Stopped-flow Spectrophotometer Research Articles

Kinetics and mechanistic investigation of the substitution from dinuclear Pt(II) complexes bridged by N,N’-bis(2-pyridylcarboxamide)phenylenediamines

The rates of substitution of the chloride ligand in dinuclear complexes, [Pt2(N,N'-bis(2-pyridylcarboxamide)-1,3-phenylenediamine)Cl4] (1), [Pt2(N,N'-bis(2-pyridylcarboxamide)-1,4-phenylenediamine)Cl4] (2), [Pt2(N,N'-bis(3-isoquinolylcarboxamide)-1,3-phenylenediamine)Cl4] (3) and [Pt2(N,N'-bis(3-isoquinolylcarboxamide)-1,4-phenylenediamine)Cl4] (4), by three bio-relevant nucleophiles, thiourea (TU), N,N-dimethylthiourea (DMTU) and N,N,N’,N’-tetramethylthiourea (TMTU), were investigated. The mononuclear analogue [Pt(N-phenylpyridine-2-carboxamide)Cl2] (5) was included to compare the results. The kinetics of the reactions were studied under pseudo first-order conditions in a methanol solution (I = 0.1 M LiCl) as a function of concentration and temperature using the stopped-flow spectrophotometer. The observed pseudo first-order rate constants for the substitution reactions of all the complexes were calculated from two well-separated steps and obey the rate law k obs (1 st /2 nd ) = k 2 (1 st /2 nd )[Nu]. The reactivity of the complexes decreases in the order 2 > 1 > 4 > 3 > 5. The conformational symmetry of the complexes as controlled by the phenylenediamine bridges, steric hindrance due to the bridges as well as the σ-donor capacity of the coordinated groups around the metal centers influence the reactivity of the dinuclear complexes. The low enthalpy (ΔH #) and negative intrinsic entropy (ΔS #) values support an associative mechanism of substitution. The kinetic data are supported by DFT calculations.

Substitution reactions of cis-platinum(II) complexes containing bidentate N,N-donor pyridinecarboxamide ligands with different substituents

Substitution reactions of [2-(pyridinecarboxamide)dichloride Pt(II)] [PtCl2 ], [N-phenyl-(2-pyridinecarboxamide)dichloride Pt(II)] [PhPtCl2 ], [N-(4-methylphenyl)-2-pyridinecarboxamide)dichloride Pt(II)] [CH3PhPtCl2 ], [N-(4-methoxyphenyl)-(2-pyridine-carboxamide)dichloride Pt(II)] [CH3OPhPtCl2 ] and [N-(4-fluorophenyl)-(2-pyridinecarboxamide)dichloride Pt(II)] [FPhPtCl2 ], with nucleophiles; thiourea (TU), N,N′-dimethylthiourea (DMTU) and N,N,N′,N′-tetramethylthiourea (TMTU) were studied under pseudo first-order conditions. The rates of substitution were investigated as a function of nucleophile concentration and temperature using stopped-flow and UV-visible absorption spectrophotometers. Substitutions of the two coordinated chloride ligands of the Pt(II) complexes occur consecutively, with the first substitution occurring opposite the coordinated pyridyl. The observed pseudo first-order rate constants regressed linearly with concentration of the incoming nucleophiles according to the equation k obs = k 2[Nu]. The highest substitution rates were measured for PtCl2, which is attributed to the unsubstituted non-leaving carboxamide ligand. The Pt(II) center of this complex is the most electrophilic as a result of the strong withdrawal of electron density through π-resonance by the carboxamide group. The introduction of a 4′-substituted phenyl group on the amido N of the carboxamide reduces the reactivity of the complexes due to proportional electronic effects from the ancillary substituents on the phenyl ring as well as its conformational disposition with respect to the plane of the complex. The order of reactivity of studied nucleophiles is TU > DMTU > TMTU. The substitution is associatively activated as supported by the negative entropy of activation values for the reactions.

Understanding the role of flexible alkyl-α,ω-diamine linkers on the substitution behaviour of dinuclear trans-platinum(II) complexes: A kinetic and mechanistic study

An investigation of the role of flexible alkyl-α,ω-diamine linkers on the substitution behaviour of dinuclear trans-platinum(II) complexes. The substitution reactions of four dinuclear trans-platinum(II) complexes viz. [PtNH3Cl2]2-µ-NH2(CH2)2NH2] (P12), [PtNH3Cl2]2-µ-NH2(CH2)3NH2] (P13), [PtNH3Cl2]2-µ-NH2(CH2)4NH2] (P14) and [PtNH3Cl2]2-µ-NH2(CH2)5NH2] (P15) with three neutral thiourea-based nucleophiles specifically: thiourea (TU), N-methyl-2-thiourea (MTU) and N,N-dimethyl-2-thiourea (DMTU) were studied quantitatively under pseudo-first-order condition as function of concentration and temperature by conventional UV–Visible and stopped-flow spectrophotometers. The ligand substitution reaction of the complexes proceeds in veritably three consecutive steps. Each step follows first-order kinetics with the respective complex and nucleophile. The pseudo first order rate constants, kobs(1/2/3), for sequential substitution of the chlorido ligands, the ammine, and subsequent displacement of the linker obeyed the rate law: kobs(1/2/3) = k(1/2/3)[NU]. The ligand substitution reactions were driven by both electronic and steric factors. However, our findings revealed that upon the substitution of the chlorido ligands by the nucleophiles at the platinum centres, the σ-donor capacity via inductive effect of these electron-rich nucleophiles over compensate the steric strain imposed by the nucleophiles and by the alkanediamine linker at the substitution sites. Consequently, electronic factors governed the overall reaction pattern of these complexes. 195Pt NMR results confirmed the simultaneous substitution of all the chlorido ligands by thiourea-based nucleophiles, followed by the subsequent but successive displacement of the ammine groups and the flexible alkanediamine linker from the metal centres. The order of reactivity of the nucleophiles with the complexes decreases with the increase in steric bulk in the nucleophiles: TU > MTU > DMTU. The small positive enthalpy and the large but negative entropy confirm the associative mode of activation for all the studied complexes. Computational modelling using density functional theory (DFT) calculations was employed to rationalise the kinetic trends.

Kinetic solvent viscosity effects reveal a protein isomerization in the reductive half-reaction of Neurospora crassa class II nitronate monooxygenase

Nitronate monooxygenase from Neurospora crassa (NcNMO) is an FMN-dependent enzyme that oxidizes nitronates. The enzyme belongs to Group H of flavin monooxygenases. Previous biochemical and mechanistic studies on the enzyme showed that NcNMO oxidizes both anionic nitronates and neutral nitroalkanes, a feature distinguishing NcNMO from bacterial and other fungal NMOs which are active exclusively on nitronates. Recently, NMOs have been shown to oxidize propionate 3-nitronate (P3N), a toxic nitro acid present in legumes, fungi, and leaf beetles; P3N is an irreversible inhibitor of succinate dehydrogenase causing anywhere from neurological disorders to death in livestock and humans. In this study, we report the first kinetic investigation of NcNMO with P3N and its conjugated acid 3-nitropropionic acid (3NPA), and a mechanistic investigation with 3NPA using kinetic solvent viscosity effects. The kcat value with P3N (300 s−1) was 7-times larger than with 3NPA and the kcat/KP3N value (1,700,000 M−1s−1) was ~500-times larger than the kcat/K3NPA value, consistent with P3N being a faster and better substrate than 3NPA. The normalized kcat and kcat/K3NPA values showed inverse hyperbolic dependences on the relative solvent viscosity, consistent with an internal isomerization of the enzyme-substrate complex. A similar inverse hyperbolic pattern of the normalized kred value for the rate constant of flavin reduction determined anaerobically in a stopped-flow spectrophotometer suggested that the internal enzyme isomerization occurs before the flavin reduction step in the reductive half-reaction.

The effect of the chelate ligand on the rate of chloride substitution from Pt(II) complexes with picolyl and isoquinolinylcarboxamide moieties on the non-leaving ligand

Substitution reactions of four Pt(II) complexes, namely; N-(2-picolyl)picolinamide-platinum(II) chloride (Pt3), N-(8-quinolyl)pyridine-2-carboxamide platinum(II) chloride (Pt4), N-(8-quinolyl)-3-isoquinolinecarboxamide platinum(II) chloride (Pt5) and N-(8-quinolinyl)-1-isoquinolinecarboxamide platinum(II) chloride (Pt6), were studied with biorelevant nucleophiles, viz. thiourea (TU), N,N′-dimethylthiourea (DMTU) and N,N,N′,N′-tetramethylthiourea (TMTU) under pseudo first order conditions as a function of concentration and temperature using the stopped flow spectrophotometer. The observed pseudo first order rate constants for the substitution reactions obey the rate law kobs = k2[Nu]. The reactivity of the studied complexes depends on strength of π-backbonding of the attached ligands, which is enhanced by the strong electron withdrawing nature of the carboxamide group on the non-leaving ligands of Pt(II) complexes. Quinoline moieties of Pt5 and Pt6 lie out-of-the square plane, resulting in enhanced reactivity due to the aided entrapment of the nucleophile by the non-planar groups of the ligand. Negative activation entropies and positive enthalpies of activation support an associative mode of activation.

A Rapid Fenton treatment of bio-treated dyeing and finishing wastewater at second-scale intervals: kinetics by stopped-flow technique and application in a full-scale plant

A rapid Fenton treatment at second-scale intervals was investigated for further removal of organic compounds in the effluent of bio-treated dyeing and finishing wastewater (BDFW). The decolorization kinetics was studied using a stopped-flow spectrophotometer (SFS) at second-scale intervals. A combined first-order model was found to fit well for the decrease of both methylene blue and rhodamine B in SFS as well as SCOD (soluble chemical oxygen demand) and DOC (dissolved organic carbon) in real BDFW in batch test during Fenton oxidation. A full-scale plant with treatment capacity of 400,000 m3·d−1 was designed and has been run continuously based on the results of the stopped-flow study to treat the effluent of BDFW using Fenton oxidation in 16 pipeline reactors, each with a volume of 6.9 m3 and 24 s of reaction time since 2014. The COD, SCOD and DOC decreased from 140, 110 and 35 mg·L−1 to 77, 71 and 26 mg·L−1 respectively, which can meet the latest strict discharge limitations. The natural fluorescent substances detected in the BDFW were completely removed. The main organic pollutants in the BDFW can be significantly reduced using both gas chromatography-mass spectrometry and ultrahigh-performance liquid chromatography-hybrid quadrupole time-of-flight mass spectrometry. The rapid Fenton reaction applied in pipeline reactors at second intervals has several advantages over the conventional Fenton’s process such as much shorter reaction time at second scale intervals, no need to build extra pH adjustment or reaction tanks, simple operation, low capital cost, etc.

Further understanding the involvement of Fe(IV) in peroxydisulfate and peroxymonosulfate activation by Fe(II) for oxidative water treatment

Very recently, we have interestingly found that the Fe(II)-activated peroxydisulfate (PDS) process can oxidize methyl phenyl sulfoxide (PMSO) to its oxygen transfer product methyl phenyl sulfone (PMSO2), suggesting the possible occurrence of a ferryl intermediate (Fe(IV)) rather than long recognized sulfate radical (SO4−) in this process. In this work, the formation of PMSO2 was also observed when PMSO was treated by the Fe(II)/peroxymonosulfate (PMS) system over the pH range of 3–7, and the yield of PMSO2 (i.e., the molar ratio of PMSO2 formed to PMSO lost) was quantified to be approximately 100% under acidic conditions, similar to the Fe(II)/PDS system. Moreover, 18O from isotope-labeled water (H218O) was successfully incorporated into PMSO2 product when PMSO was oxidized by the Fe(II)/PDS and Fe(II)/PMS systems in H218O, likely due to the competitive oxygen exchange of Fe(IV)-oxo species with H218O. Further, Fe(IV) evolution profiles in both systems were predicted by a Fe(IV)-mediated kinetic model, and these results are in good agreement with the experimental data obtained with a stopped-flow spectrophotometer.

Glutathione transferase catalyzes the addition of glutathione to nitro‐conjugated linoleic acid

Nitroalkene fatty acids are formed in vivo and are currently being tested in two Phase 2 clinical trials. They modulate cellular responses through Michael addition reactions with thiol‐containing proteins. Nitro‐conjugated linoleic acid (NO2‐CLA) is formed preferentially both in vivo and in vitro. There are two main positional isomers of NO2‐CLA, 9‐ and 12‐NO2‐CLA. They both react with low molecular weight thiols at two electrophilic sites located on the β‐ and δ‐carbons giving rise to reversible biphasic kinetics. The fast reaction corresponds to the formation of the β‐adduct (the kinetic product), while the slow reaction corresponds to the δ‐adduct (the thermodynamic product) (Turell, Vitturi, J. Biol. Chem., 2017). With the aim of further characterizing the biological chemistry of this bifunctional nitroalkene, we evaluated if its reaction with glutathione (GSH) is catalyzed by glutathione transferases (GST). The reaction was followed by monitoring NO2‐CLA decay at 330 nm using a stopped flow spectrophotometer. A mu‐type glutathione transferase from the parasitic platyhelminth Echinococcus granulosus (EgGST1) was used. Catalysis was observed in both phases when using a mixture of NO2‐CLA isomers. Remarkably, the δ‐addition was accelerated □13 times more than the β‐addition, 4.4 × 106 vs 3.4 × 105 M−1, respectively, at 2 mM GSH and 10 μM NO2‐CLA (25 °C, pH 7.4), suggesting specificity. To evaluate whether there is a preference of the enzyme for the NO2‐CLA isomer, purified 9‐ or 12‐ NO2‐CLA were used. No difference was observed between them. The reaction between GSH and nitro‐oleic acid (NO2‐OA), a nitroalkene fatty acid with only one electrophilic site in β, was also catalyzed by this enzyme. An acceleration of 1 × 105 M−1 was observed (2 mM GSH, 20 μM NO2‐OA), in line with the value obtained for the β‐addition of NO2‐CLA. We are now further characterizing this reaction which is expected to affect nitroalkene fatty acids metabolism and excretion and thus impact their signaling actions.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Performing anaerobic stopped-flow spectrophotometry inside of an anaerobic chamber.

The catalytic cycle of most flavin-dependent enzymes can be divided into oxidative and reductive half-reactions. Although some enzymes are oxidized by electron carrier proteins or organic compounds, many use oxygen as the final electron acceptor. In order to properly study the reductive half-reaction of flavin-dependent enzyme that react with oxygen, as in the case of oxidases and monooxygenases, it is necessary to establish anaerobic conditions that will only allow the reduction process to be monitored. The reduced flavoenzyme can be further studied by exposing it to oxygen to monitor the oxidative half-reaction. Anaerobic chambers provide an ideal environment for performing these experiments as they reliably maintain an anaerobic atmosphere inside a large workspace. A common tool used to study flavin-dependent enzymes is the stopped-flow spectrophotometry. This chapter describes methods for performing stopped-flow experiments in an anaerobic chamber. We include information about the chamber components, setting up a stopped-flow spectrophotometer inside of a chamber, preparing anaerobic solutions, and performing experiments to measure the reductive and oxidative half-reactions of flavin-dependent monooxygenases.

Cell-Free Expression of Unnatural Amino Acid Incorporated Aquaporin SS9 with Improved Separation Performance in Biomimetic Membranes.

Aquaporins (AQPs) are widely applied in biomimetic membranes for water recycling and desalination. In this study, a novel aquaporin was isolated from Photobacterium profundum SS9 (AQP SS9), which showed high water permeability and potential for practical water purification applications. To improve the stability of the AQP SS9 embedded biomimetic membranes, a modified AQP SS9 was obtained by incorporation of an unnatural amino acid (p-propargyloxyphenylalanine, pPpa) (P-AQP SS9) in vitro using a mutated Methanocaldococcus jannaschii tyrosyl-tRNA synthetase (TyrRS) and the cell-free expression system. The modified AQP SS9 can covalently link with phospholipids and hence significantly improve the stability of biomimetic membranes. The concentration of Mg2+ and fusion expression with signal peptides were evaluated to enhance the expression level of P-AQP SS9, resulting in a highest yield of 49 mg/L. The modified AQP SS9 was then reconstituted into DOPC liposomes and analyzed by a stopped-flow spectrophotometer. The obtained water permeability coefficient (Pf) of 7.46×10−4 m/s was 5.7 times higher than that of proteoliposomes with the wild-type AQP SS9 (Pf=1.31×10−4 m/s) and 12.1 times higher than that of the DOPC liposomes (Pf=6.15×10−5m/s). This study demonstrates the development of a cell-free system for the expression of membrane proteins with much higher stability and the potential application of the modified aquaporins for water filtration.

Rapid oxidation of iodide and hypoiodous acid with ferrate and no formation of iodoform and monoiodoacetic acid in the ferrate/I−/HA system

Toxic and odorous iodinated disinfection byproducts (I-DBPs) could form in the chemical oxidation of iodine-containing water. A critical step for controlling the hazardous I-DBPs is to convert the iodine species into stable and harmless iodate (IO3−) while inhibiting the accumulation of highly reactive hypoiodous acid (HOI). Herein, the oxidation of I− and HOI with ferrate was investigated, and the formation profile of HOI was determined based on 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) coloring method through a stopped-flow spectrophotometer. The second-order rate constants (kapp) of ferrate with HOI decreased from 1.6 × 105 M−1s−1 to 8.3 × 102 M−1s−1 as the solution pH varied from 5.3 to 10.3, which were 7.5, 7.2 and 13.8 times higher than that of ferrate with I− at pH 6.0, 7.0 and 8.0, respectively. Compared with other oxidants such as ozone, hypochlorous acid, chloramine and potassium permanganate, ferrate would swiftly oxidize HOI formed in the I− oxidation process. For the ferrate oxidation of I-containing water, HOI was swiftly oxidized to IO3− from pH 5.0 to 9.0. Phosphate buffer promoted the oxidation of I− while inhibited the oxidation of HOI with ferrate. When 5 mgC/L of humic acids (HA) existed in the solution, no formation of iodoform and monoiodoacetic acid (MIAA) was observed in the oxidation of iodide (20 μM) with ferrate (from 10 μM to 80 μM). These results suggested that ferrate oxidation could be an effective method for the control of I-DBPs in iodine-containing water treatment.

Third-Order Kinetics for Interaction of Glutathione with a Dinuclear Pd(II) Complex and Their Mechanism, DNA Binding and DFT Study

A kinetics and mechanistic study of the interaction between the bridged dimer \( [{\text{Pd}}({\text{pic}})({\text{OH}})]_{2}^{2 + } \) (where pic = 2-aminomethylpyridine) and glutathione (GSH) has been performed under pseudo-first order conditions using a stopped-flow spectrophotometer. The reaction follows a distinctive third-order kinetics via two consecutive steps. The first step follows the rate law kobs = k1[GSH]2, whereas step two is independent of the GSH concentration. The activation parameters, ΔH‡ and ΔS‡ for both steps, were determined and an associative mode of activation is suggested for the substitution process. Geometry optimizations and HOMO–LUMO energy calculations were done by DFT. The UV spectra of the complexes were compared with theoretically obtained TD-DFT spectra. Complex 2 and 3 interact non-covalently with calf-thymus DNA with binding constants (Kb and KSV) of the order of 104 L·mol−1.

Stepwise Hydrogen Atom and Proton Transfers in Dioxygen Reduction by Aryl-Alcohol Oxidase.

The mechanism of dioxygen reduction by the flavoenzyme aryl-alcohol oxidase was investigated with kinetic isotope, viscosity, and pL (pH/pD) effects in rapid kinetics experiments by stopped-flow spectrophotometry of the oxidative half-reaction of the enzyme. Double mixing of the enzyme in a stopped-flow spectrophotometer with [α-2H2]- p-methoxybenzyl alcohol and oxygen at varying aging times established a slow rate constant of 0.0023 s-1 for the wash-out of the D atom from the N5 atom of the reduced flavin. Thus, the deuterated substrate could be used to probe the cleavage of the N-H bond of the reduced flavin in the oxidative half-reaction. A significant and pH-independent substrate kinetic isotope effect (KIE) of 1.5 between pH 5.0 and 8.0 demonstrated that H transfer is partially limiting the oxidative half-reaction of the enzyme; a negligible solvent KIE of 1.0 between pD 5.0 and 8.0 proved a fast H+ transfer reaction that does not contribute to determining the flavin oxidation rates. Thus, a mechanism for dioxygen reduction in which the H atom originating from the reduced flavin and a H+ from a solvent exchangeable site are transferred in separate kinetic steps is proposed. The spectroscopic and kinetic data presented also showed a lack of stabilization of transient flavin intermediates. The substantial differences in the mechanistic details of O2 reduction by aryl-alcohol oxidase with respect to other alcohol oxidases like choline oxidase, pyranose 2-oxidase, and glucose oxidase further demonstrate the high level of versatility of the flavin cofactor in flavoenzymes.

A Reversible, Charge-Induced Intramolecular C4a-S-Cysteinyl-Flavin in Choline Oxidase Variant S101C.

Choline oxidase serves as a paradigm for alcohol oxidation catalyzed by flavin-dependent enzymes. In its active site, S101 is 4 Å from the flavin C4a atom on an extended loop. Enzyme variants substituted at S101 were generated in a previous study and investigated mechanistically [Yuan, H., and Gadda, G. (2011) Biochemistry 50, 770-779]. In this study, the typical ultraviolet-visible (UV-vis) absorption spectrum of oxidized flavin was observed for the S101C enzyme in HEPES, TES, or sodium phosphate, whereas an absorption spectrum suggesting the presence of a C4a-flavin adduct with cysteine was obtained in Tris-HCl at pH 8.0. pH titrations of the UV-vis absorption spectrum of the wild-type, S101A, S101C, and H99N enzymes in the presence and absence of Tris allowed for the determination of two pKa values that define a pH range in which the C4a-S-cysteinyl flavin is stabilized. Inhibition studies and stopped-flow kinetics demonstrated that binding of protonated Tris in the active site of the S101C enzyme is required to form the C4a-S-cysteinyl flavin. Deuterium kinetic isotope effects and proton inventories of the S101C enzyme mixed in a stopped-flow spectrophotometer with Tris established a mechanism for the reversible formation of the C4a-S-cysteinyl flavin. This study provides a detailed mechanistic analysis of the reversible formation of a bicovalent C4a-S-cysteinyl-8α-N3-histidyl flavin in choline oxidase, identifying an optimal pH range and a mechanistic rationale for the stabilization of de novo C4a-S-cysteinyl-flavins. Moreover, it presents an example of an intramolecular reaction of an enzyme-bound flavin without a substrate.

Substitutions of S101 decrease proton and hydride transfers in the oxidation of betaine aldehyde by choline oxidase

Choline oxidase oxidizes choline to glycine betaine, with two flavin-mediated reactions to convert the alcohol substrate to the carbon acid product. Proton abstraction from choline or hydrated betaine aldehyde in the wild-type enzyme occurs in the mixing time of the stopped-flow spectrophotometer, thereby precluding a mechanistic investigation. Mutagenesis of S101 rendered the proton transfer reaction amenable to study. Here, we have investigated the aldehyde oxidation reaction catalyzed by the mutant enzymes using steady-state and rapid kinetics with betaine aldehyde. Stopped-flow traces for the reductive half-reaction of the S101T/V/C variants were biphasic, corresponding to the reactions of proton abstraction and hydride transfer. In contrast, the S101A enzyme yielded monophasic traces like wild-type choline oxidase. The rate constants for proton transfer in the S101T/C/V variants decreased logarithmically with increasing hydrophobicity of residue 101, indicating a behavior different from that seen previously with choline for which no correlation was determined. The rate constants for hydride transfer also showed a logarithmic decrease with increasing hydrophobicity at position 101, which was similar to previous results with choline as a substrate for the enzyme. Thus, the hydrophilic character of S101 is necessary not only for efficient hydride transfer but also for the proton abstraction reaction.

Evidence for proton tunneling and a transient covalent flavin-substrate adduct in choline oxidase S101A

The effect of temperature on the reaction of alcohol oxidation catalyzed by choline oxidase was investigated with the S101A variant of choline oxidase. Anaerobic enzyme reduction in a stopped-flow spectrophotometer was biphasic using either choline or 1,2-[2H4]-choline as a substrate. The limiting rate constants klim1 and klim2 at saturating substrate were well separated (klim1/klim2>9), and were >15-fold slower than for wild-type choline oxidase. Solvent deuterium kinetic isotope effects (KIEs) ~4 established that klim1 probes the proton transfer from the substrate hydroxyl to a catalytic base. Primary substrate deuterium KIEs ≥7 demonstrated that klim2 reports on hydride transfer from the choline alkoxide to the flavin. Between 15°C and 39°C the klim1 and klim2 values increased with increasing temperature, allowing for the analyses of H+ and H– transfers using Eyring and Arrhenius formalisms. Temperature-independent KIE on the klim1 value (H2Oklim1/D2Oklim1) suggests that proton transfer occurs within a highly reorganized tunneling-ready-state with a narrow distribution of donor-acceptor distances. Eyring analysis of the klim2 value gave lines with the slope(choline)>slope(D-choline), suggesting kinetic complexity. Spectral evidence for the transient occurrence of a covalent flavin-substrate adduct during the first phase of the anaerobic reaction of S101A CHO with choline is presented, supporting the notion that an important role of amino acid residues in the active site of flavin-dependent enzymes is to eliminate alternative reactions of the versatile enzyme-bound flavin for the reaction that needs to be catalyzed.

Red blood cell membrane water permeability increases with length of ex vivo storage

Water transport across the red blood cell (RBC) membrane is an essential cell function that needs to be preserved during ex vivo storage. Progressive biochemical depletion during storage can result in significant conformational and compositional changes to the membrane. Characterizing the changes to RBC water permeability can help in evaluating the quality of stored blood products and aid in the development of improved methods for the cryopreservation of red blood cells. This study aimed to characterize the water permeability (Lp), osmotically inactive fraction (b), and Arrhenius activation energy (Ea) at defined storage time-points throughout storage and to correlate the observed results with other in vitro RBC quality parameters. RBCs were collected from age- and sex-matched blood donors. A stopped flow spectrophotometer was used to determine Lp and b by monitoring changes in hemoglobin autofluorescence when RBCs were exposed to anisotonic solutions. Experimental values of Lp were characterized at three different temperatures (4, 20 and 37 °C) to determine the Ea. Results showed that Lp, b, and Ea of stored RBCs significantly increase by day 21 of storage. Degradation of the RBC membrane with length of storage was seen as an increase in hemolysis and supernatant potassium, and a decrease in deformability, mean corpuscular hemoglobin concentration and supernatant sodium. RBC osmotic characteristics were shown to change with storage and correlate with changes in RBC membrane quality metrics. Monitoring water parameters is a predictor of membrane damage and loss of membrane integrity in ex vivo stored RBCs.

34 - The Kinetic Study on the Reactivity of HNO (Azanone) Towards Nitrite Ions, Hydroxylamine, Selected Nitrosocompounds and Selected Phosphines

Nitroxyl (HNO, IUPAC name azanone), being formally the protonated product of one-electron reduction of nitric oxide (NO), is an elusive reactive nitrogen species possessing interesting biological chemistry and high pharmacological importance. 1,2 Contrary to nitric oxide, HNO is a strong electrophile highly reactive towards nucleophiles. Here, we present the study on the kinetics of HNO reaction with nitrate ions, hydroxylamine, selected nitrosocompounds and selected phosphines. In this study we applied previously described 4 competition kinetics method based on two parallel, competing HNO reactions – with thiol and with molecular oxygen. The latter, relatively fast reaction (k = (1.8 ± 0.3) ∙ 10 4 M -1 s -1 ), 3 results in the formation of peroxynitrite, 3 that can be easily detected fluorometrically with the use of fluorogenic boronate derivative of resorufin, PC1. PC1 reacts rapidly and directly with ONOO – (k = 1∙10 6 M -1 s -1 ) with the formation of fluorescent resorufin as the major product. In this study the formation of resorufin was monitored by Applied Photophysics SX20 stopped-flow spectrophotometer equipped with a fluorescence detector. HNO released from Angeli's salt (3 μM) reacts either with the azanone scavenger, or with molecular oxygen (225 μM) forming peroxynitrite detected by PC1 (25 μM). Table 1. The determined values of the second-order rate constant of HNO reaction. HNO scavenger k (M -1 s -1 ) Nitrite ions (4,8 ± 0,2) × 10 3 Hydroxyloamine (2,0 ± 0,1) × 10 4 2-Nitroso-1-naphtol (9,8 ± 0,2) × 10 5 Nitrosobenzene >1,1× 10 5 S-Nitrosoglutathione 2,9 × 10 5 Triphenylophosphine > 6,1 × 10 6 Triphenylphosphine-3,3′,3′′-trisulfonic acid trisodium salt (1,5 ± 0,1) × 10 7 References 1: J. M. Fukuto, M. D. Bartberger et al., Chem. Res. Toxicol., 2005, 18 , 790. 2: N. Paolocci, M. I. Jackson et al. , Pharmacol. Ther. , 2007, 113 , 442. 3: R. Smulik, D. Debski et al. , J. Biol. Chem., 2014, 289 , 35570.

Carbon Dioxide Sequestration by Using a Model Carbonic Anhydrase Complex in Tertiary Amine Medium.

Globally, the elevation of carbon dioxide (CO2 ) levels due to the anthropogenic effect poses a serious threat to the ecosystem. Hence, it is important to control and/or mitigate the level of CO2 in the atmosphere, which necessitates novel tools. Herein, it is proposed to improve CO2 sequestration by using model complexes based on the enzyme carbonic anhydrase (CA) in aqueous tertiary amine medium. The effect of substituents on the model CA model complexes on CO2 absorption and desorption was determined by using a stopped-flow spectrophotometer to follow pH changes through coupling to pH indicator and a continuous stirred-tank reactor (CSTR). The CO2 hydration rate constants were determined under basic conditions and compound 6, which contained a hydrophilic group, showed the highest absorption or hydration levels of CO2 (2.860×10(3) L mol(-1) s(-1) ). In addition, CSTR results for the absorption and desorption of CO2 suggest that simple model CA complexes could be used in post-combustion processing.

Energy Landscapes and Catalysis in Nitric-oxide Synthase

Nitric oxide (NO) plays diverse roles in mammalian physiology. It is involved in blood pressure regulation, neurotransmission, and immune response, and is generated through complex electron transfer reactions catalyzed by NO synthases (NOS). In neuronal NOS (nNOS), protein domain dynamics and calmodulin binding are implicated in regulating electron flow from NADPH, through the FAD and FMN cofactors, to the heme oxygenase domain, the site of NO generation. Simple models based on crystal structures of nNOS reductase have invoked a role for large scale motions of the FMN-binding domain in shuttling electrons from the FAD-binding domain to the heme oxygenase domain. However, molecular level insight of the dynamic structural transitions in NOS enzymes during enzyme catalysis is lacking. We use pulsed electron-electron double resonance spectroscopy to derive inter-domain distance relationships in multiple conformational states of nNOS. These distance relationships are correlated with enzymatic activity through variable pressure kinetic studies of electron transfer and turnover. The binding of NADPH and calmodulin are shown to influence interdomain distance relationships as well as reaction chemistry. An important effect of calmodulin binding is to suppress adventitious electron transfer from nNOS to molecular oxygen and thereby preventing accumulation of reactive oxygen species. A complex landscape of conformations is required for nNOS catalysis beyond the simple models derived from static crystal structures of nNOS reductase. Detailed understanding of this landscape advances our understanding of nNOS catalysis/electron transfer, and could provide new opportunities for the discovery of small molecule inhibitors that bind at dynamic protein interfaces of this multidimensional energy landscape.