Activation and Reaction Volumes in Solution. 3

A Kinetic Study on the Nitric Oxide Trapping by the Fe(III)-Dithiocarbamate-Nitroxyl Complex: Influence of the Ligand Structure and External Pressure on the Trapping Rate

The Fe(III)-dithiocarbamate-nitroxyl (Fe-dtc-nitroxyl) complex, where nitroxyl represents nitroxyl free radical (R-N(O)-R’), has been shown to trap nitric oxide (NO) in aqueous solution, replacing the nitroxyl ligand. As a result, non-complexed nitroxyl radical was released to the solution. Fe(III)-dithiocarbamate (Fe-dtc) complexes without the nitroxyl ligand also traps NO, producing NO-Fe-dtc complex. These two reactions have been used for the purpose of NO detection. We investigated how the ligand structure and external pressure influence NO trapping rates in these complexes. The ratios of NO trapping rates (k 1/k 2) between various Fe-dtc-nitroxyl complexes and the Fe-dtc complex were determined by using a competitive NO trapping method. The ratio k 1/k 2 was 29.8±2.8 for Fe-dtc-nitroxyl and Fe-dtc, where dtc = N-(dithiocarboxy) sarcosine (DTCS) and nitroxyl = 2,2,6,6-tetramethyl- piperidine-1-oxyl (TEMPO). For another combination, dtc = N-methyl-D-glucamine dithiocarbamate (MGD)/nitroxyl = TEMPO, k 1/k 2 was 7.19±0.25. Overall, NO trapping rate of the Fe-dtc-nitroxyl complex was faster than that of the Fe-dtc complex, and k 1/k 2 for Fe-MGD complex was dependent on the electron-withdrawing or -repelling nature of the functional group in the ligand. Based on pressure dependence experiments for the competitive reaction, we obtained large negative activation volumes for NO trapping of the Fe-dtc-nitroxyl complex as well as the difference in activation volumes (–45 to –26 cm3 mol–1) between the NO trapping reactions by the two Fe complexes. These data sets with different nature allowed us to speculate the reaction mechanism for NO trapping of the Fe-dtc-nitroxyl complex.

Characterization of Water-Soluble Cucurbit[7]uril in Alcohol-Water Mixtures by High-Pressure Studies on the Inclusion Complexation with New Methylene Blue

The inclusion complexation of new methylene blue (NMB) with cucurbit[7]uril (CB[7]) was studied spectrophotometrically. It was found that CB[7] forms a 1:1 inclusion complex with NMB in alcohol-water mixtures and the structure of the inclusion complex has been established by 1H-NMR measurements. We have determined the association constants of CB[7] with NMB under high pressures. The association constants of CB[7] increase as the external pressure and solvent polarity increase. Pressure dependence experiments enabled us to calculate the reaction volume for inclusion of CB[7] to range from -15.8 to -10.8cm3 mol-1. From analyses of the high-pressure results, the volume change caused by water and/or alcohol molecules repelled from the CB[7] cavity in the alcohol-water mixtures was evaluated, which is indicative of the presence of water and/or alcohol molecules in the CB[7] cavity in the alcohol-water mixtures. The volume of molecules repelled from the CB[7] cavity plays an important role in the volume change upon inclusion.

Ionic liquids: just Molten salts after all?

While there has been much effort in recent years to characterise ionic liquids in terms of parameters that are well described for molecular solvents, using these to explain reaction outcomes remains problematic. Herein we propose that many reaction outcomes in ionic liquids may be explained by considering the electrostatic interactions present in the solution; that is, by recognising that ionic liquids are salts. This is supported by evidence in the literature, along with studies presented here.

Mechanistic insight into formation of oxo-iron(IV) porphyrin pi-cation radicals from enzyme mimics of cytochrome P450 in organic solvents

Two new models for cytochrome P450 in which the thiolate axial ligand is replaced by a RSO(3)(-) group, form oxo-iron(IV) porphyrin pi-cation radicals as sole oxidation products in "peroxo shunt" reactions independent of the nature of the employed solvent (polar or non-polar) and electronic nature of the porphyrin rings. Although the properties of the solvent and push-pull effects from the porphyrin rings do not affect the mode of the O-O bond cleavage (heterolytic or homolytic) in these models, they strongly affect the rate and mechanism of each reaction step leading to the formation of the high-valent iron intermediates. This article reports the results of mechanistic studies involving the measurements of the rate of oxo-iron(IV) porphyrin pi-cation radical formation from the enzyme mimics of P450 for different oxidant concentration, temperature and pressure in selected organic solvents. Extraction of the appropriate rate constants and activation parameters for the reactions studied enable a detailed discussion of the effects of solvent and electronic nature of the porphyrin rings on the position of the first pre-equilibrium involving formation of the acylperoxo-iron(III) porphyrin intermediate, as well as on the rate of heterolytic O-O bond cleavage leading to the formation of the high-valent iron species. Furthermore, an unusual effect of solvent on the kinetics of oxo-iron(IV) porphyrin pi-cation radical formation in methanol is demonstrated and discussed in the present work.

Hydrophobicity and mixing effects on select heterogeneous, water-accelerated synthetic reactions

The influence of aqueous reaction media on organic reactions is a topic of long-standing interest, particularly as it affects the rate or selectivity of synthetic reactions. Sometimes such reactions appear homogeneous, typically in dilute solution, whereas others are obviously heterogeneous, typically in concentrated solution that is more characteristic of a preparative synthetic reaction. The latter situation has been termed "on water." Here, it is demonstrated that the rates of heterogeneous ene reactions, Passerini reactions, and Ugi reactions in pure water and in aqueous solutions are dependent on the mixing method and reactant polarity, consistent with the involvement of hydrophobic interactions in their acceleration.

Application of high pressure laser flash photolysis in studies on selected hemoprotein reactions

This article focuses on the application of high pressure laser flash photolysis for studies on selected hemoprotein reactions with the objective to establish details of the underlying reaction mechanisms. In this context, particular attention is given to the reactions of small molecules such as dioxygen, carbon monoxide, and nitric oxide with selected hemoproteins (hemoglobin, myoglobin, neuroglobin and cytochrome P450(cam)), as well as to photo-induced electron transfer reactions occurring in hemoproteins (particularly in various types of cytochromes). Mechanistic conclusions based on the interpretation of the obtained activation volumes are discussed in this account.

Searching for Stable, Five-Coordinate Aquated Al(III) Species. Water Exchange Mechanism and Effect of pH

Density functional theory calculations have been performed for the water exchange mechanism of aquated Al(III). The effect of pH was considered by studying the exchange processes for [Al(H2O)6]3+ and its conjugated base, [Al(H2O)5OH]2+. Both complexes were found to exchange water in a dissociative way with activation energies (EA) of 15.9 and 10.2 kcal/mol, respectively. The influence of solvent molecules on the gas-phase cluster model was considered by the addition of up to four water molecules to the model system. The stabilizing effect of the solvent on the transition state decreases EA to 8.6 (hexa-aqua complex) and 7.6 (monohydroxo complex) kcal/mol, whereas EA for all hydroxo species is consistently significantly lower than those for the related aqua systems, which indicates a much faster water exchange rate. For the hydroxo complex, all calculated five-coordinate intermediates, nH2O.[Al(H2O)4(OH)]2+ (n = 1, 2, 3, 4, 5), are more stable than the corresponding six-coordinate reactants. Our results therefore suggest the presence of a stable five-coordinate species of aquated Al(III), namely, the [Al(H2O)4(OH)]2+ complex.

Constraining molecules at the closest approach: chemistry at high pressure

The purpose of this tutorial review is to illustrate the effects that the application of high pressures can have on chemical reactions involving highly compressible molecular materials. The essentials of the high-pressure technology (generation and in situ control of high pressures) are described with particular attention to the versatile diamond anvil cell (DAC) apparatus. The general effects of pressure on chemical equilibrium, reaction rate and reaction mechanism are discussed. The motivation for application of high-pressure methods (in the 1-300 MPa range) to chemical synthesis and in biochemistry are illustrated focusing the attention on environmental effects and with an excursus on developing biotechnological applications. The peculiarities and the unexpected outcomes of chemical reactions occurring at very high pressures (>or=300 MPa) are discussed considering the extraordinary results obtained in polymerization and amorphization of simple molecules and of unsaturated hydrocarbons. The possible connection of the high temperature-high pressure thresholds for chemical reactions with microscopic counterparts (intermolecular distances, molecular orientations) is also discussed.

Characterization of dual-wavelength seminaphthofluorescein and seminapthorhodafluor dyes for pH sensing under high hydrostatic pressures

Hydrostatic pressure is an important physical parameter in biology, with pressures in the few-hundred-atm range having significant effects on cellular morphology, metabolism, and viability. To ensure valid results when studying pressure effects using fluorescence spectroscopy and imaging methods, metabolic probes need to be characterized for high-pressure use. Of interest is the sensing of pH at high pressures due to the key role that pH plays in cellular function. Despite the availability of pH-sensitive dyes, only a few have been characterized for high-pressure use. Here we present the effects of pressure on the acid-base equilibria of four dual-wavelength seminaphthorhodafluor and seminaphthofluorescein dyes (pK(a)=6.6-7.8). Using phosphate buffers as high-pressure pH references, we investigate the pressure dependence of pK(a) for these dyes and determine the volume change associated with the acid-dissociation reaction. We find that if pressure-induced pK(a) changes are not accounted for during interpretation of emission spectra, systematic errors of up to 0.02 pH units per 100atm would result, comparable to previously measured pressure-induced pH changes in vivo. Results are validated by correctly sensing pH changes in Tris and acetate solutions. Methods presented here are applicable to other metabolic probes utilizing dual-wavelength ratiometric sensing modes.

Kinetic, Thermodynamic, and Mechanistic Patterns for Free (Unbound) Cytochromec at Au/SAM Junctions: Impact of Electronic Coupling, Hydrostatic Pressure, and Stabilizing/Denaturing Additives

Combined kinetic (electrochemical) and thermodynamic (calorimetric) investigations were performed for an unbound (intact native-like) cytochrome c (CytC) freely diffusing to and from gold electrodes modified by hydroxyl-terminated self-assembled monolayer films (SAMs), under a unique broad range of experimental conditions. Our approach included: 1) fine-tuning of the charge-transfer (CT) distance by using the extended set of Au-deposited hydroxyl-terminated alkanethiol SAMs [-S-(CH(2))(n)-OH] of variable thickness (n=2, 3, 4, 6, 11); 2) application of a high-pressure (up to 150 MPa) kinetic strategy toward the representative Au/SAM/CytC assemblies (n=3, 4, 6); 3) complementary electrochemical and microcalorimetric studies on the impact of some stabilizing and denaturing additives. We report for the first time a mechanistic changeover detected for "free" CytC by three independent kinetic methods, manifested through 1) the abrupt change in the dependence of the shape of the electron exchange standard rate constant (k(o)) versus the SAM thickness (resulting in a variation of estimated actual CT range within ca. 15 to 25 A including ca. 11 A of an "effective" heme-to-omega-hydroxyl distance). The corresponding values of the electronic coupling matrix element vary within the range from ca. 3 to 0.02 cm(-1); 2) the change in activation volume from +6.7 (n=3), to approximately 0 (n=4), and -5.5 (n=6) cm(3) mol(-1) (disclosing at n=3 a direct pressure effect on the protein's internal viscosity); 3) a "full" Kramers-type viscosity dependence for k(o) at n=2 and 3 (demonstrating control of an intraglobular friction through the external dynamic properties), and its gradual transformation to the viscosity independent (nonadiabatic) regime at n=6 and 11. Multilateral cross-testing of "free" CytC in a native-like, glucose-stabilized and urea-destabilized (molten-globule-like) states revealed novel intrinsic links between local/global structural and functional characteristics. Importantly, our results on the high-pressure and solution-viscosity effects, together with matching literature data, strongly support the concept of "dynamic slaving", which implies that fluctuations involving "small" solution components control the proteins' intrinsic dynamics and function in a highly cooperative manner as far as CT processes under adiabatic conditions are concerned.

High static pressure alters spin trapping rates in solution. Dependence on the structure of nitrone spin traps

Using a competitive spin trapping method, relative spin trapping rates were quantified for various short-lived radicals (methyl, ethyl, and phenyl radicals). High static pressure was applied to the competitive spin-trapping system by employing high-pressure electron spin resonance (ESR) equipment. Under high pressure (490 bar), spin trapping rate constants for alkyl and phenyl radicals increased by 10 to 40%, and the increase was dependent on the structure of nitrone spin traps. A maximum increase was obtained when tert-butyl(4-pyridinylmethylene)amine N-oxide (4-POBN) was used as a spin trap. Activation volumes (DeltaDeltaV(double dagger)) for the two spin trapping reactions were calculated to be -17-(-9) cm(3) mol(-1) for the 4-POBN system.

The direct determination of partial molar volumes and reaction volumes in ultra-dilute non-reactive and reactive multi-component systems using a combined spectroscopic and modified response surface model approach

Two experimental multi-component organometallic systems were studied, namely, (1) a non-reactive system consisting of [Mo(CO)(6)], [Mn(2)(CO)(10)], and [Re(2)(CO)(10)] in toluene under argon at 298.15 K and 0.1 MPa and (2) a reactive system consisting of [Rh(4)(CO)(12)] + PPh(3)--> [Rh(4)(CO)(11)PPh(3)] + CO in n-hexane under argon at 298.15 K and 0.1 MPa. The mole fractions of all solutes were less than 140 x 10(-6) in system (1) and less than 65 x 10(-6) in system (2). Simultaneous in-situ FTIR spectroscopic measurements and on-line oscillatory U-tube density measurements were performed on the multi-component solutions. A newly developed response surface methodology was applied to the data sets to determine the individual limiting partial molar volumes of all constituents present as well as the reaction volume. The limiting partial molar volumes obtained for system (1) were 176.4 +/- 2.5, 265.1 +/- 2.4, and 276.8 +/- 2.4 cm(3) mol(-1) for [Mo(CO)(6)], [Mn(2)(CO)(10)], and [Re(2)(CO)(10)], respectively and are consistent with independent binary experiments. The limiting partial molar volumes obtained for system (2) were 310.7 +/- 2.7, 219.8 +/- 2.2 and 461.5 +/- 4.5 cm(3) mol(-1) for [Rh(4)(CO)(12)], PPh(3) and [Rh(4)(CO)(11)PPh(3)], respectively. In addition, a reaction volume Delta(r)V equal to -17.0 +/- 5.7 cm(3) mol(-1) was obtained. The present results demonstrate that both partial molar volumes and reaction volumes can be obtained directly from multi-component organometallic solutions. This development provides a new tool for physico-chemical determinations relevant to a variety of solutes and their reactions.

Mechanistic Information on the Reductive Elimination from Cationic Trimethylplatinum(IV) Complexes to Form Carbon−Carbon Bonds

Cationic complexes of the type fac-[(L(2))Pt(IV)Me(3)(pyr-X)][OTf] (pyr-X = 4-substituted pyridines; L(2) = diphosphine, viz., dppe = bis(diphenylphosphino)ethane and dppbz = o-bis(diphenylphosphino)benzene; OTf = trifluoromethanesulfonate) undergo C-C reductive elimination reactions to form [L(2)Pt(II)Me(pyr-X)][OTf] and ethane. Detailed studies indicate that these reactions proceed by a two-step pathway, viz., initial reversible dissociation of the pyridine ligand from the cationic complex to generate a five-coordinate Pt(IV) intermediate, followed by irreversible concerted C-C bond formation. The reaction is inhibited by pyridine. The highly positive values for DeltaS()(obs) = +180 +/- 30 J K(-1) mol(-1), DeltaH(obs) = 160 +/- 10 kJ mol(-1), and DeltaV()(obs) = +16 +/- 1 cm(3) mol(-1) can be accounted for in terms of significant bond cleavage and/or partial reduction from Pt(IV) to Pt(II) in going from the ground to the transition state. These cationic complexes have provided the first opportunity to carry out detailed studies of C-C reductive elimination from cationic Pt(IV) complexes in a variety of solvents. The absence of a significant solvent effect for this reaction provides strong evidence that the C-C reductive coupling occurs from an unsaturated five-coordinate Pt(IV) intermediate rather than from a six-coordinate Pt(IV) solvento species.

High-pressure studies of aggregation of recombinant human interleukin-1 receptor antagonist: thermodynamics, kinetics, and application to accelerated formulation studies

Recombinant human interleukin-1 receptor antagonist (IL-1ra) in aqueous solutions unfolds and aggregates when subjected to hydrostatic pressures greater than about 180 MPa. This study examined the mechanism and thermodynamics of pressure-induced unfolding and aggregation of IL-1ra. The activation free energy for growth of aggregates (DeltaG-/+(aggregation)) was found to be 37 +/- 3 kJ/mol, whereas the activation volume (DeltaV-/+(aggregation)) was -120 +/- 20 mL/mol. These values compare closely with equilibrium values for denaturation: The free energy for denaturation, DeltaG(denaturation), was 20 +/- 5 kJ/mol, whereas the partial specific volume change for denaturation, DeltaV(denaturation), was -110 +/- 30 mL/mol. When IL-1ra begins to denature at pressures near 140 MPa, cysteines that are normally buried in the native state become exposed. Under oxidizing conditions, this results in the formation of covalently cross-linked aggregates containing nonnative, intermolecular disulfide bonds. The apparent activation free energy for nucleation of aggregates, DeltaG-/+(nuc), was 42 +/- 4 kJ/mol, and the activation volume for nucleation, DeltaV-/+(nuc),was -175 +/- 37 mL/mol, suggesting that a highly solvent-exposed conformation is needed for nucleation. We hypothesize that the large specific volume of IL-1ra, 0.752 +/- 0.004 mL/g, coupled with its relatively low conformational stability, leads to its susceptibility to denaturation at relatively low pressures. The positive partial specific adiabatic compressibility of IL-1ra, 4.5 +/- 0.7 +/- 10(-12) cm2/dyn, suggests that a significant component of the DeltaV(denaturation) is attributable to the elimination of solvent-free cavities. Lastly, we propose that hydrostatic pressure is a useful variable to conduct accelerated formulation studies of therapeutic proteins.

Pressure modulation, a new dynamic technique for the electrochemical determination of adsorption, reaction and activation volumes

A new dynamic method for the measurement of pressure dependent kinetic and thermodynamic quantities is described and its successful operation demonstrated for two example systems. The pressure was modulated with an amplitude of only +/-1 bar or less by means of a piezo-transducer. The small effect on the reaction rate, potential or charge of the electrode can be detected using the lock in technique. The determination of the reaction volume of the redox couple Fe(CN)6(4-)/Fe(CN)6(3-) served as a control of the validity of the measurement and the reliability of the experimental approach. As a first model system the adsorption of hydrogen on polycrystalline Pt was studied. A volume of adsorption of 4.8 cm3 mol(-1) was found. The partial molar volume for protons in water (upsilon+ = -5.4 cm3 mol(-)) measured by Zana and Yeager, J. Phys. Chem., 1967, 71, 521-536, was used to establish that the partial molar volume of the adsorbed hydrogen was close to zero.

Optical fiber-based on-line UV/Vis spectroscopic monitoring of chemical reaction kinetics under high pressure in a capillary microreactor

With a miniaturized (3 microL volume) fiber-optics based system for on-line measurement by UV/Vis spectroscopy, the reaction rate constants (at different pressures) and the activation volumes (deltaV(not =)) were determined for a nucleophilic aromatic substitution and an aza Diels-Alder reaction in a capillary microreactor.

Kinetic and thermodynamic studies on ligand substitution reactions and base-on/base-off equilibria of cyanoimidazolylcobamide, a vitamin B12 analog with an imidazole axial nucleoside

Ligand substitution reactions of the vitamin B12 analog cyanoimidazolylcobamide, CN(Im)Cbl, with cyanide were studied. Cyanide substitutes imidazole (Im) in the alpha-position more slowly than it substitutes dimethylbenzimidazole in cyanocobalamin (vitamin B12). The kinetics of the displacement of Im by CN- showed saturation behaviour at high cyanide concentration; the limiting rate constant was found to be 0.0264 s(-1) at 25 degrees C and is characterized by the activation parameters: DeltaH(not =) = 111 +/- 2 kJ mol(-1), DeltaS(not =) = +97 +/- 6 J K(-1) mol(-1), and DeltaV(not =) = +9.3 +/- 0.3 cm3 mol(-1). These parameters are interpreted in terms of an I(d) mechanism. The equilibrium constant for the reaction of CN(Im)Cbl with CN- was found to be 861 +/- 75 M(-1), which is significantly less than that obtained for the reaction of cyanocobalamin with CN- (viz. 10(4) M(-1)). pKbase-off for the base-on/base-off equilibrium was determined spectrophotometrically and found to be 0.99 +/- 0.05, which is about 0.9 pH units higher than that obtained previously in the case of cyanocobalamin. In addition, the kinetics of the base-on/base-off reaction was studied using a pH-jump technique and the data obtained revealed evidence for an acid catalyzed reaction path. The results obtained in this study are discussed in reference to those reported previously for cyanocobalamin.

Activation parameters for the spontaneous and pressure-induced phases of the dissociation of single-ring GroEL (SR1) chaperonin

We investigated the dissociation of single-ring heptameric GroEL (SR1) by high hydrostatic pressure in the range 0.5-3.0 kbar. The kinetics were studied as a function of temperature in the range 15-35 degrees C. The dissociation processes at each pressure and temperature showed biphasic behavior. The slower rate (k1,obs) was confirmed to be the self-dissociation of SR1 at any specific temperature at atmospheric pressure. This dissociation was pressure independent and followed concentration-dependent first-order kinetics. The self-dissociation rates followed normal Eyring plots (In k1,obs/T vs. 1/T) from which the free energy of activation (deltaG++ = 22 +/- 0.3 kcal mol(-1)), enthalpy of activation (deltaH++ = 18 +/- 0.5 kcal mol(-1)), and entropy of activation (deltaS++ = -15 +/- 1 kcal mol(-1)) were evaluated. The effect of pressure on the dissociation rates resulted in nonlinear behavior (ln k2,obs vs. pressure) at all the temperatures studied indicating that the activation volumes were pressure dependent. Activation volumes at zero pressure (V++o) and compressibility factors (beta++) for the dissociation rates at the specific temperatures were calculated. This is the first systematic study where the self-dissociation of an oligomeric chaperonin as well as its activation parameters are reported.

Effects of pressure and solvents on the infrared absorption intensity of the O-H stretching mode of phenol in solutions

Effects of pressure and solvents on the infrared spectrum of phenol in solutions have been investigated using a hydrostatic high-pressure cell with synthetic diamond windows. For the first time, we performed a quantitative investigation of the effect of pressure on the absolute intensity of O-H stretching mode up to 150 MPa (in CCl4) and 200 MPa (in CS2). For comparison, we measured the effect of solvents on the absorption intensity. The Polo-Wilson theory, which is the most traditional theory for medium effects on the intensity, was tested for present results. The pressure dependence was in sufficient agreement with their formula, while the solvent dependence is unsatisfactory. This suggests that the traditional intensity correction by Polo-Wilson's formula is practically valid for pressure-tuning infrared experiments.

Mechanistic Elucidation of the Substitution Behavior of Alkyl Cobaloximes in Water and Methanol as Solvents

The ligand substitution behavior of trans-[Co(Hdmg)2(R)S] (R = CH3, PhCH2; Hdmg = dimethylglyoximate; S = H2O and/or MeOH) was studied for imidazole, pyrazole, 1,2,4-triazole, N-acetylimidazole, 5-chloro-1-methylimidazole, NO2-, Ph3P, Ph3As, and Ph3Sb as entering ligands. In all cases, except for Ph3As and Ph3Sb, the observed kinetics shows a linear dependence on the entering nucleophile concentration with no evidence for a back reaction. In the case of Ph3As and Ph3Sb as entering nucleophiles, kinetic evidence for a reverse solvolysis reaction is at hand. Activation parameters (DeltaH++, DeltaS++, and DeltaV++) were determined from the temperature and pressure dependence of all studied reactions and support the operation of a dissociatively activated substitution mechanism. The rate and activation parameters show that there is an increase in the dissociative character from a dissociative interchange to a limiting dissociative mechanism that depends on the nature of R and the entering nucleophile. The crystal structure of trans-[Co(en)2(Me)H2O]2+ was determined by X-ray analysis. The Co-O and Co-C bond lengths were found to be 2.153(6) and 1.995(10) angstroms, respectively. The kinetic and structural data are discussed in reference to a series of earlier studied systems for which data are reported in the literature.