Integration of electrochemical generator and detector of nitric oxide with the aid of the iron porphyrin

An electrochemical cell of generating nitric oxide (NO) was constructed in order to realize the catalytic reaction of nitrite by iron porphyrin [Y. Chi, J. Chen, K. Aoki, Inorg. Chem. 43 (2004) 8437]. The cell was composed of the generator of the iron porphyrin-including Nafion^®-coated electrode, the ion-exchange membrane, and the electrochemical detector of NO. The reduction of iron porphyrin in the presence of nitrite at the generator coordinates nitrite to form the NO-included complex, of which oxidation releases NO. The water-dissolved NO diffused to the detector through the membrane to block nitrite. It was detected by voltammetry with the aid of the oxidative catalytic reaction of iron porphyrin.     

Definitive assignment of the g tensor of [Fe(OEP)(NO)] by single-crystal EPR

Single-crystal EPR measurements have been performed on the triclinic form of [Fe(OEP)(NO)] (Ellison, M. K.; Scheidt, W. R. J. Am. Chem. Soc. 1997, 119, 7404) and on the isomorphous cobalt derivative [Co(OEP)(NO)] (Ellison, M. K.; Scheidt, W. R. Inorg. Chem. 1998, 37, 382) which has been doped with [Fe(OEP)(NO)]. Principal values of the g tensor determined at room temperature are gmax = 2.106, gmid = 2.057, and gmin = 2.015. The principal direction associated with the minimum g value lies 8 degrees from the Fe-N(NO) direction, 2 degrees from the normal to the heme plane, and 42 degrees from the N-O direction. The direction associated with the maximum g value lies 9 degrees from the normal to the Fe-N-O plane. The fact that the direction of gmin is near the Fe-N(NO) direction is consistent with the dominant role of spin-orbit coupling at the iron atom in determining the g tensor and with the picture of the electronic structure of the compound from restricted calculations, which makes the half-filled orbital mostly dz2 on the iron atom. The hyperfine tensor is nearly isotropic and was only resolved in the doped samples. Principal values of the A tensor determined at room temperature are 40.9, 49.7, and 42.7 MHz. Principal values of the g tensor determined from the doped samples at 77 K are gmax = 2.110, gmid = 2.040, and gmin = 2.012. Principal values of the A tensor are 42.5, 52.8, and 44.1 MHz at 77 K. The small change in g values with temperature is in contrast to the large temperature dependence on g values observed in samples of MbNO (Hori et al. J. Biol. Chem. 1981, 256, 7849).     

Arginine deiminase uses an active-site cysteine in nucleophilic catalysis of L-arginine hydrolysis

Arginine deiminase (EC 3.5.3.6) catalyzes the hydrolysis of l-arginine to citrulline and ammonium ion, which is the first step of the microbial l-arginine degradation pathway. The deiminase conserves the active-site Cys-His-Asp motif found in several related enzymes that catalyze group-transfer reactions from the guanidinium center of arginine-containing substrates. For each of these enzymes, nucleophilic catalysis by the conserved Cys has been postulated but never tested. In this communication we report the results from rapid quench studies of single-turnover reactions carried out with recombinant Pseudomonas aeruginosa arginine deiminase and limiting [14C-1]l-arginine. The citrulline-formation and arginine-decay curves measured at 25 degrees C were fitted to yield apparent rate constants k = 3.6 +/- 0.1 s-1 and k = 4.2 +/- 0.1 s-1, respectively. The time course for the formation (k =13 s-1) and decay (k = 6.5 s-1) of 14C-labeled enzyme defined a kinetically competent intermediate. Under the same reaction conditions, the Cys406Ser mutant failed to form the 14C-labeled enzyme intermediate. These results, along with the recently reported enzyme X-ray structure (Galkin, A.; Kulakova, L.; Sarikaya, E.; Lim, K.; Howard, A.; Herzberg, O. J. Biol. Chem. 2004, 279, 14001-14008, evidence a reaction pathway in which l-arginine deimination proceeds via a covalent enzyme intermediate formed by ammonia displacement from the arginine guanidinum carbon by the active-site Cys406.     

Methanol oxidation in a flow reactor: Implications for the branching ratio of the CH3OH+OH reaction

The oxidation of methanol in a flow reactor has been studied experimentally under diluted, fuel-lean conditions at 650–1350 K, over a wide range of O₂ concentrations (1%–16%), and with and without the presence of nitric oxide. The reaction is initiated above 900 K, with the oxidation rate decreasing slightly with the increasing O₂ concentration. Addition of NO results in a mutually promoted oxidation of CH₃OH and NO in the 750–1100 K range. The experimental results are interpreted in terms of a revised chemical kinetic model. Owing to the high sensitivity of the mutual sensitization of CH₃OH and NO oxidation to the partitioning of CH₃O and CH₂OH, the CH₃OH + OH branching fraction could be estimated as α = 0.10 ± 0.05 at 990 K. Combined with low-temperature measurements, this value implies a branching fraction that is largely independent of temperature. It is in good agreement with recent theoretical estimates, but considerably lower than values employed in previous modeling studies. Modeling predictions with the present chemical kinetic model is in quantitative agreement with experimental results below 1100 K, but at higher temperatures and high O₂ concentration the model underpredicts the oxidation rate. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 423–441, 2008     

An amperometric biosensor for glucose based on electrodeposited redox polymer/glucose oxidase film on a gold electrode.

In this paper, we described a glucose biosensor based on the co-electrodeposition of a poly(vinylimidazole) complex of [Os(bpy)2Cl](+/2+) (PVI-Os) and glucose oxidase (GOX) on a gold electrode surface. The one-step co-electrodeposition method provided a better control on the sensor construction, especially when it was applied to microsensor construction. The modified electrode exhibited the classical features of a kinetically fast redox couple bound to an electrode surface and the redox potential of the redox polymer/enzyme film was 0.14 V (vs. SCE). For a scan rate of up to 200 mV s(-1), the peak-to-peak potential separation was less than 25 mV. In the presence of glucose, a typical catalytic oxidation current was observed, which reached a plateau at 0.25 V (vs. SCE). Under the optimal experimental conditions, the steady-state electrooxidation current measured at 0.30 V (vs. SCE) was linear to the glucose concentration in the range of 0-30 mM. Successful attempts were made in blood sample analysis.     

Simultaneous Determination of Ascorbic Acid,Dopamine and Uric Acid Based on Gold Nanoparticles‐PTCA‐Cys Composites Modified Electrodes

In this study, a nanocomposite of 3, 4, 9, 10‐perylenetetracarboxylic acid and L‐cysteine (PTCA‐Cys) with satisfactory water‐solubility and film‐forming ability was prepared and worked as substrate for modifying the glassy carbon electrode. Then, gold nanoparticles (AuNPs) were immobilized to achieve a PTCA‐Cys‐AuNPs modified electrode which provided more reaction positions on the sensor. Scanning electron microscopy, transmission electron microscopy, cyclic voltammetry and different pulse voltammetry were employed to characterize the assembly process of the sensor. The constructed sensor displayed desirable sensitivity, selectivity and stability towards the simultaneous detection of ascorbic acid (AA), dopamine (DA) and uric acid (UA). Under the optimal experimental conditions, the oxidation peaks of AA, DA and UA appeared at 64, 240 and 376 mV, respectively. The corresponding linear response ranges were 3.2–435, 0.04–100 and 0.80–297 μM, and the detection limits were 1.1, 0.010 and 0.27 μM (S/N=3), respectively.     

Measurements of the kinetics of the OH-initiated oxidation of β-pinene: Radical propagation in the OH + β-pinene + O2 + NO reaction system

The mechanism of the OH-initiated oxidation of β-pinene in the presence of NO has been investigated using a discharge-flow system at 5 Torr and 300 K. OH radical concentrations were measured as a function of reaction time by laser-induced fluorescence (LIF). The rate constant for the OH +β-pinene reaction was measured to be (7.68 ± 0.72) ×10⁻¹¹ cm³ molecule⁻¹ s⁻¹. OH radical propagation was observed after the addition of O₂ and NO, and the measured OH concentration profiles were compared to simulations based on both the Master Chemical Mechanism and the Regional Atmospheric Chemistry Mechanism for β-pinene oxidation in order to determine the ability of these mechanisms to describe the observed efficiency of radical propagation. Both models are able to reproduce the observed OH concentrations profiles to within 15%. Expanding the MCM to include isomerization of the β-hydroxy alkoxy radicals improves the agreement with the experimental observations. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 522–531, 2005     

Citric acid oxidation by vanadium(V) in sulfuric acid medium: Kinetic and mechanistic study

The citric acid oxidation by vanadium(V) in sulfuric acid medium at 303 K is reported. The reaction rate was determined spectrophotometrically by monitoring the formation of vanadium(IV) at 760 nm. The oxidation showed a first‐order dependence with respect to vanadium(V) concentration and fractional order with respect to citric acid concentrations, with no control and with constant ionic strength. The reaction is also first order with respect to sulfuric acid concentration with no control and of fractional order at constant ionic strength. The reaction rate is enhanced by an increase of ionic strength and increased by a decrease of the dielectric constant. The activation parameters were calculated based on the rate constants determined in the 293 to 313 K interval. The proposed oxidation mechanisms and the derived rate laws are consistent with the experimental rate laws. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 566–572, 2000     

Is the ruthenium analogue of compound I of cytochrome p450 an efficient oxidant? A theoretical investigation of the methane hydroxylation reaction

High-valent metal-oxo complexes catalyze C-H bond activation by oxygen insertion, with an efficiency that depends on the identity of the transition metal and its oxidation state. Our study uses density functional calculations and theoretical analysis to derive fundamental factors of catalytic activity, by comparison of a ruthenium-oxo catalyst with its iron-oxo analogue toward methane hydroxylation. The study focuses on the ruthenium analogue of the active species of the enzyme cytochrome P450, which is known to be among the most potent catalysts for C-H activation. The computed reaction pathways reveal one high-spin (HS) and two low-spin (LS) mechanisms, all nascent from the low-lying states of the ruthenium-oxo catalyst (Ogliaro, F.; de Visser, S. P.; Groves, J. T.; Shaik, S. Angew. Chem. Int. Ed. 2001, 40, 2874-2878). These mechanisms involve a bond activation phase, in which the transition states (TS's) appear as hydrogen abstraction species, followed by a C-O bond making phase, through a rebound of the methyl radical on the metal-hydroxo complex. However, while the HS mechanism has a significant rebound barrier, and hence a long lifetime of the radical intermediate, by contrast, the LS ones are effectively concerted with small barriers to rebound, if at all. Unlike the iron catalyst, the hydroxylation reaction for the ruthenium analogue is expected to follow largely a single-state reactivity on the LS surface, due to a very large rebound barrier of the HS process and to the more efficient spin crossover expected for ruthenium. As such, ruthenium-oxo catalysts (Groves, J. T.; Shalyaev, K.; Lee, J. In The Porphyrin Handbook; Biochemistry and Binding: Activation of Small Molecules, Vol. 4; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New York, 2000; pp 17-40) are expected to lead to more stereoselective hydroxylations compared with the corresponding iron-oxo reactions. It is reasoned that the ruthenium-oxo catalyst should have larger turnover numbers compared with the iron-oxo analogue, due to lesser production of suicidal side products that destroy the catalyst (Ortiz de Montellano, P. R.; Beilan, H. S.; Kunze, K. L.; Mico, B. A. J. Biol. Chem. 1981, 256, 4395-4399). The computations reveal also that the ruthenium complex is more electrophilic than its iron analogue, having lower hydrogen abstraction barriers. These reactivity features of the ruthenium-oxo system are analyzed and shown to originate from a key fundamental factor, namely, the strong 4d(Ru)-2p(O,N) overlaps, which produce high-lying pi(Ru-O), sigma(Ru-O), and sigma(Ru-N) orbitals and thereby to lead to a preference of ruthenium for higher-valent oxidation states with higher electrophilicity, for the effectively concerted LS hydroxylation mechanism, and for less suicidal complexes. As such, the ruthenium-oxo species is predicted to be a more robust catalyst than its iron-oxo analogue.     

Interprotein electron transfer in a confined space: uncoupling protein dynamics from electron transfer by sol-gel encapsulation

In this paper, we describe the first observations of photoinitiated interprotein electron transfer (ET) within sol-gels. We have encapsulated three protein-protein complexes, specifically selected because they represent a full range of affinities, are sensitive to different types of dynamic processes, and thus are expected to respond differently to sol-gel encapsulation. The three systems are (i) the [Zn, Fe(3+)L] mixed-metal hemoglobin hybrids, where the alpha(1)-Zn and beta(2)-Fe subunits correspond to a "predocked" protein-protein complex with a crystallographically defined interface (Natan, M. J.; Baxter, W. W.; Kuila, D.; Gingrich, D. J.; Martin, G. S.; Hoffman, B. M. Adv. Chem. Ser. 1991, 228 (Electron-Transfer Inorg., Org., Biol. Syst.), 201-213), (ii) the Zn-cytochrome c peroxidase complex with cytochrome c, [ZnCcP, Fe(3+)Cc], having an intermediate affinity between its partners (Nocek, J. M.; Zhou, J. S.; De Forest, S.; Priyadarshy, S.; Beratan, D. N.; Onuchic, J. N.; Hoffman, B. M. Chem. Rev. 1996, 96, 2459-2489), and (iii) the [Zn-deuteromyoglobin, ferricytochrome b(5)] complex, [ZnDMb, Fe(3+)b(5)], which is loosely bound and highly dynamic (Liang, Z.-X.; Nocek, J.; Huang, K.; Hayes, R. T.; Kurnikov, I. V.; Beratan, D. N.; Hoffman, B. M. J. Am. Chem. Soc. 2002, 124, 6849-6859. Intersubunit ET within the hybrid does not involve second-order processes or subunit rearrangements, and thus is influenced only by perturbations of high-frequency motions coupled to ET. For the latter two complexes, sol-gel encapsulation eliminates second-order processes: protein partners encapsulated as a complex must stay together throughout a photoinitiated ET cycle, while proteins encapsulated alone cannot acquire a partner. It further modulates intracomplex motions of the two partners.     

Measurements of the kinetics of the OH‐initiated oxidation of isoprene: Radical propagation in the OH + isoprene + O2 + NO reaction system

The mechanism of the OH‐initiated oxidation of isoprene in the presence of NO and O₂ has been investigated using a discharge‐flow system at 298 K and 2 torr total pressure. OH radical concentration profiles were measured using laser‐induced fluorescence as a function of reaction time. The rate constant for the reaction of OH + isoprene was measured to be (1.10 ± 0.05) × 10⁻¹⁰ cm³ mol⁻¹ s⁻¹. In the presence of NO and O₂, regeneration of OH radicals by the reaction of isoprene‐based peroxy radicals with NO was measured and compared to simulations of the kinetics of this system. The results of these experiments are consistent with an overall rate constant of 9 × 10⁻¹² cm³ mol⁻¹ mol⁻¹ (with an uncertainty factor of 2) for the reaction of isoprene‐based hydroxyalkyl peroxy radicals with NO. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 637–643, 1999     

Syntheses and properties of liquid crystalline N-substituted pyrroles and their polymers

Three liquid crystalline N-substituted pyrroles were synthesized from 6-(1-pyrrolyl)hexanol with phenolic derivatives having a mesogenic core of cyclohexylbenzene or biphenyl by Mitsunobu reaction. These pyrroles had two anodic peaks at 1.4 and 1.8 V (vs. SCE). The former was due to an oxidation of the pyrrole moiety and the latter was due to an oxidation of the mesogenic moiety. These pyrrole monomers were polymerized by electrochemical and chemical methods. The potentiostatic method and the chemical method using FeCl₃ gave a soluble and fusible polymer, respectively. A polymer having a mesogenic core of cyclohexyl benzene obtained by the chemical method and a polymer having a mesogenic core of biphenylketone obtained by the potentiostatic method had a liquid-crystalline phase. The phase was identified as smectic A by polarizing microscopy and XRD analysis. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 2691–2698, 1998     

Hamiltonian dynamics of chemical reactions. Consecutive first-order reactions and reactions possessing one autocatalytic intermediate

A recently proposed Hamiltonian approach to phenomenological chemical kinetics [T. Georgian and G.L. Findley, Int. J. Quantum Chem., Quantum Biol. Symp. 10 , 331 (1983); T. Georgian, J.M. Halpin, and G.L. Findley, Int. J. Quantum Chem., Quantum Biol. Symp. 11 , 347 (1984)] is applied to all consecutive first-order, single-step reactions, and to all reactions possessing one autocatalytic intermediate. The reaction Hamiltonians presented are shown to be consistent with the phenomenological rate equations and the relationship between reaction form and the form of the reaction potential is discussed. In particular, we show: (1) that the interaction between consecutive reactions manifests itself as a coupling term in the reaction potential, a term which may be eliminated via transition to “normal reaction coordinates” for the chemical system; and (2) that coupled sets of autocatalytic reactions give rise to coupling terms in the reaction Hamiltonian which are characteristic of the reaction mechanism.     

The k-j-j' vector correlation in inelastic and reactive scattering

Quasi-classical trajectory (QCT) methods are presented which allow characterization of the angular momentum depolarization of the products of inelastic and reactive scattering. The particular emphasis of the theory is on three-vector correlations, and on the connection with the two-vector correlation between the initial and final angular momenta, j and j', which is amenable to experimental measurement. The formal classical theory is presented, and computational results for NO(A) + He are used to illustrate the type of mechanistic information provided by analysis of the two- and three-vector correlations. The classical j-j' two-vector correlation results are compared with quantum mechanical calculations, and are shown to be in good agreement. The data for NO(A) + He support previous conclusions [M. Brouard, H. Chadwick, Y.-P. Chang, R. Cireasa, C. J. Eyles, A. O. L. Via, N. Screen, F. J. Aoiz, and J. K?os, J. Chem. Phys. 131, 104307 (2009)] that this system is only weakly depolarizing. Furthermore, it is shown that the projection of j along the kinematic apse is nearly conserved for this system under thermal collision energy conditions.     

Experimental and kinetic modeling study of the effect of NO and SO2 on the oxidation of CO?H2 mixtures

The effect of NO and SO₂ on the oxidation of a CO? H₂ mixture was studied in a jet‐stirred reactor at atmospheric pressure and for various equivalence ratios (0.1, 1, and 2) and initial concentrations of NO and SO₂ (0–5000 ppm). The experiments were performed at fixed residence time and variable temperature ranging from 800 to 1400 K. Additional experiments were conducted in a laminar flow reactor on the effect of SO₂ on CO? H₂ oxidation in the same temperature range for stoichiometric and reducing conditions. It was demonstrated that in fuel‐lean conditions, the addition of NO increases the oxidation of the CO? H₂ mixture below 1000 K and has no significant effect at higher temperatures, whereas the addition of SO₂ has a small inhibiting effect. Under stoichiometric and fuel‐rich conditions, both NO and SO₂ inhibit the oxidation of the CO? H₂ mixture. The results show that a CO? H₂ mixture has a limited NO reduction potential in the investigated temperature range and rule out a significant conversion of HNO to NH through reactions like HNO + CO ?? NH + CO₂ or HNO + H₂ ?? NH + H₂O. The chain terminating effect of SO₂ under stoichiometric and reducing conditions was found to be much more pronounced than previously reported under flow reactor conditions and the present results support a high rate constant for the H + SO₂ + M ?? HOSO + M reaction. The reactor experiments were used to validate a comprehensive kinetic reaction mechanism also used to simulate the reduction of NO by natural gas blends and pure C₁ to C₄ hydrocarbons. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 564–575, 2003     

iVI-TD-DFT: An iterative vector interaction method for exterior/interior roots of TD-DFT

The recently proposed iterative vector interaction (iVI) method for large Hermitian eigenvalue problems (Huang et al., J. Comput. Chem. 2017, 38, 2481) is extended to generalized eigenvalue problems, HC = SCE , with the metric S being either positive definite or not. Although, it works with a fixed-dimensional search subspace, iVI can converge quickly and monotonically from above to the exact exterior/interior roots. The algorithms are further specialized to nonrelativistic and relativistic time-dependent density functional theories (TD-DFT) by taking the orbital Hessian as the metric (i.e., the inverse TD-DFT eigenvalue problem) and incorporating explicitly the paired structure into the trial vectors. The efficacy of iVI-TD-DFT is demonstrated by various examples. © 2018 Wiley Periodicals, Inc.     

Detection of NADH and Ethanol at Titanium Containing MCM‐41 with Low Overpotential

Titanium‐containing MCM‐41 (Ti‐MCM‐41) modified glassy carbon electrode (GCE) can exhibit an excellent electrocatalytic activity towards the oxidation of β‐Nicotinamide adenine dinucleotide (NADH). A dramatic decrease in the overvoltage of NADH oxidation reaction is observed at 0.28 V vs. SCE. The application in the amperometric biosensing of ethanol using alcohol dehydrogenase enzyme (ADH) also has been demonstrated with this material. The proposed sensor shows a highly sensitivity, an acceptable reproducibility and a good stability. The linear range of ethanol is 25–1000 μM and the detection limit is 8.0 μM. Ti‐MCM‐41 modified electrode not only can be used to detect the concentration of NADH in biochemical reaction, but also as the potential matrix for the construction of dehydrogenases sensor.     

Mechanistic insight into the nitrosylation of the [4Fe-4S] cluster of WhiB-like proteins

The reactivity of protein bound iron-sulfur clusters with nitric oxide (NO) is well documented, but little is known about the actual mechanism of cluster nitrosylation. Here, we report studies of members of the Wbl family of [4Fe-4S] containing proteins, which play key roles in regulating developmental processes in actinomycetes, including Streptomyces and Mycobacteria, and have been shown to be NO responsive. Streptomyces coelicolor WhiD and Mycobacterium tuberculosis WhiB1 react extremely rapidly with NO in a multiphasic reaction involving, remarkably, 8 NO molecules per [4Fe-4S] cluster. The reaction is 10(4)-fold faster than that observed with O(2) and is by far the most rapid iron-sulfur cluster nitrosylation reaction reported to date. An overall stoichiometry of [Fe(4)S(4)(Cys)(4)](2-) + 8NO → 2[Fe(I)(2)(NO)(4)(Cys)(2)](0) + S(2-) + 3S(0) has been established by determination of the sulfur products and their oxidation states. Kinetic analysis leads to a four-step mechanism that accounts for the observed NO dependence. DFT calculations suggest the possibility that the nitrosylation product is a novel cluster [Fe(I)(4)(NO)(8)(Cys)(4)](0) derived by dimerization of a pair of Roussin's red ester (RRE) complexes.     

Spectroscopy and electrochemistry of cytochrome P450 BM3-surfactant film assemblies

We report analyses of electrochemical and spectroscopic measurements on cytochrome P450 BM3 (BM3) in didodecyldimethylammonium bromide (DDAB) surfactant films. Electronic absorption spectra of BM3-DDAB films on silica slides reveal the characteristic low-spin FeIII heme absorption maximum at 418 nm. A prominent peak in the absorption spectrum of BM3 FeII-CO in a DDAB dispersion is at 448 nm; in spectra of aged samples, a shoulder at approximately 420 nm is present. Infrared absorption spectra of the BM3 FeII-CO complex in DDAB dispersions feature a time-dependent shift of the carbonyl stretching frequency from 1950 to 2080 cm(-1). Voltammetry of BM3-DDAB films on graphite electrodes gave the following results: FeIII/II E(1/2) at -260 mV (vs SCE), approximately 300 mV positive of the value measured in solution; DeltaS degrees (rc), DeltaS degrees , and DeltaH degrees values for water-ligated BM3 in DDAB are -98 J mol(-1) K(-1), -163 J mol(-1) K(-1), and -47 kJ mol(-1), respectively; values for the imidazole-ligated enzyme are -8 J mol(-1) K(-1), -73 J mol(-1) K(-1), and -21 kJ mol(-1). Taken together, the data suggest that BM3 adopts a compact conformation within DDAB that in turn strengthens hydrogen bonding interactions with the heme axial cysteine, producing a P420-like species with decreased electron density around the metal center.