( K D) of 36 nM. A glutamate residue was substituted for the wild-type threonine at position 290 in dSlo. The mutant channel was expressed in COS-7 cells and reconstituted into lipid bilayers for single channel analysis. The mutant channel bound wild-type, recombinant CTX with high affinity and in a bimolecular fashion, and displayed a half-blocking concentration ( K D) of 23 nM. Changing just one residue from threonine to glutamate at position 290 in dSlo changed the affinity of the channel’s CTX-receptor over 100-fold. Kinetic analysis revealed that this large increase in affinity was due to decreasing the dissociation rate of the toxin. These results suggest that a CTX receptor does exist in the dSlo channel mouth and that the threonine at position 290 destabilizes the toxin on the binding site.

Proton probing of the charybdotoxin binding site of Shaker K+ channels.

We have investigated the interaction of charybdotoxin (CTX) with Shaker K channels. We substituted a histidine residue for the wild-type phenylalanine (at position 425) in an inactivation-removed channel. The nature of the imidazole ring of the histidine provides the ability to change the charge on this amino acid side chain with solution hydrogen ion concentration. Wild-type, recombinant CTX blocked wild-type Shaker channels in a bimolecular fashion with a half-blocking concentration (Kd) of 650 nM (at a membrane potential of 0 mV). The F425H mutant channels were much more sensitive to CTX block with an apparent Kd (at pH 7.0) of 75 nM. Block of F425H but not wild-type channels was strongly pH sensitive. A pH change from 7 to 5.5 rendered the F425H channels >200-fold less sensitive to CTX. The pH dependence of CTX block was steeper than expected for inhibition produced by H+ ions binding to identical, independent sites. The data were consistent with H+ ions interacting with subunits of the channel homotetrameric structure. The in situ pK for the imidazole group on the histidine at channel position 425 was determined to be near 6.4 and the dissociation constant for binding of toxin to the unprotonated channel was near 50 nM. We estimate that the binding of a H+ ion to each subunit adds 0.8 kcal/mol or more of interaction energy with CTX. We used mutant toxins to test electrostatic and steric interactions between specific CTX residues and channel position 425. Our results are consistent with a model in which protons on F425H channel subunits interact with three positive charges on CTX at an effective distance 6-7 A from this channel position.

Zur Katalyse der stereospezifischen Butadienpolymerisation mit den Katalysatorsystemen aus Nd(η3‐C3H5)3 · dioxan und methylaluminoxan (MAO) sowie Hexaisobutylaluminoxan (HIBAO)

AbstractThe activation of the tris(allyl)neodymium complex Nd(η3‐C3H5)3 · dioxane with alkylaluminoxanes (MAO or HIBAO) results in highly selective catalysts for the 1,4‐cis‐polymerization of butadiene (cis‐selectivity up to 80%). Under standard conditions (50°C, toluene), the turnover frequency (TOF) of the catalyst/MAO system amounts to 10–15000 mol butadiene/(mol Nd · h). Molecular weight determinations indicate the formation of only one polymer chain per neodymium center as in a living polymerization reaction, and for the catalyst/HIBAO system the rate law rp = kp [Nd][C4H6] with kp = 8,7 · 10−2 mol/(L · s) (at 25°C) has been derived. As the catalytically active species, a cationic monobutenyl neodymium(III) complex is discussed, which is stabilized through coordinative interaction with the counter anion as well as the growing polybutadiene chain. This cationic complex reacts under insertion with butadiene in a bimolecular fashion.

Ebselen as a Glutathione Peroxidase Mimic and as a Scavenger of Peroxynitrite

Publisher Summary The compound ebselen is considered capable of contributing to the antioxidant defense in tissues. Many of the biological properties of ebselen were explainable on the basis of its property as an enzyme mimic, carrying out the function of the selenoenzyme, GSH peroxidase (GPx), and of phospholipid hydroperoxide GSH peroxidase (PHGPx). It is obvious that the biological functions of ebselen might extend to other reactions in addition to the reduction of hydroperoxides. Biologically occurring selenoproteins deactivates peroxynitrite. A further aspect resides in the high reactivity of ebselen with thiol groups, leading to the inactivation or inhibition of enzymes. This chapter discusses synthesis of ebselen, molecular activities of ebselen, activity in biological model systems, metabolism, clinical assessment, and other selenoorganic compounds. An assay systems for the detection of GPx-like activity have been described—coupled enzymatic assay, assay of GSH removal, and assay of hydroperoxide removal. There is scavenging of peroxynitrite—peroxynitrite (ONOO-IONOOH), a strong oxidizing species, is generated in inflammatory processes from the superoxide anion radicaland nitric oxide. The formation of peroxynitrite by various cell types, such as macrophages and endothelial cells has been detected. The peroxynitrite-induced luminol chemiluminescence emitted from Kupffer cells was inhibited in the presence of low concentrations of ebselen, and ebselen rapidly reacts with peroxynitrite in a bimolecular fashion, yielding the selenoxide of the parent molecule, ebselenSe-oxide[2-phenyl-l,2-benzisoselenazol-3(2H)-one I-oxide], as the sole selenium-containing product at 1:1 stoichiometry. Radical scavenging and singlet oxygen quenching have been mentioned, along with ebselen's reactivity with protein thiols. GPx-like activity of ebselen is involved in the protection of intact cells against lipid peroxidation. Ebselen protects against different types of oxidative attack by acting in its capacity as a GPx mimic, and, in particular, as a PHGPx mimic. It is capable of preventing copper-dependent LDL oxidation in the presence of GSH.

Cyclohexadienes from the rearrangement of O-aroyl-N-acetyl-N-(2,6-dimethylphenyl)hydroxylamines. Reaction in aqueous solution to meta- and para-substituted 2,6-dimethylacetanilides

O-Aroyl-N-acetyl-N-(2,6-dimethylphenyl)hydroxylamines (aroyl = benzoyl, 3-nitrobenzoyl, and pentafluorobenzoyl) rearrange in acetonitrile to 1,5-dimethyl-5-aroyloxy-6-N-acetyliminocyclohexa-1,3-dienes that can be isolated as pure compounds. These cyclohexadienes react in aqueous solutions, producing m-aroyloxy- and m-hydroxy-2,6-dimethylacetanilides in an H+-catalysed reaction and the corresponding para products in a non-catalysed reaction. Analysis of the effect of the aroyloxy group suggests that the latter reaction involves heterolysis to a reactive, non-selective, ion pair that collapses to the para product, or reacts at this position with water. The meta products arise from the N-protonated cyclohexadiene reacting with solvent in a conjugate addition to give the meta-substituted phenol, or in an intramolecular reaction with the carbonyl group to give the rearranged ester. This latter reaction is proposed to proceed via an intermediate 1,3-dioxolan-2-ylium ion. The nucleophiles azide, phenylsulfinate and the methyl thioglycolate anion react in a bimolecular fashion to give meta-substituted 2,6-dimethylacetanilides. With phenylsulfinate this requires H+, while methyl thioglycolate anion reacts with the neutral cyclohexadiene. Azide exhibits reaction by both processes. These reactions are proposed to involve conjugate additions, either to the N-protonated cyclohexadiene, or, with better nucleophiles, directly on the neutral compound. These cyclohexadienes model intermediates that may form during the metabolism of certain carcinogenic amines. These results establish the presence of three electrophilic species capable of reacting with cellular nucleophiles—the highly reactive and non-selective cation formed in the heterolysis, the less reactive and more selective N-protonated species, and the neutral cyclohexadiene itself. The last electrophile is relatively unreactive, but can be a target of very good nucleophiles such as thiol anions.

Oxidation of oxymyoglobin to metmyoglobin with hydrogen peroxide: involvement of ferryl intermediate.

Hydrogen peroxide, one of the potent oxidants in muscle tissues, can induce very rapid oxidation of oxymyoglobin (MbO2) to metmyoglobin (metMb) with an apparent rate constant of 7.5 X 10(4) h-1 M-1 (i.e., 20.8 s-1 M-1) over the wide pH range of 5.5-10.2 in 0.1 M buffer at 25 degrees C. Its molecular mechanism, however, is quite different from that of the autoxidation of MbO2 to metMb. Kinetic analysis has revealed that the hydrogen peroxide oxidation proceeds through the formation of ferryl-Mb(IV) from deoxy-Mb(II), which is in equilibrium with MbO2, by a two-equivalent oxidation with H2O2. Once the ferryl species is formed, it reacts rapidly with another deoxy-Mb(II) in a bimolecular fashion so as to yield 2 mol of metMb(III). Under physiological conditions, the rate-determining step was the oxidation of the deoxy species by H2O2, its rate constant being estimated to be on the order of 3.6 X 10(3) s-1 M-1 at 25 degrees C. These findings leads us to the view that a good supply of dioxygen provides rather an important defense against the oxidation of myoglobin with hydrogen peroxide in cardiac and skeletal muscle tissues.

Evidence for a reactive cysteine at the nucleotide binding site of spinach ribulose-5-phosphate kinase

Ribulose-5-phosphate kinase from spinach was rapidly inactivated by N-bromoacetylethanolamine phosphate in a bimolecular fashion with a k 2 of 2.0 m −1 s −1 at 2 °C and pH 8.0. Ribulose 5-phosphate had little effect on the rate of inactivation, whereas complete protection was afforded by ADP or ATP. The extent of incorporation as determined with 14C-labeled reagent was about 1 molar equivalent per subunit in the presence of ATP with full retention of enzymatic activity, and about 2 molar equivalents per subunit in the completely inactivated enzyme. Amino acid analyses of enzyme derivatized with 14C-labeled reagent reveal that all of the covalently incorporated reagent was associated with cysteinyl residues. Hence two sulfhydryls are reactive, but the inactivation correlates with alkylation of one cysteinyl residue at or near the enzyme's nucleotide binding site. The kinase was also extremely sensitive to the sulfhydryl reagents 5,5′-dithiobis(2-nitrobenzoic acid) and N-ethylmaleimide. The reactive sulfhydryl groups are likely those generated by reduction of a disulfide during activation.

Base catalysed rearrangements involving ylide intermediates. Part 9. The rearrangement reactions of cyclic allylic ammonium and sulphonium ylides

The allylic ammonium ylides (22) and (47) and sulphonium ylide (53) are isolable, usually as crystalline solids, because their [3,2] sigmatropic rearrangements are inhibited by the strain associated with the bicyclic transition state of a concerted reaction mechanism. The tetrahydropyridinium (22) and dihydrothiopyranium (29) ylides undergo a thermal [3,2] rearrangement, but, the pyrrolinium ylide (47) rearranges by a [1,2], rather than a [3,2], sigmatropic pathway. The dihydrothiophenium ylide (53) does not undergo either a [1,2] or [3,2] sigmatropic rearrangement but instead reacts in a bimolecular fashion to give eventually buta-1,3-diene, 2,5-dihydrothiophen, and the heterocycle (55). The [3,2] rearrangements of the ylides (22) and (29) follow predominantly an endo-pathway leading to a single diastereoisomer of the products (23) and (31); this strong endo preference is not shown by analogous acyclic ammonium ylides.

THE REDUCING RADICALS PRODUCED IN WATER RADIOLYSIS: SOLUTIONS OF OXYGEN—HYDROGEN PEROXIDE—HYDROGEN ION1

The reducing radical H or H2O- arising from the gamma radiolysis of water is shown to react competitively with dissolved O2 and H2O2; the ratio of the respective rate constants is 2.0. lt also reacts in a simple competitive bimolecular fashion with hydrogen ion, H+, to give an acid form of the radical which reacts several thousand ttmes as fast with O2 as with H2O2. The rate constants for reaction of H with O2 and with H+ are equal. The yield of H deduced from the O2-H2O2 competition in neutral solution is 2.85, which is equal to the standard value of the total yield of reducing radicals. Thus essentially none of the reducing radicals appear to be generated initially in the acid form, a conclusion which disagrees with recently proposed interpretations of some other radiolytic reactions.