Cytochrome P450 119 Compounds I Formed by Chemical Oxidation and Photooxidation Are the Same Species

Compound I from cytochrome P450 119 prepared by the photooxidation method involving peroxynitrite oxidation of the resting enzyme to Compound II followed by photooxidation to Compound I was compared to Compound I generated by m-chloroperoxybenzoic acid (MCPBA) oxidation of the resting enzyme. The two methods gave the same UV/Visible spectra, the same products from oxidations of lauric acid and palmitic acid and their (ω-2,ω-2,ω-3,ω-3)-tetradeuterated analogues, and the same kinetics for oxidations of lauric acid and caprylic acid. The experimental identities between the transients produced by the two methods leave no doubt that the same Compound I species is formed by the two methods.

Tear fluid extracellular DNA: diagnostic and therapeutic implications in dry eye disease

PURPOSE

To determine the abundance of extracellular DNA (eDNA) in tear fluid of patients with dry eye disease (DED) and to report clinical outcomes after DNase I eyedrops use to reduce excessive tear fluid eDNA.

METHODS

Tear fluid was collected from healthy control subjects and patients with DED. The eDNA abundance was determined with the PicoGreen dye assay. The DED symptoms and clinical signs were recorded and correlated with eDNA abundance. Two patients with DED having excessive eDNA in tear fluid were treated with DNase I eyedrops.

RESULTS

The PicoGreen dye assay measures tear fluid eDNA abundance after a 2-minute incubation time. With longer incubations, admixed cells also contribute to eDNA measurements. The mean (SE) eDNA abundance in healthy control subjects' tear fluid was 1.4 (0.2) μg/mL. The mean (SE) eDNA abundance in tear fluid of patients with nonautoimmune DED, autoimmune DED, and graft versus host disease was significantly higher: the values were 2.9 (0.6), 5.2 (1.2), and 9.1 (2.3) μg/mL, respectively (P < 0.05). In most of these patients, the PicoGreen dye kinetic assay of tear fluid showed an increase in fluorescence signal due to the presence of viable cells in tear fluid. Tear fluid eDNA had the best correlation with corneal Rose Bengal staining (r = 0.55). Treatment of patients having DED with DNase I eyedrops reduced eDNA abundance, abrogated signal increase, and improved comfort.

CONCLUSIONS

Excessive eDNA is present in tear fluid of patients with dry eyes. A novel therapeutic approach for managing DED may be to measure eDNA abundance in tear fluid with the PicoGreen dye assay and reduce excessive amounts with DNase I eyedrops.

Bioluminescent detection of peroxynitrite with a boronic acid-caged luciferin

Peroxynitrite, a highly reactive biological oxidant, is formed under pathophysiologic conditions from the diffusion-limited reaction of nitric oxide and superoxide radical anion. Peroxynitrite has been implicated as the mediator of nitric oxide toxicity in many diseases and as an important signaling disrupting molecule (L. Liaudet et al., Front. Biosci.14, 4809-4814, 2009) [1]. Biosensors effective at capturing peroxynitrite in a specific and fast enough manner for detection, along with readouts compatible with in vivo studies, are lacking. Here we report that the boronic acid-based bioluminescent system PCL-1 (peroxy-caged luciferin-1), previously reported as a chemoselective sensor for hydrogen peroxide (G.C. Van de Bittner et al., Proc. Natl. Acad. Sci. USA107, 21316-21321, 2010) [2], reacts with peroxynitrite stoichiometrically with a rate constant of 9.8±0.3×10(5)M(-1)s(-1) and a bioluminescence detection limit of 16nM, compared to values of 1.2±0.3M(-1)s(-1) and 231nM for hydrogen peroxide. Further, we demonstrate bioluminescent detection of peroxynitrite in the presence of physiological competitors: carbon dioxide, glutathione, albumin, and catalase. We also demonstrate the utility of this method to assess peroxynitrite formation in mammalian cells by measuring peroxynitrite generated under normal culture conditions after stimulation of macrophages with bacterial endotoxin lipopolysaccharide. Thus, the PCL-1 method for measuring peroxynitrite generation shows superior selectivity over other oxidants under in vivo conditions.

Ocular surface extracellular DNA and nuclease activity imbalance: a new paradigm for inflammation in dry eye disease

PURPOSE

We determined whether nucleases are deficient in the tear fluid of dry eye disease (DED) patients, and whether this causes extracellular DNA (eDNA) and neutrophil extracellular trap (NET) accumulation in the precorneal tear film, thus causing ocular surface inflammation.

METHODS

Exfoliated cells adhered to Schirmer test strips were collected on glass slides, and immunofluorescence confocal microscopy was used to evaluate neutrophils, eDNA, NETs, and their molecular components. Similar experiments were performed with mucoid films collected from the inferior conjunctival fornix or bulbar conjunctiva. We used quantitative PCR to evaluate eDNA signaling pathways and inflammatory cytokine expression. We also determined the amount of ocular surface eDNA and evaluated tear fluid nuclease activity.

RESULTS

eDNA, NETs, and neutrophils were present on the ocular surface in DED patients and abundant in mucoid films. NETs consisted of eDNA, histones, cathelicidin, and neutrophil elastase. Tear fluid nuclease activity was decreased significantly in DED patients, whereas the amount of eDNA on the ocular surface was increased significantly. Expression of genes downstream of eDNA signaling, such as TLR9, MyD88, and type I interferon, as well as the inflammatory cytokines interleukin-6 and tumor necrosis factor-α, was significantly increased in DED patients.

CONCLUSIONS

Extracellular DNA production and clearance mechanisms are dysregulated in DED. Nuclease deficiency in tear fluid allows eDNA and NETs to accumulate in precorneal tear film, and results in ocular surface inflammation. These findings point to novel therapeutic interventions in severe DED based on clearance of eDNA, NETs, and other molecular components from the ocular surface.

Rates of fatty acid oxidations by P450 compound I are pH dependent

The rates of oxidation of fatty acids by CYP119 compound I were dependent on the pH of the medium. The plot shows log k values for reactions of acids as a function of pH, where the slopes indicate mixed third-order and fourth-order dependence on base concentration. For palmitic acid, the rate increased 50-fold over the pH range 6.8-7.3.

Oxidation of 10-undecenoic acid by cytochrome P450(BM-3) and its Compound I transient

Oxidations of 10-undecenoic acid by cytochrome P450(BM-3) and its Compound I transient were studied. The only product formed in Compound I oxidations was 10,11-epoxyundecanoic acid, whereas the enzyme under turnover conditions gave the epoxide and 9-hydroxy-10-undecenoic acid in a 10 : 90 ratio. Kinetic studies at 0 °C of oxidations by Compounds I formed by MCPBA oxidation and by a photo-oxidation pathway gave the same results, displaying saturation kinetics that yielded equilibrium binding constants and first-order oxidation rate constants that were experimentally indistinguishable. Oxidation of 10-undecenoic acid by Compound I from CYP119 generated by MCBPA oxidation also gave 10,11-epoxyundecanoic acid as the only product. CYP119 Compound I bound the substrate less strongly but reacted with a faster oxidation rate constant than P450(BM-3) Compound I. The kinetic parameters for oxidation of the substrate by P450(BM-3) under turnover conditions were similar to those of the Compound I transient even though the products differed.

Low temperature photo-oxidation of chloroperoxidase Compound II

Oxidation of the heme-thiolate enzyme chloroperoxidase (CPO) from Caldariomyces fumago with peroxynitrite (PN) gave the Compound II intermediate, which was photo-oxidized with 365 nm light to give a reactive oxidizing species. Cryo-solvents at pH ≈ 6 were employed, and reactions were conducted at temperatures as low as -50° C. The activity of CPO as evaluated by the chlorodimedone assay was unaltered by treatment with PN or by production of the oxidizing transient and subsequent reaction with styrene. EPR spectra at 77K gave the amount of ferric protein at each stage in the reaction sequence. The PN oxidation step gave a 6:1 mixture of Compound II and ferric CPO, the photolysis step gave an approximate 1:1 mixture of active oxidant and ferric CPO, and the final mixture after reaction with excess styrene contained ferric CPO in 80% yield. In single turnover reactions at -50°C, styrene was oxidized to styrene oxide in high yield. Kinetic studies of styrene oxidation at -50°C displayed saturation kinetics with an equilibrium constant for formation of the complex of K(bind)=3.8 x 10(4)M(-1) and an oxidation rate constant of k(ox)=0.30s(-1). UV-Visible spectra of mixtures formed in the photo-oxidation sequence at ca. -50° C did not contain the signature Q-band absorbance at 690 nm ascribed to CPO Compound I prepared by chemical oxidation of the enzyme, indicating that different species were formed in the chemical oxidation and the photo-oxidation sequence.

Quantitative production of compound I from a cytochrome P450 enzyme at low temperatures. Kinetics, activation parameters, and kinetic isotope effects for oxidation of benzyl alcohol

Cytochrome P450 enzymes are commonly thought to oxidize substrates via an iron(IV)-oxo porphyrin radical cation transient termed Compound I, but kinetic studies of P450 Compounds I are essentially nonexistent. We report production of Compound I from cytochrome P450 119 (CYP119) in high conversion from the corresponding Compound II species at low temperatures in buffer mixtures containing 50% glycerol by photolysis with 365 nm light from a pulsed lamp. Compound I was studied as a reagent in oxidations of benzyl alcohol and its benzylic mono- and dideuterio isotopomers. Pseudo-first-order rate constants obtained at -50 degrees C with concentrations of substrates between 1.0 and 6.0 mM displayed saturation kinetics that gave binding constants for the substrate in the Compound I species (K(bind)) and first-order rate constants for the oxidation reactions (k(ox)). Representative results are K(bind) = 214 M(-1) and k(ox) = 0.48 s(-1) for oxidation of benzyl alcohol. For the dideuterated substrate C(6)H(5)CD(2)OH, kinetics were studied between -50 and -25 degrees C, and a van't Hoff plot for complexation and an Arrhenius plot for the oxidation reaction were constructed. The H/D kinetic isotope effects (KIEs) at -50 degrees C were resolved into a large primary KIE (P = 11.9) and a small, inverse secondary KIE (S = 0.96). Comparison of values extrapolated to 22 degrees C of both the rate constant for oxidation of C(6)H(5)CD(2)OH and the KIE for the nondeuterated and dideuterated substrates to values obtained previously in laser flash photolysis experiments suggested that tunneling could be a significant component of the total rate constant at -50 degrees C.

Kinetics and activation parameters for oxidations of styrene by Compounds I from the cytochrome P450(BM-3) (CYP102A1) heme domain and from CYP119

Cytochrome P450 (CYP or P450) enzymes are ubiquitous in nature where they catalyze a vast array of oxidation reactions. The active oxidants in P450s have long been assumed to be iron(IV)-oxo porphyrin radical cations termed Compounds I, but P450 Compounds I have proven to be difficult to prepare. The recent development of an entry to these transients by photo-oxidation of the corresponding iron(IV)-oxo neutral porphyrin species (Compounds II) permits spectroscopic and kinetic studies. We report here application of the photo-oxidation method for production of Compound I from the heme domain of CYP102A1 (cytochrome P450(BM-3)), and product and kinetic studies of reactions of styrene with this Compound I transient and also Compound I from CYP119. The studies were performed at low temperatures in 1:1 (v:v) mixtures of glycerol and phosphate buffer. Single-turnover reactions at 0 degrees C gave styrene oxide in good yields. In kinetic studies conducted between -10 and -50 degrees C, both Compounds I displayed saturation kinetics permitting determinations of binding constants and first-order oxidation rate constants. Temperature-dependent functions for the binding constants and rate constants were determined for both Compounds I. In the temperature range studied, the Compound I transient from the CYP102A1 heme domain bound styrene more strongly than Compound I from CYP119, but the rate constants for oxidations of styrene by the latter were somewhat larger than those for the former. The temperature-dependent functions for the first-order oxidation reactions are as follows: log k = 13.2-15.2/2.303RT and log k = 13.3-14.6/2.303RT (kilocalories per mole) for Compounds I from the CYP102A1 heme domain and CYP119, respectively.

Kinetics of oxidation of benzphetamine by compounds I of cytochrome P450 2B4 and its mutants

Cytochromes P450 are ubiquitous heme-containing enzymes that catalyze a wide range of reactions in nature including many oxidation reactions. The active oxidant species in P450 enzymes are widely thought to be iron(IV)-oxo porphyrin radical cations, termed Compound I species, but these intermediates have not been observed under turnover conditions. We prepared Compounds I of the mammalian hepatic P450 enzyme CYP2B4 and three mutants (E301Q, T302A, and F429H) by laser flash photolysis of the Compound II species that, in turn, were prepared by reaction of the resting enzymes with peroxynitrite. The PN treatment resulted in a small amount of nitration of the P450 as determined by mass spectrometry but no change in reactivity of the P450 in a test reaction. CYP2B4 Compound I oxidized benzphetamine to norbenzphetamine in high yield in bulk studies. In direct kinetic studies of benzphetamine oxidations, Compounds I displayed saturation kinetics with similar binding equilibrium constants (K(bind)) for each. The first-order oxidation rate constants (k(ox)) were comparable for Compounds I of CYP2B4, the E301Q mutant, and the T302A mutant, whereas the k(ox) for Compound I of the F429H mutant was reduced by a factor of 2. CYP119 Compound I, studied for comparison purposes, reacted with benzphetamine with a binding constant that was nearly an order of magnitude smaller than that of CYP2B4 but a rate constant that was similar. Substrate binding constants for P450 Compound I are important for controlling overall rates of oxidation reactions, and the intrinsic reactivities of Compounds I from various P450 enzymes are comparable.

Highly reactive porphyrin-iron-oxo derivatives produced by photolyses of metastable porphyrin-iron(IV) diperchlorates

Photolyses of metastable porphyrin-iron(IV) diperchlorates in laser flash photolysis reactions gave highly reactive transients. The systems studied were 5,10,15,20-tetraphenylporphyrin (TPP), 5,10,15,20-tetramesitylporphyrin (TMP), and 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP). The new species, which decayed within milliseconds in acetonitrile solutions, were shown to react with organic substrates by oxo-transfer reactions involving insertions into carbon-carbon double bonds of alkenes and styrenes or benzylic carbon-hydrogen bonds of arenes. The order of reactivity was OEP > TPP > TMP. Second-order rate constants for reactions with several substrates at 22 degrees C were determined; representative values of rate constants for the TPP derivative were k = 8.6 x 10(5) M(-1) s(-1) for styrene, k = 2.5 x 10(6) M(-1) s(-1) for cyclohexene, and k = 7.7 x 10(4) M(-1) s(-1) for ethylbenzene. These porphyrin-iron-oxo transients reacted 4-5 orders of magnitude faster than the corresponding iron(IV)-oxo porphyrin radical cations with rate constants similar to those of porphyrin-manganese(V)-oxo derivatives. Rate constants for oxidations of benzylic C-H positions of arenes correlated with the C-H bond dissociation energies, and Hammett correlations for reactions with substituted styrenes had rho(+) values ranging from -0.5 to -0.7, reflecting electrophilic character of the oxidants and their high reactivity. On the basis of their unique UV-visible spectra, high reactivities, and oxo-transfer properties, the new transients are tentatively identified as porphyrin-iron(V)-oxo perchlorates, electronic isomers (or valence tautomers) of well-known iron(IV)-oxo porphyrin radical cations.

Spectra and kinetic studies of the compound I derivative of cytochrome P450 119

The Compound I derivative of cytochrome P450 119 (CYP119) was produced by laser flash photolysis of the corresponding Compound II derivative, which was first prepared by reaction of the resting enzyme with peroxynitrite. The UV-vis spectrum of the Compound I species contained an asymmetric Soret band that could be resolved into overlapping transitions centered at approximately 367 and approximately 416 nm and a Q band with lambda(max) approximately 650 nm. Reactions of the Compound I derivative with organic substrates gave epoxidized (alkene oxidation) and hydroxylated (C-H oxidation) products, as demonstrated by product studies and oxygen-18 labeling studies. The kinetics of oxidations by CYP119 Compound I were measured directly; the reactions included hydroxylations of benzyl alcohol, ethylbenzene, Tris buffer, lauric acid, and methyl laurate and epoxidations of styrene and 10-undecenoic acid. Apparent second-order rate constants, equal to the product of the equilibrium binding constant (K(bind)) and the first-order oxidation rate constant (k(ox)), were obtained for all of the substrates. The oxidations of lauric acid and methyl laurate displayed saturation kinetic behavior, which permitted the determination of both K(bind) and k(ox) for these substrates. The unactivated C-H positions of lauric acid reacted with a rate constant of k(ox) = 0.8 s(-1) at room temperature. The CYP119 Compound I derivative is more reactive than model Compound I species [iron(IV)-oxo porphyrin radical cations] and similar in reactivity to the Compound I derivative of the heme-thiolate enzyme chloroperoxidase. Kinetic isotope effects (kH/kD) for oxidations of benzyl alcohol and ethylbenzene were small, reflecting the increased reactivity of the Compound I derivative in comparison to models. Nonetheless, CYP119 Compound I apparently is much less reactive than the oxidizing species formed in the P450 cam reaction cycle. Studies of competition kinetics employing CYP119 activated by hydrogen peroxide indicated that the same oxidizing transient is formed in the photochemical reaction and in the hydrogen peroxide shunt reaction.

Tunneling in C-H oxidation reactions by an oxoiron(IV) porphyrin radical cation: direct measurements of very large H/D kinetic isotope effects

Rate constants for oxidations of benzyl alcohol-d0 and -d7 by oxoiron(IV) tetramesitylporphyrin radical cation perchlorate in acetonitrile were measured in single turnover kinetic studies. The kinetic isotope effect (kH/kD) increased from 28 at 23 degrees C to 360 at -30 degrees C due to extensive hydrogen atom tunneling that was analyzed in terms of a parabolic energy barrier to tunneling. Similarly, large KIE values were found for oxidations of ethylbenzene-d0 and -d10 at room temperature. The large KIE values are a function of the porphyrin identity, and porphyrins containing electron-withdrawing groups display normal KIEs. KIEs found under catalytic turnover conditions are somewhat smaller than those obtained in single turnover reactions. The results should serve as benchmarks for computational studies of C-H oxidations by porphyrin and heme-iron-oxo systems.

Cytochrome p450 compound I

Cytochrome P450 enzymes (P450s) comprise a large class of enzymes that effect numerous oxidations in nature. The active oxidants in P450s are thought to be iron(IV)-oxo porphyrin radical cations termed Compounds I, and these intermediates have been sought since the discovery of P450s 40 years ago. We report formation of the Compound I derivative of a P450 enzyme by laser flash photolysis oxidation of the corresponding Compound II species, an iron(IV)-oxo neutral porphyrin intermediate. The Compound II derivative in turn was produced by oxidation of the P450 with peroxynitrite, which effected a net one-electron, oxo-transfer reaction to the iron(III) atom of the resting enzyme. For the P450 studied in this work, CYP119 from the thermophile Sulfolobus solfactaricus, the P450 Compound II derivative was stable for seconds at ambient temperature, and the Compound I transient decayed with a lifetime of ca. 200 ms.

Radical heterolysis reactions. Dynamics of formation, collapse, and solvation of ion pairs determined by a competition kinetic method

The kinetics of radical heterolysis reactions, including rate constants for radical cation-anion contact ion pair formation, collapse of the contact pair back to the parent radical, and separation of the contact pair to a solvent-separated ion pair or free ions were obtained in several solvents for a beta-mesyloxy radical. Rate constants were determined from indirect kinetic studies using thiophenol as both a radical trapping agent via H-atom transfer and an alkene radical cation trapping agent via electron transfer. [reaction: see text].

Kinetic studies of a fast, reversible alkene radical cation cyclization reaction

The radical cation formed by mesylate heterolysis from the 1,1-dimethyl-7,7-diphenyl-2-mesyloxy-6-heptenyl radical was studied in several solvents. Computational results suggest that the initially formed acyclic radical cation is a resonance hybrid with partial positive charge in both double bonds of 1,1-diphenyl-7-methyl-1,6-octadiene (10). Thiophenol trapping was used as the competing reaction for kinetic determinations. The acyclic radical cation rapidly equilibrates with a cyclic distonic radical cation, and thiophenol trapping gives acyclic product 10 and cyclic products, mainly trans-1-(diphenylmethyl)-2-(1-methylethenyl)cyclopentane (11). The rate constants for cyclization at ambient temperature were k = (0.5-2) x 10(10)(s-1), and those for ring opening were k = (1.5-9) x 10(10)(s-1). Laser flash photolysis studies in several solvents show relatively slow processes (k = (2.5-260) x 10(5)(s-1) that involve rate-limiting trapping reactions for the equilibrating radical cations. In mixtures of fluoroalcohols RfCH2OH in trifluoromethylbenzene, variable-temperature studies display small, and in one case a negative, activation energies, requiring equilibration reactions prior to the rate-limiting processes. Fast equilibration of acyclic and cyclic radical cations implies that product ratios can be controlled by the populations of the acyclic and cyclic species and relative rate constants for trapping each.

The hook effect: a need for constant vigilance

The hook effect is a well-recognized problem that can occur in assays of most tumour markers, including alpha feto-protein (AFP). We present a case of hepatoblastoma in a baby. The diagnosis was delayed as a result of unrecognized 'hooking' of a very high AFP concentration in the automated immunoassay method used. The falsely low result obtained was considered normal for the patient's age and supported the diagnosis of benign haemangioendothelioma. Liaison between clinical and laboratory staff was critical in obtaining an accurate AFP result, proceeding to liver biopsy and establishing the definitive diagnosis of hepatoblastoma. While the reasons and solutions for hook effect have been well researched and published, we believe the presence of extremely high serum AFP concentration in some hepatoblastoma patients means that the hook effect remains a problem and can generate erroneously low AFP results despite assay reformulation by manufacturers. Therefore constant vigilance by laboratory staff is still needed.

Formation of compound I by photo-oxidation of compound II

[reaction: see text] Compound I is the heme-iron(IV)-oxo porphyrin radical cation formed in peroxidase and catalase enzymes by reaction with hydrogen peroxide. As an alternative to chemical oxidations of porphyrin-iron(III) species, various compound I species were produced by 355 nm laser flash photolysis photooxidation of the corresponding compound II species, porphyrin-iron(IV)-oxo derivatives. The method is demonstrated by production and kinetic studies of the compound I species from 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin-iron, from horseradish peroxidase, and from wild-type horse skeletal myoglobin.

Laser Flash Photolysis Generation and Kinetic Studies of Porphyrin−Manganese−Oxo Intermediates. Rate Constants for Oxidations Effected by Porphyrin−MnV−Oxo Species and Apparent Disproportionation Equilibrium Constants for Porphyrin−MnIV−Oxo Species

Porphyrin-manganese(V)-oxo and porphyrin-manganese(IV)-oxo species were produced in organic solvents by laser flash photolysis (LFP) of the corresponding porphyrin-manganese(III) perchlorate and chlorate complexes, respectively, permitting direct kinetic studies. The porphyrin systems studied were 5,10,15,20-tetraphenylporphyrin (TPP), 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (TPFPP), and 5,10,15,20-tetrakis(4-methylpyridinium)porphyrin (TMPyP). The order of reactivity for (porphyrin)Mn(V)(O) derivatives in self-decay reactions in acetonitrile and in oxidations of substrates was (TPFPP) > (TMPyP) > (TPP). Representative rate constants for reaction of (TPFPP)Mn(V)(O) in acetonitrile are k = 6.1 x 10(5) M(-1) s(-1) for cis-stilbene and k = 1.4 x 10(5) M(-1) s(-1) for diphenylmethane, and the kinetic isotope effect in oxidation of ethylbenzene and ethylbenzene-d(10) is k(H)/k(D) = 2.3. Competitive oxidation reactions conducted under catalytic conditions display approximately the same relative rate constants as were found in the LFP studies of (porphyrin)Mn(V)(O) derivatives. The apparent rate constants for reactions of (porphyrin)Mn(IV)(O) species show inverted reactivity order with (TPFPP) < (TMPyP) < (TPP) in reactions with cis-stilbene, triphenylamine, and triphenylphosphine. The inverted reactivity results because (porphyrin)Mn(IV)(O) disproportionates to (porphyrin)Mn(III)X and (porphyrin)Mn(V)(O), which is the primary oxidant, and the equilibrium constants for disproportionation of (porphyrin)Mn(IV)(O) are in the order (TPFPP) < (TMPyP) < (TPP). The fast comproportionation reaction of (TPFPP)Mn(V)(O) with (TPFPP)Mn(III)Cl to give (TPFPP)Mn(IV)(O) (k = 5 x 10(8) M(-1) s(-1)) and disproportionation reaction of (TPP)Mn(IV)(O) to give (TPP)Mn(V)(O) and (TPP)Mn(III)X (k approximately 2.5 x 10(9) M(-1) s(-1)) were observed. The relative populations of (porphyrin)Mn(V)(O) and (porphyrin)Mn(IV)(O) were determined from the ratios of observed rate constants for self-decay reactions in acetonitrile and oxidation reactions of cis-stilbene by the two oxo derivatives, and apparent disproportionation equilibrium constants for the three systems in acetonitrile were estimated. A model for oxidations under catalytic conditions is presented.

Kinetics of radical heterolysis reactions forming alkene radical cations

Rate constants for heterolytic fragmentation of beta-(ester)alkyl radicals were determined by a combination of direct laser flash photolysis studies and indirect kinetic studies. The 1,1-dimethyl-2-mesyloxyhexyl radical (4a) fragments in acetonitrile at ambient temperature with a rate constant of k(het) > 5 x 10(9) s(-1) to give the radical cation from 2-methyl-2-heptene (6), which reacts with acetonitrile with a pseudo-first-order rate constant of k = 1 x 10(6) s(-1) and is trapped by methanol in acetonitrile in a reversible reaction. The 1,1-dimethyl-2-(diphenylphosphatoxy)hexyl radical (4b) heterolyzes in acetonitrile to give radical cation 6 in an ion pair with a rate constant of k(het) = 4 x 10(6) s(-1), and the ion pair collapses with a rate constant of k < or = 1 x 10(9) s(-1). Rate constants for heterolysis of the 1,1-dimethyl-2-(2,2-diphenylcyclopropyl)-2-(diphenylphosphatoxy)ethyl radical (5a) and the 1,1-dimethyl-2-(2,2-diphenylcyclopropyl)-2-(trifluoroacetoxy)ethyl radical (5b) were measured in various solvents, and an Arrhenius function for reaction of 5a in THF was determined (log k = 11.16-5.39/2.3RT in kcal/mol). The cyclopropyl reporter group imparts a 35-fold acceleration in the rate of heterolysis of 5a in comparison to 4b. The combined results were used to generate a predictive scale for heterolysis reactions of alkyl radicals containing beta-mesyloxy, beta-diphenylphosphatoxy, and beta-trifluoroacetoxy groups as a function of solvent polarity as determined on the E(T)(30) solvent polarity scale.

Solvent polarity effects and limited acid catalysis in rearrangements of model radicals for the methylmalonyl-CoA mutase- and isobutyryl-CoA mutase-catalyzed isomerization reactions

The kinetics of reactions of models for the intermediate radicals formed in the methylmalonyl-CoA mutase- and isobutyryl-CoA mutase-catalyzed rearrangements were studied by laser flash photolysis methods. The aldehyde-containing model analogous to the propanal-3-yl radical reacted via 3-exo cyclization with rate constants that varied with solvent polarity (k in the range 2 x 105 to 1 x 107 s-1). The analogous methyl ketone-containing radical reacted 2 orders of magnitude less rapidly, and the ethylthiocarbonyl-containing radical analogue reacted too slowly for kinetic measurements. No acid catalysis was observed in acetic acid, but the CF3CO2H-complexed radicals reacted 1 order of magnitude faster than the uncomplexed radicals. The results indicate that catalysis of the 3-exo radical cyclizations of the radicals formed in the enzymes by hydrogen bonding to an acid, so-called "partial protonation", is not adequate for acceleration of the reactions to the point of kinetic competence. A dissociative mechanism for the radical rearrangements in nature is considered as an alternative.

Direct Detection of Ion Pair Formation and Collapse in a Migration Reaction of a β-Phosphate Radical

In solutions of trifluorotoluene or toluene containing 2,2,2-trifluoroethanol, the beta-phosphate radical heterolyzed to give a detectable ion pair, identified as a solvent-separated species. Rate constants for the radical fragmentation reaction forming the ion pair, for ion pair collapse, and for diffusive escape to free ions were measured. The kinetics and entropy of activation for fragmentation indicate that the rearrangement reaction occurs by a heterolysis pathway in all solvents. [reaction: see text]

Carboxylate-substituted radicals from phenylselenide derivatives. Designs on models for coenzyme B12-dependent enzyme-catalyzed rearrangements

Laser flash photolysis (266 nm) of alpha- and beta-phenylselenyl esters, carboxylic acids, and carboxylates in aqueous acetonitrile media gave the corresponding radicals by homolytic cleavage of the phenylselenyl groups. In the beta-substituted systems, acid and carboxylate radicals reacted in intramolecular reporter reactions with approximately equal rate constants. For the alpha-substituted systems, an ester- and carboxylic acid-substituted radical reacted in an intramolecular reporter reaction with the same rate constants, but the analogous alpha-carboxylate radical, a radical anion, reacted an order of magnitude less rapidly and with an activation energy that is 3 kcal/mol greater than that found for analogues. A kinetic titration of the equilibrating alpha-acid and alpha-carboxylate radicals gave pKa = 4.6. The results indicate that alpha-ester and alpha-carboxylic acid radicals are unlikely to be appropriate models for alpha-carboxylate radicals, the intermediates formed in a large subset of coenzyme B12-dependent enzyme-catalyzed reactions.

Efficient production of enol ether radical cations by heterolytic cleavage of beta-mesylate radicals

[reaction: see text] alpha-Methoxy-beta-mesyloxy radicals were produced in laser flash photolysis reactions, and yields of enol ether radical cations formed by heterolytic fragmentation of the mesylate group were determined. The mesylate heterolysis reaction is faster than heterolyses of phosphate and bromide groups in analogous radicals and highly efficient in medium-polarity solvents.

p-Nitrobenzenesulfenate esters as precursors for laser flash photolysis studies of alkyl radicals

A series of p-nitrobenzenesulfenate esters was used in laser flash photolysis (LFP) studies to generate alkoxyl radicals that fragmented to give the (2,2-diphenylcyclopropyl)methyl radical. Rate constants for the beta-scission reactions increased as a function of the carbonyl compound produced in the fragmentation reaction in the order CH2O < MeCHO < Me2CO < PhCHO < Ph2CO and increased with increasing solvent polarity. For alkoxyl radicals that fragment to produce benzaldehyde and benzophenone, the beta-scission reactions are faster than 1,5-hydrogen atom abstractions when the incipient carbon radical is as stable as a secondary alkyl radical, and this entry to carbon radicals can be used in LFP kinetic studies.

Laser flash photolysis kinetic studies of enol ether radical cations. Rate constants for heterolysis of alpha-methoxy-beta-phosphatoxyalkyl radicals and for cyclizations of enol ether radical cations

Two series of enol ether radical cations were studied by laser flash photolysis methods. The radical cations were produced by heterolyses of the phosphate groups from the corresponding alpha-methoxy-beta-diethylphosphatoxy or beta-diphenylphosphatoxy radicals that were produced by 355 nm photolysis of N-hydroxypryidine-2-thione (PTOC) ester radical precursors. Syntheses of the radical precursors are described. Cyclizations of enol ether radical cations 1 gave distonic radical cations containing the diphenylalkyl radical, whereas cyclizations of enol ether radical cations 2 gave distonic radical cation products containing a diphenylcyclopropylcarbinyl radical moiety that rapidly ring-opened to a diphenylalkyl radical product. For 5-exo cyclizations, the heterolysis reactions were rate limiting, whereas for 6-exo and 7-exo cyclizations, the heterolyses were fast and the cyclizations were rate limiting. Rate constants were measured in acetonitrile and in acetonitrile solutions containing 2,2,2-trifluoroethanol, and several Arrhenius functions were determined. The heterolysis reactions showed a strong solvent polarity effect, whereas the cyclization reactions that gave distonic radical cation products did not. Recombination reactions or deprotonations of the radical cation within the first-formed ion pair compete with diffusive escape of the ions, and the yields of distonic radical cation products were a function of solvent polarity and increased in more polar solvent mixtures. The 5-exo cyclizations were fast enough to compete efficiently with other reactions within the ion pair (k approximately 2 x 10(9) s(-1) at 20 degrees C). The 6-exo cyclization reactions of the enol ether radical cations are 100 times faster (radical cations 1) and 10 000 times faster (radical cations 2) than cyclizations of the corresponding radicals (k approximately 4 x 10(7) s(-1) at 20 degrees C). Second-order rate constants were determined for reactions of one enol ether radical cation with water and with methanol; the rate constants at ambient temperature are 1.1 x 10(6) and 1.4 x 10(6) M(-1) s(-1), respectively.

Product studies and laser flash photolysis on alkyl radicals containing two different beta-leaving groups are consonant with the formation of an olefin cation radical

1-Bromo-2-methoxy-1-phenylpropan-2-yl (3) and 2-methoxy-1-phenyl-1-diphenylphosphatopropan-2-yl (4) were generated under continual photolysis from the respective PTOC precursors in a mixture of acetonitrile and methanol. The radicals undergo heterolytic fragmentation of the substituent in the beta-position to generate the olefin cation radical (5). Z-2-Methoxy-1-phenylpropene (15) is the major product formed in the presence of 1,4-cyclohexadiene, and is believed to result from hydrogen atom transfer to the oxygen of the olefin cation radical, followed by deprotonation. Laser flash photolysis experiments indicate that reaction between 5 and 1,4-cyclohexadiene occurs with a rate constant of approximately 6 x 10(5) M(-1) s(-1). 2,2-Dimethoxy-1-phenylpropane (18) is observed as a minor product. Laser flash photolysis experiments place an upper limit on methanol trapping of 5 at k <1 x 10(3) M(-1) s(-1) and do not provide any evidence for the formation of reactive intermediates other than 5. The use of two PTOC precursors containing different leaving groups to generate a common olefin cation radical enables one to utilize product analysis to probe for the intermediacy of other reactive intermediates. The ratio of 15:18 is dependent upon hydrogen atom donor concentration, but is independent of the PTOC precursor. These observations are consistent with the proposal that both products result from trapping of 5 that is formed via heterolysis of 3 and 4.

Heterolytic cleavage of a beta-phosphatoxyalkyl radical resulting in phosphate migration or radical cation formation as a function of solvent polarity

[formula: see text] The 2-(diethylphosphatoxy)-2-(p-methoxyphenyl)-1,1-dimethylethyl radical (1) reacted to give the benzylic radical product from phosphate migration or a radical cation (or a mixture of the two) as a function of solvent. Smooth acceleration in rates of reactions of 1 in solvents of increasing polarity and consistent entropies of activation indicate that radical 1 reacts by common mechanism irrespective of the final products formed, specifically by initial heterolysis to a radical cation-phosphate anion pair.