Saturation Transfer NMR Research Articles

"Electronic lock" in chromiumtricarbonyl complex of cycloheptatrienyltin.

The fast sigmatropic [1,5]-Sn migration in cycloheptatrienyl(triphenyl)tin is effectively locked in the chromiumtricarbonyl complex. A much slower [1,3]-Sn + [1,2]-Cr diatropic rearrangement was detected by spin saturation transfer NMR.

Transgenic livers expressing mitochondrial and cytosolic CK: mitochondrial CK modulates free ADP levels.

The function of creatine kinase (CK) and its effect on phosphorus metabolites was studied in livers of transgenic mice expressing human ubiquitous mitochondrial CK (CK-Mit) and rat brain CK (CK-B) isoenzymes and their combination. (31)P NMR spectroscopy and saturation transfer were recorded in livers of anesthetized mice to measure high-energy phosphates and hepatic CK activity. CK reaction velocity was related to total enzyme activity irrespective of the isoenzyme expressed, and it increased with increasing concentrations of creatine (Cr). The fluxes mediated by both isoenzymes in both directions (phosphocreatine or ATP synthesis) were equal. Over a 20-fold increase in CK-Mit activity (28-560 micromol. g wet wt(-1). min(-1)), the fraction of phosphorylated Cr increased 1.6-fold. Hepatic free ADP concentrations calculated by assuming equilibrium of the CK-catalyzed reaction in vivo decreased from 84 +/- 9 to 38 +/- 4 nmol/g wet wt. Calculated free ADP levels in mice expressing high levels of CK-B (920-1,635 micromol. g wet wt(-1). min(-1)) were 52 +/- 6 nmol/g wet wt. Mice expressing both isoenzymes had calculated free ADP levels of 36 +/- 4 nmol/g wet wt. These findings indicate that CK-Mit catalyzes its reaction equally well in both directions and can lower hepatic apparent free ADP concentrations.

Magnetization‒transfer 31P NMR of biochemical exchange in vivo: Application to creatine kinase kinetics

Phosphorus‒31 saturation‒transfer NMR spectroscopy provides an elegant means to study fluxes through the creatine kinase reaction in human skeletal muscle. To obtain reliable quantitative kinetic information, experimental imperfections, such as incomplete saturation and radiofrequency bleed over need to be addressed appropriately. In resting muscle, creatine kinase was near equilibrium both in normal controls and in a patient with impaired oxidative phosphorylation. Oral intake of high doses of creatine monohydrate for several days resulted in significantly increased concentrations of phosphocreatine but had no measurable effect on the phosphocreatine resynthesis rate in resting muscle.

Creatine loading and resting skeletal muscle phosphocreatine flux: a saturation-transfer NMR study

Creatine loading and resting skeletal muscle phosphocreatine flux: a saturation-transfer NMR study

Creatine loading and resting skeletal muscle phosphocreatine flux: a saturation-transfer NMR study.

31P saturation-transfer nuclear magnetic resonance spectroscopy was used to study skeletal muscle phosphocreatine (PCr) flux in healthy male volunteers. Data analysis included consideration of effects from incomplete saturation and radiofrequency spillover. Spectra were recorded from the resting gastrocnemius muscle before and after 6 days of creatine monohydrate (Cr-H2O) intake (20 g/day). Parallel to an improved muscle performance during maximal intermittent exercise following Cr-H2O supplementation, the concentration of PCr increased (P=0.01) by 23% (34.9+/-2.8 mmol/l vs. 28.6+/-2.7 mmol/l), whereas other metabolites were unaffected (inorganic phosphate: 4.3+/-1.4 mmol/l, free intracellular Mg(2+): 1.1+/-0.7 mmol/l, cytosolic pH: 7.04+/-0.02). Forward and reverse fluxes through the creatine kinase (CK) reaction did not change significantly from their baseline levels (v(for): 11.8+/-5.4 mmol/l per second vs. 15.3+/-6.8 mmol/l per second, (v(rev): 9.5+/-3.4 mmol/l per second vs. 10.9+/-3.7 mmol/l per second). The rate of PCr resynthesis in resting muscle is not limited by the CK reaction, which is near equilibrium. Consequently, the post-load increase in total creatine has no effect on the unidirectional CK reaction rates.

Assessment of mitochondrial energy coupling in vivo by 13C/31P NMR.

The recently cloned uncoupling protein homolog UCP3 is expressed primarily in muscle and therefore may play a significant role in the regulation of energy expenditure and body weight. However, investigation into the regulation of uncoupling protein has been hampered by the inability to assess its activity in vivo. In this report, we demonstrate the use of a noninvasive NMR technique to assess mitochondrial energy uncoupling in skeletal muscle of awake rats by combining (13)C NMR to measure rates of mitochondrial substrate oxidation with (31)P NMR to assess unidirectional ATP synthesis flux. These combined (31)P/(13)C NMR measurements were performed in control, 10-day triiodo-l-thyronine (T(3))-treated (model of increased UCP3 expression), and acute 2,4-dinitrophenol (DNP)-treated (protonophore and mitochondrial uncoupler) rats. UCP3 mRNA and protein levels increased 8.1-fold (+/- 1.1) and 2.8-fold (+/- 0.8), respectively, in the T(3)-treated vs. control rat gastrocnemius muscle. (13)C NMR measurements of tricarboxylic acid cycle flux as an index of mitochondrial substrate oxidation were 61 +/- 21, 148 +/- 25, and 310 +/- 48 nmol/g per min in the control, T(3), and DNP groups, respectively. (31)P NMR saturation transfer measurements of unidirectional ATP synthesis flux were 83 +/- 14, 84 +/- 14, and 73 +/- 7 nmol/g per s in the control, T(3), and DNP groups, respectively. Together, these flux measurements, when normalized to the control group, suggest that acute administration of DNP (mitochondrial uncoupler) and chronic administration of T(3) decrease energy coupling by approximately 80% and approximately 60%, respectively, and that the latter treatment correlates with an increase in UCP3 mRNA and protein expression. This NMR approach could prove useful for exploring the regulation of uncoupling protein activity in vivo and elucidating its role in energy metabolism and obesity.

N-Alkyl-N-methylacetamidinium Ions. Isomerization and Water Catalyzed Exchange Rates in D2O

first_page settings Order Article Reprints Font Type: Arial Georgia Verdana Font Size: Aa Aa Aa Line Spacing:    Column Width:    Background: Open AccessAbstract N-Alkyl-N-methylacetamidinium Ions. Isomerization and Water Catalyzed Exchange Rates in D2O by Angel Dacosta, Sarah V. Pekerar and Oswaldo Núñez Laboratorio de Fisicoquímica Orgánica, Departamento de Procesos y Sistemas Universidad Simón Bolívar, Apartado Postal 89000, Caracas, Venezuela Molecules 2000, 5(3), 309-310; https://doi.org/10.3390/50300309 Published: 22 March 2000 Download Download PDF Download PDF with Cover Download XML Download Epub Versions Notes Previously we had reported results on the rate of stereoisomerization (see figure) in D2O of N-benzyl-N´-methyllacetamidinium ion in an ample pD range (D2O is used as a solvent).We found that the sigmoided-type profile of kobs. vs. pD plot, fits the rate expression : kobs = (k1[D+] + k2Ka)/ (Ka + [D+]), where k1 and k2 are the rates of isomerization of the acetamidinium ion and the acetamidine respectively and Ka is the acidity equilibrium constant of the acetamidinium ion. These constants were evaluated by measuring the rates at each pD using dynamic NMR(H) (line shape analysis and saturation transfer). Based on the low barrier of 19.7 Kcal/mol at 25°C (k1 = 0.02 s-1) it was suggested that the isomerization (EZ-ZE) of the acetamidinium form procceds throw rotation of the C-N partial double bond. We argued that this relatively low barrier is due the steric repulsion of the N-benzyl group that destabilizes the ground state (planar amidinium) relative to its twisted transition state. On the other hand, our result supports that the E-syn-Z-anti isomerization of the acetamidine form proceeds via rotation about a C-N single bond as it had been proposed previously. We stated that the proposed mechanism was in agreement with the measured low barrier of 14.7 Kcal/mol at 25°C (k1= 126 s-1). In order to test the proposed mechanisms and in view of the biological importance of the acetamidines we have undertaken a systematic study on the isomerization of N´-alkyl-N-methylacetamidines as a complementary support to our previous results on the N-benzyl.N´-methylacetamidine. Therefore we have prepared the following acetamidines: N-allyl-N´-methylacetamidine (R=CH2-CH=CH2), N-trifluoroethyl-N´-methylacetamidine (R=CH2CF3) and N-methoxiethyl-N´-methylacetamidine (R=CH2OCH3). The purpose of this study is to measure the isomerization rates under pD conditions in which both forms, acetamidinium ion and acetamidine, participate. Therefore, the rates k1 and k2 and the equilibrium, Ka of each of these compounds was determined. We then explore the existence of a structure-reactivitity relationship for each parameter. Finally we search on the rate of proton exchange, at low pD, were the D2O acts as a base and we found the relative acidity between the –NH sites at the acetamidinium ions. Share and Cite MDPI and ACS Style Dacosta, A.; Pekerar, S.V.; Núñez, O. N-Alkyl-N-methylacetamidinium Ions. Isomerization and Water Catalyzed Exchange Rates in D2O. Molecules 2000, 5, 309-310. https://doi.org/10.3390/50300309 AMA Style Dacosta A, Pekerar SV, Núñez O. N-Alkyl-N-methylacetamidinium Ions. Isomerization and Water Catalyzed Exchange Rates in D2O. Molecules. 2000; 5(3):309-310. https://doi.org/10.3390/50300309 Chicago/Turabian Style Dacosta, Angel, Sarah V. Pekerar, and Oswaldo Núñez. 2000. "N-Alkyl-N-methylacetamidinium Ions. Isomerization and Water Catalyzed Exchange Rates in D2O" Molecules 5, no. 3: 309-310. https://doi.org/10.3390/50300309 Find Other Styles Article Metrics No No Article Access Statistics For more information on the journal statistics, click here. Multiple requests from the same IP address are counted as one view.

Effects of Some Charged Amino Acid Mutations on the Electron Self-Exchange Kinetics of Cytochrome b5

Two mutants of trypsin-solubilized bovine liver cytochrome b5 (cyt b5) have been prepared to elucidate the function of charged residues near the heme exposed edge in the electron self-exchange kinetics of cyt b5. The Glu44, Glu56, and Asp60 were mutated into alanines in mutant I (E44A/E56A/D60A) while Glu44, Glu48, Glu56, and Asp60 were mutated into alanines in mutant II (E44A/E48A/E56A/D60A). The electron self-exchange rates of cyt b5 mutants I and II have been measured as a function of temperature and ionic strength using the one-dimensional 1H NMR saturation transfer method. Under the condition of μ = 0.10 M and [cyt b5] = 0.50 mM, the rate constant of the cyt b5 mutant I is (4.0 ± 0.4) × 103 M-1 s-1 at 20 °C. The value rises to (7.2 ± 0.7) × 103 M-1 s-1 at 35 °C; data from 20 to 35 °C gave ΔH⧧ = 6.8 ± 0.8 kcal mol-1 and ΔS⧧ = −19 ± 3 eu. The rate constant for cyt b5 mutant II is (8.4 ± 0.7) × 103 M-1 s-1 at 20 °C with μ = 0.10 M and [cyt b5] = 0.60 mM. The value rises to (17 ± 1) × 103 M-1 s-1 at 35 °...

Temperature dependence of the creatine kinase reaction measured in rat brain in vivo by31P NMR saturation transfer

Creatine kinase (CK) catalyzes the reversible phosphorylation of MgADP by phosphocreatine and thus regulates cellular concentrations of ADP and ATP. The temperature dependence of this reaction has been determined in rat brain in vivo between 30 and 40°C using31P NMR saturation transfer measurements. The pseudo-first-order rate constant for the forward CK reaction, kf, varies little with temperature over this range, with an apparent activation energy Ea= 14.2 ± 4.9 kJ/mol. This is considerably lower than the values of Eafor isolated CK enzymes. However, when changes in [MgADP] and [H+] with temperature are considered, a substrate concentration-independent value of Ea= 65.3 ± 9.7 kJ/mol is obtained for the maximum forward reaction velocity Vmax. This agrees well with literature values for the isolated brain-type isoform of CK.Key words: creatine kinase, activation energy, temperature, brain, rat.

Effect of Benzylic Methyl Groups on Kinetic Basicity of Amine Ligand in o-Boron Substituted N,N-Dimethylbenzylamines

Abstract Rates of the dissociation of the intramolecular B–N coordination bond in two series of phenylborane derivatives, the boronate and diethylborane complexes, with –CHMeNMe2 or –CMe2NMe2 group at the o-position were determined by the NMR lineshape analysis or saturation transfer method. The new organoboron compounds were synthesized from the corresponding organolithium compounds with appropriate boron reagents. Comparison of the kinetic data with those of the –CH2NMe2 compounds reveals that the barrier height to the dissociation, namely the kinetic basicity of the amine ligand, is increased as the molecule possesses more methyl groups at the benzylic position for both of the series of boron compounds. The X-ray structure of one of the boronate complexes and the NMR titration measurements of model amines indicate that the basicity of the amine ligand is not affected much by the methyl substitution in the coordinated form. Therefore, the substituent effect on the kinetic basicity is mainly ascribed to the destabilization of the transition state by the geminal dimethyl groups rather than to any inductive or steric effects at the initial state, especially for the –CMe2NMe2 compounds.

Divinyldisiloxane and divinylsilane complexes of rhodium(I)

Reaction of the tetrakis(cyclooctene)rhodium(I) complex [{Rh(C 8H 14- c) 2(μ-Cl)} 2] with the appropriate divinyldisiloxane molecules (ViSiR 2) 2O (R=Me or Ph) yields, by displacement of the cycloctene ligands, the complexes [{Rh(ViSiR 2) 2O(μ-Cl)} 2] (R=Me ( 1) or Ph ( 2)). These react further with a tertiary phosphine PR 3 to give cis-[Rh{(ViSiR 2) 2O}(PR′ 3)Cl] (R′=Ph or C 6H 4Me- p). The complex cis-[{Rh(Vi 2SiMe 2)(μ-Cl)} 2] ( 7) was similarly prepared by the displacement of ethylene from [{Rh(C 2H 4) 2(μ-Cl)} 2] by the divinyldimethylsilane Vi 2SiMe 2. X-ray molecular structures of the crystalline complexes 1, 2 and 7 show a distorted square planar Rh(I) environment, the CH 2CH groups being orthogonal to this plane; 1 and 2 have the Rh–(ViSiR 2) 2O metallacycle in the chair conformation, but differ in the nature of the central Rh(Cl)RhCl core, which is planar for 1 and puckered for 2, but each of 1 and 2 is the rac-diastereoisomer, whereas 7 has the meso-configuration. In solution 1 and 2 exist as a mixture of isomers, probably the rac- and meso-pairs as established by multinuclear NMR spectral studies. A series of saturation transfer NMR spectroscopic experiments showed that the divinyldisiloxane ligands in [{Rh(ViSiPh 2) 2O(μ-Cl)} 2] underwent a dynamic process involving the dissociation, rotation and then reassociation of the vinyl groups.

Temperature dependence of the creatine kinase reaction measured in rat brain in vivo by <sup>31</sup>P NMR saturation transfer

Temperature dependence of the creatine kinase reaction measured in rat brain in vivo by <sup>31</sup>P NMR saturation transfer

31P NMR studies of creatine kinase flux in M-creatine kinase-deficient mouse heart.

Hearts of wild-type and cytosolic muscle creatine kinase (M-CK)-knockout mice were perfused with Krebs-Henseleit buffer containing 10 mM glucose and 5 mM pyruvate and studied during pacing at 400 and 600 beats/min and during K+ arrest. Phosphocreatine (PCr) and ATP concentrations in M-CK-deficient hearts were not significantly different from those in wild-type hearts. With the use of 31P NMR saturation transfer, the flux mediated predominantly by mitochondrial creatine kinase (Mi-CK) was clearly detected in M-CK-deficient hearts. Mi-CK flux was 4.8 +/- 0.6 and 4.5 +/- 0.6 mM/s during pacing at 400 and 600 beats/min, respectively, and was 3. 5 +/- 0.4 mM/s during cardiac arrest. In control hearts total CK flux was 7.8 +/- 1.1 and 6.6 +/- 1.3 mM/s during pacing at 400 and 600 beats/min, respectively, and decreased to 3.8 +/- 0.5 mM/s during arrest. It is suggested that the relative contribution of Mi-CK to the total NMR-measured CK flux in the wild-type heart is higher than that of the homodimeric M-CK isoform (MM-CK).

Determination of mitochondrial creatine kinase fluxes in intact heart mitochondria using 31P-saturation transfer nuclear magnetic resonance spectroscopy

Forward (→ATP) and reverse (→CrP) fluxes through the creatine kinase reaction were determined in isolated rat and bovine heart mitochondria and with soluble MM-CK from rabbit skeletal muscle, using 31P-saturation transfer NMR. With soluble MM-CK forward and reverse fluxes were identical in the absence and presence of BSA or rat liver mitochondria. Addition of liver mitochondria decreased fluxes with increasing mitochondria concentration. The flux f/ V max (f) ratio was 0.006 with 10 mg BSA and 0.04 with 10 mg rat liver mitochondria, respectively. With heart mitochondria, flux r was considerably higher than flux f and the flux f/ V max (f) ratio was 1.7 for rat heart and 0.22 for bovine heart. It is concluded that in the presence of isolated mitochondria, the flux through the creatine kinase is driven by the mitochondrial ATP–ADP turnover. Therefore the flux f/ V max (f) ratio is highest for rat heart mitochondria with a high ATP–ADP turnover, intermediate for bovine heart mitochondria and low for MM-CK in the presence of liver mitochondria. It is lowest with MM-CK alone, where the creatine kinase reaction is at equilibrium and external ATP–ADP turnover is absent. The higher reverse than forward fluxes of mitochondrial creatine kinase determined at steady state by saturation transfer NMR, are caused mainly by a high ATP↔P i exchange in heart mitochondria preparations, having a high ATPase activity, compared to liver mitochondria.

Off-Resonance Irradiation Effect in Steady-State NMR Saturation Transfer

The off-resonance irradiation effect (spill-over effect), occurring in steady-state NMR saturation-transfer experiments, is studied theoretically and experimentally for a two-spin system in chemical exchange, when a contralateral irradiation is applied to record the reference spectrum. The relevant parameter chosen for this study is the saturation-transfer ratio. It is defined as the ratio between the value of one exchanging magnetization, obtained when saturating the other, and that of the same magnetization measured when applying a contralateral irradiation. The theoretical study is carried out via a model based on the Bloch equations modified for chemical exchange and expressed in a doubly tilted single rotating frame. The saturation-transfer ratio is expressed as a function of the saturating RF field magnitude. It is shown that the RF field applied off-resonance during the acquisition of the reference spectrum does not correct the experimental saturation-transfer ratio for the spill-over effect. In fact, the saturation-transfer ratio increases with the magnitude of this field. This result is qualitatively explained by the consequence of the effective magnetic fields' relative orientations upon the amount of exchanged magnetization. The validity of the theoretical description is tested experimentally with a solution ofN,N-dimethylacetamide in which chemical exchange arises from internal hindered rotation. An experimental protocol is proposed to detect spill-over and correct it when necessary. The way to describe spill-over theoretically when more than two spins interact by chemical exchange and/or dipolar coupling is also given.

Effects of Hypoxia and Toxicant Exposure on Arginine Kinase Function as Measured by 31P-NMR Magnetization Transfer in Living Abalone

The activity of arginine kinase (AK) was evaluated by saturation transfer NMR in red abalone ( Haliotis rufescens) in response to hypoxia, sodium azide (NaN 3; an inhibitor of cytochrome c oxidase), or pentachlorophenol (PCP; an uncoupler of oxidative phosphorylation) exposure. Pseudo-first order rate constants ( K for) in the forward (ATP forming) reaction direction showed maximal increases from basal values of 0.025 s −1 to 0.095, 0.114, 0.126 s −1 for NaN 3 hypoxia, and PCP exposures, respectively. Increases in K for were inversely correlated (r 2 = 1.00) to declines in ATP concentration in all exposed animals. Flux (the product of K for and phosphoarginine concentration) appeared to converge on a common value, from basal flux values of 0.257 mM PA s −1 to 0.703, 0.770, and 0.627 mM PA s −1 for NaN 3, hypoxia, and PCP exposures, respectively. It seems likely that all three stresses were equally effective at inhibiting mitochondrial ATP formation, which may account for the similarity in flux increase, possibly to maximal rates of AK-mediated ATP formation. Differences in K for are related to declines in ATP concentrations, which appear to be stress specific, and likely indicate additional mechanisms of toxicity for NaN 3 and PCP.

31P NMR saturation transfer study of the creatine kinase reaction in human skeletal muscle at rest and during exercise.

The creatine kinase reaction has been studied by 31P NMR in exercising human calf muscle. Quantitative analysis of high energy phosphates and saturation transfer study of the creatine kinase flux in the direction of ATP synthesis (Vfor) were performed at rest and during exercise. As expected, exercise induced a [PCr] decrease (from 28.5 +/- 0.9 to 21.9 +/- 1.5 mM, P < 0.01) matched by a Pi increase (from 4.5 +/- 0.2 to 8.9 +/- 1.8 mM, P = 0.06). pHi and [ATP] remained unchanged. Vfor did not change from rest (12.4 +/- 0.9 mM s(-1)) to moderate exercise and decreased at the highest exercise level (8.4 +/- 1.4 mM s(-1), P = 0.006). This observation differs from the prediction of the creatine kinase rate equation, showing an increase in the flux with exercise intensity. Computations suggest that this discrepancy arises from metabolite compartmentalization and/or from the reaction kinetics of a dead end complex stabilized by planar anions.

Evidence for DirecttransInsertion in a Hydrido‐Olefin Rhodium Complex—Free Nitrogen as a Trap in a Migratory Insertion Process

Reduction of the hydrido chloride complex [Rh(H)Cl{CH3 C(CH2 CH2 -P(tBu)2 )2 }] (4) with NaH under a nitrogen atmosphere results in formation of two products: the dinitrogen complex [Rh(N2 )-{CH3 C(CH2 P(tBu)2 )2 }] (2) and the unusual low-valent hydrido-olefin complex, [RhH{CH2 C(CH2 CH2 P(tBu)2 )2 }] (3). In the presence of N2 , complexes 2 and 3 are in equilibrium in solution; 2 is about 2.9kcalmol(-1) more stable than 3 + N2 . Both complexes co-crystallize in the solid state; they occupy the same crystallographic site in the crystal lattice (P2-(1)/c; Z = 4; a = 12.173(2), b = 14.121 (3), c = 15.367 (3); α = 90, β = 106.50(3), γ = 90°). The mechanism of the reversible interconversion of 2 and 3 has been studied in detail. Complex 3 undergoes rapid olefin insertion/β-hydrogen elimination processes. The insertion rates were measured at different temperatures by saturation transfer NMR experiments, providing evidence for a highly organized late transition state (δS≠≈︂ - 40 e.u.), which can be caused by a concerted "trans migration". This theoretically unfavorable process is assisted by a distortion from the ideal square-planar configuration, including a decrease of the P-Rh-P angle and some bias of the double bond toward the hydride as indicated by the X-ray crystal structure of 3. Under a nitrogen atmosphere, the intermediate formed upon olefin insertion is slowly trapped by free dinitrogen to form complex 2. The dinitrogen dissociation from 2 was found to be the rate-determining step for the overall interconversion of 2 and 3 (δG≠298 = 24.1 kcalmol(-1) ).

Reaction of (μ-H)2Os3(CO)9(PPh3) with Acetylene and Ethylene. Structures, Dynamics, and Interconversion of Two Isomers of the Vinyl Complex (μ-H)Os3(CO)9(PPh3)(μ-CHCH2)

As reported previously by Brown and Evans, the reaction of (μ-H)2Os3(CO)9(PPh3) with acetylene provides two isomers of the hydrido vinyl compound (μ-H)Os3(CO)9(PPh3)(μ-η2-CHCH2) (2). The solid-state structures of these two isomers now have been defined by single-crystal X-ray crystallography. The major isomer 2a contains a triangular array of osmium atoms with the vinyl ligand bridging the Os(2)−Os(3) edge, forming a σ-bond to Os(3) and a π-bond to Os(2), and the triphenylphosphine ligand coordinating to Os(2) in an “in-plane” position. The presence of the hydride ligand also bridging the Os(2)−Os(3) edge was deduced indirectly. The minor isomer 2b also contains a triangular Os3 array with the vinyl ligand bridging the Os(2)−Os(3) edge and forming a σ-bond to Os(2) and a π-bond to Os(3). The triphenylphosphine ligand is bound to Os(1). The presence of the bridging hydride on the Os(1)−Os(3) edge was not directly detected but was supported by analysis of the structure; the hydride ligand therefore does not bridge the same edge as the vinyl moiety. The solution structures and dynamics of 2a and 2b have been investigated by variable-temperature 13C NMR and 13C spin saturation transfer experiments; isomer 2a exhibits evidence for nondegenerate σ/π bond interchange in the bridging vinyl group. Separation of 2a and 2b can be effected by thin-layer chromatography (silica; CH2Cl2/C6H14, 1:3); subsequent 1H NMR spectra reveal slow equilibration of 2a and 2b (t1/2 = 85 min, 2a:2b = 2.2:1 at equilibrium). The same mixture of isomers is formed from the reaction of 1 with ethylene at high pressure (500 psig), with the previously characterized complex (μ-H)2Os3(CO)9(PPh3)(μ-CHCH3) as an intermediate.

Kinetic studies of electron transfer in the cyt b 5 : metHb system

The kinetics of methemoglobin reduction by cytochrome b 5 has been studied by stopped-flow and saturation transfer NMR. A forward rate constant k f = 2.44×104 M–1 s–1 and a reverse rate constant k b = 540 M–1s–1 have been observed at 10 mm, pH 6.20, 25 °C. The ratio k f/k b = k eq = 43.6 is in good agreement with the equilibrium constant calculated from the electrochemical potential between cyt b 5 and methemoglobin. A bimolecular collisional mechanism is proposed for the electron transfer from cyt b 5 to methemoglobin based on the kinetic data analysis. The dependence of the rate constants on ionic strengths supports such collisional mechanism. It is also found that the reaction rate strongly depends on the conformations of methemoglobin.