Substantial Solvent Research Articles

Study of solvent-solute and solute-solute interaction effects on naphthalene in carbon dioxide by NMR measurement of rotational ang angular momentum correlation times in CO 2 at sub- and supercritical temperatures

The effect of temperature variation from subcritical to supercritical conditions on the rotational and angular momentum correlation times of naphthalene dissolved in carbon dioxide is reported. The rotational correlation time, τ c is found to be only slightly dependent upon solution viscosity, unaffected by the CO 2 phase change at about 32°C and is essentially equal to its value in acetone at the same temperature. τ c is approximately twice its gas phase free rotor time. These results are interpreted to indicate that ground state solute-solute interactions are unimportant. The angular momentum correlation time, τ j , undergoes a dramatic increase at the temperature of the phase change indicating an approximately 3.2 fold solvent density augmentation around the naphthalene compared to the bulk gas phase CO 2 density under the experimental conditions beyond the critical temperature. These results indicate that a substantial solvent density augmentation occurs that does not cause measurable changes in the rotational properties of the solute. The solvent augmentation is found to disappear within approximately 2°C on either side of the critical temperature.

Correlation between gas-phase electron affinities, electrode potentials, and catalytic activities of halogenated metalloporphyrins

The gas-phase electron affinities (EAs) of a series of halogenated iron tetraphenyl porphyrins determined using ion cyclotron resonance mass spectrometry are reported. The EAs of the Fe(II) species vary approximately linearly with their condensed-phase reduction potentials. The EAs of the Fe(III) species with axial Cl ligands vary approximately linearly with condensed-phase reduction potentials. The lines for the Fe(III) and Fe(II) data sets are quite different, however, suggesting substantial solvent effects on the condensed-phase reduction process

Electrostatic solvent effect on the ketene-imine cycloaddition reaction

The ketene + formaldimine cycloaddition reaction to give 2-azetidinone has been studied as a model of one of the most interesting methods for the synthesis of β-lactams. The role of the solvent has been analyzed using a self-consistent reaction field method in which the solvation energy is obtained using a multipole expansion of the molecular charge distribution. Substantial solvent effects are predicted both for the reaction mechanism and for the reaction rate. Zero-point energy corrections have been shown to also be important.

Kinetics and mechanism of the acid-catalyzed hydrolysis of 1,1-dimethoxyethene (ketene dimethyl acetal) and trimethoxyethene in aqueous solution

Rates of acid-catalyzed hydrolysis of 1,1-dimethoxyethene and trimethoxyethene were measured in wholly aqueous solutions of perchloric acid, sodium hydroxide, and a series of carboxylic acid buffer solutions at 25 o C. The reactions show general acid catalysis and give substantial solvent isotope effects in the normal direction (k H /k D >1)

The rate of formation of transition-state analogs in the active site of adenosine deaminase is encounter-controlled: implications for the mechanism

We have previously shown that purine riboside, when bound to adenosine deaminase, forms a complex in which C-6 of the purine is tetrahedral [Kurz, L. C., & Frieden, C. (1987) Biochemistry 26, 8450]. We now report the rates of formation of enzyme-inhibitor complexes of two types, those which do and those which do not form such tetrahedral intermediates. In both cases, the rates are encounter-controlled since the progress curves for formation of the complexes are well-described by a simple second-order approach to equilibrium and the rate constants show an inverse solvent viscosity dependence. Assuming that the formation of the intermediate-analogue complex is preceded by an initial ground-state analogue complex, the lifetime of that ground-state complex must be less than approximately 20 microseconds. All of the enzyme-inhibitor complexes studied share three characteristics: (1) the complexes generate large UV-difference spectra; (2) a substantial solvent isotope effect is found on the enzyme's affinity for the inhibitors; and (3) a new signal appears in the CD spectra of the complexes. Two of the nucleosides studied, 1-deazapurine riboside and 1-deaza-adenosine, form complexes which appear to mimic a ground-state rather than a reactive intermediate when bound to adenosine deaminase. We find that the values for the association rate constants for those inhibitors which form intermediate analogues are very similar to that for adenosine. The presence of a significant solvent isotope effect on the affinity of all inhibitors is attributable in part to a large transfer isotope effect on the free ligand and in part to an effect on the bound ligand. This complicates use of the solvent isotope effect in applications of the multiple isotope method for estimating intrinsic isotope effects and commitment factors.

Theory of nematic order with aggregate dehydration for reversibly assembling proteins in concentrated solutions: Application to sickle-cell hemoglobin polymers.

The reversible association of globular protein molecules in concentrated solution leads to highly polydisperse fibers, e.g., actin filaments, microtubules, and sickle-cell hemoglobin fibers. At high concentrations, excluded-volume interactions between the fibers lead to spontaneous alignment analogous to that in simple lyotropic liquid crystals. However, the phase behavior of reversibly associating proteins is complicated by the threefold coupling between the growth, alignment, and hydration of the fibers. In protein systems aggregates contain substantial solvent, which may cause them to swell or shrink, depending on osmotic stress. Extending previous work, we present a model for the equilibrium phase behavior of the above-noted protein systems in terms of simple intra- and interaggregate interactions, combined with equilibration of fiber-incorporated solvent with the bulk solvent. Specifically, we compare our model results to recent osmotic pressure data for sickle-cell hemoglobin and find excellent agreement. This comparison shows that particle interactions sufficient to cause alignment are also sufficient to squeeze significant amounts of solvent out of protein fibers. In addition, the model is in accord with findings from independent sedimentation and birefringence studies on sickle-cell hemoglobin.

Rapid proton NMR method for determination of threo:erythro ratios in lignin model compounds and examination of reduction stereochemistry

Threo:erythro ratios of &aryl ether lignin model compounds are rapidly obtained by integration of a-OH proton signals at ca. 4.5 ppm in proton NMR spectra in acetone-de sovlent containing traces of water. The excellent separation of these sharp signals (ca. 0.11 ppm), which is unaffected by etherification of the phenol, and the additional dispersion of the 0-proton signals make NMR in this solvent ideal for homonuclear correlation spectroscopy. Sodium borohydride reduction of precursor ketones shows a substantial solvent effect: for example, in ethanol, isopropyl alcohol or tert-butyl alcohol the erythro isomer is mildly favored (ca. 40:60 threo:erythro), whereas in 50% aqueous alcohols the stereochemistry reverses to a marked threo preference (ca. 80:20 threo:erythro).

Decomposition of N-nitrosopeptides in strong acids

Decomposition of the N-nitrosodipeptides obtained from N-(N′-acetyl-L-prolyl)glycine and N-(N′-acetyl-L-propyl)-L-alanine in aqueous acid at 25 °C involves both deamination and denitrosation. Both reactions occur concurrently via different conjugate acid intermediates, with denitrosation being predominant at high acidity. Acidity dependences and inverse solvent deuterium isotope effects [k(H2SO4)/k(D2SO4)ca. 0.7] suggest that deamination involves rate limiting attack by H2O on an O-conjugate acid, formed in a rapid pre-equilibrium. For denitrosation, H+ transfer to the amide N-atom is considered rate limiting because of the substantial normal solvent deuterium isotope effects [k(H2SO4)/k(D2SO4)ca. 2.5] and Bunnett ω values in the range –0.1 to –0.5: the N-conjugate acid formed breaks down rapidly to products. Both the kinetics and mechanisms for decomposition of these N-nitrosodipeptides are very similar to those of alicyclic N-nitrosamides.

SPECTROSCOPIC STUDIES OF CUTANEOUS PHOTOSENSITIZING AGENTS–XV. ANTHRALIN and ITS OXIDATION PRODUCT 1,8‐DIHYDROXYANTHRAQUINONE

The photochemistry (Type I and II) of anthralin and its photo-oxidation product 1,8-dihydroxyanthraquinone (1,8-DHAQ) has been studied in ethanol, acetonitrile and dimethylsulfoxide using spin-trapping and direct detection of singlet oxygen (1O2) luminescence techniques. In ethanol, where it exists in its neutral form (AN), anthralin does not undergo either Type I or II reactions upon UV-irradiation. In contrast, irradiation of anthralin in acetonitrile, a solvent in which anthralin is partially converted to its corresponding mono-anion (AN-), generates both superoxide and singlet oxygen. Irradiation of anthralin in dimethylsulfoxide, where the AN- form is present in substantial quantity, generates superoxide and solvent derived radicals but no detectable singlet oxygen. UV-irradiation of 1,8-DHAQ in ethanol and acetonitrile produces both superoxide and singlet oxygen in significant yields. In dimethylsulfoxide, on the other hand, only superoxide and solvent derived radicals are observed. The 1O2 quantum yield for AN- and 1,8-DHAQ in acetonitrile were determined to be 0.14 and 0.88 relative to rose bengal in the same solvent. These findings suggest that the AN photosensitization occurs via Type I and II pathways, is solvent dependent and involves AN- as well as its oxidation product 1,8-DHAQ, which is a more potent generator of both singlet oxygen and superoxide.

Photoisomerization of provitamin D in dispersive systems

The kinetics of provitamin D photoisomerization in solutions and silica gel-solvent matrix upon lamp irradiation on 254 nm was measured by UV spectroscopy. The reduction of the efficiency for cis-trans isomerization in dispersive systems was revealed as compared with the solutions and the effect was established to exhibit a very substantial solvent polarity dependence. The well known concept of ground-state conformational control was utilized to explain the influence of the adsorption on the photobehaviour of provitamin D. More importantly, the results obtained reveal that the dispersive systems can be used in the certain cases to control the conformations of organic molecules and photoproducts distribution.

Structure and stability of the tetrahydridoselenonium dication, SeH4 2+

Ab initio molecular orbital theory with triple-zeta-valence plus polarization basis sets and with electron correlation incorporated at the fourth-order Moller-Plesset level has been used to study the tetrahydridoonium dications, OH42+, SH42+, and SeH42+. The tetrahydridoselenonium dication SeH42+ is predicted to have a tetrahedral (T d )structure, similar to OH42+ and SH42+, with short bonds to hydrogen (1.483 A). Although deprotonation of SeH42+ is thermodynamically favored Cby 104 kJ mol−1), such a reaction is inhibited by a large barrier (240kJmol−1]. Thus, SeH42+ lies in a deep potential well and as an isolated species should have a long lifetime in the gas phase. The estimated heat of formation, ΔH° f , for SeH42+ is very high (2483 kJ mol−1], as is the case for OH42+ and SH42+. Of the group IV onium dications (OH42+, SH42+, and SeH42+), SeH42+ displays the greatest kinetic and thermodynamic stability toward proton loss. Substantial solvent stabilization is required in order to generate SeH42+ in solution.

Dehydration of Protein Polymers in Concentrated Nematic Solutions

ABSTRACTProtein fibers (e.g., actin filaments, microtubules, and sickle cell hemoglobin polymers), formed by reversible association of quasispherical protein monomers, contain substantial solvent. At high concentrations, solutions of these fibers become non-ideal due to interactions between the elongated particles. One manifestation of this non-ideality is the spontaneous alignment of the fibers. Model calculations, involving simple intra- and inter aggregate interactions leading to polymerization and alignment, agree well with osmotic pressure measurements of sickle cell hemoglobin solutions in the region of the phase transition. However, at higher concentrations the theory cannot fit the experimental data unless solvent is gradually squeezed out of the fiber under osmotic stress. We find that a linear potential for the fiber dehydration gives osmotic pressure results consistent with the experimental data. Under these conditions, substantial fiber dehydration occurs with increasing protein concentrations beyond the alignment transition. This indicates that particle interactions sufficient to cause alignment are also sufficient to squeeze significant amounts of solvent out of the sickle cell hemoglobin fibers.

A time-resolved study of the photoinduced tautomerization of 2-aminopyridine and derivatives within specific 1:1 complexes with carboxylic acids

Double-proton transfer in electronically excited complexes of 2-aminopyridine and carboxylic acid molecules has been investigated by time-resolved fluorescence spectroscopy. We show that the 1:1 complexes are formed in dilute solutions of the components in alkanes. In these solutions the proton transfer proceeds with a rate constant larger than 1011 s–1. The activation energy for the process could not be determined owing to the formation of ion pairs consisting of a 2-aminopyridinium cation and a carboxylic anion at low temperatures.The formation of ion pairs is shown not to occur when the carboxylic acid is replaced by barbital which forms stable complexes with two 2-aminopyridine molecules in the solid state. The time-resolved fluorescence of these solid-state complexes reveals that the proton-transfer reaction rate is faster than the resolution of the detection system, even at low temperatures (77 K).Complexes of 4-amino-2-methyl-5H-[1] benzpyrano [3,4-c] pyridin-5-one (BPP) with carboxylic acids dissolved in alkanes exhibit ultrafast proton transfer both at room temperature and at 77 K. The proton-transfer rate decreases substantially in neat acetic acid as the solvent. An activation energy of 3.4 kcal mol–1 has been determined for the process in these solutions. The relatively small value of the rate constant in neat acetic acid is attributed to the fact that the cyclic structure of the solute-solvent complex, encountered in the solutions in alkanes, is disrupted owing to interaction of the solute molecule with more than one acid molecule. Substantial solvent reorganization is then required to achieve a suitable geometry for proton transfer.

The anomalous properties of the glutamate dehydrogenase-NADPH-alpha-ketoglutarate complex are not ascribable to a carbonyl addition reaction.

The glutamate dehydrogenase-NADPH-alpha-ketoglutarate complex, an active intermediate on the reaction pathway has a number of unusual properties: 1) it is the only blue-shifted natural complex of this enzyme; 2) it has an anomalously slow rate of dissociation; 3) its off-rate shows a substantial pH-independent D2O solvent isotope effect not exhibited by any other ternary complex of this enzyme; and 4) it has an unusually large enthalpy of interaction parameter. These properties must be ascribable to at least one of the two possibilities conferred on the complex by the presence of the alpha-carbonyl group of alpha-ketoglutarate; the ability to engage in carbonyl addition reactions; and/or the ability to form a specific hydrogen bond. Oxalylglycine, a competitive inhibitor of alpha-ketoglutarate in this enzyme-catalyzed reaction, provides a means of discriminating between these two modes of action. The structure of oxalylglycine provides a dicarboxylic compound which has the same intercarboxylate proton distance and has a carbonyl group in a position spatially analogous to that of alpha-ketoglutarate. Its carbonyl group, however, is that of an amide group and cannot, therefore, engage in carbonyl addition reactions, but can hydrogen bond. Therefore, any effects observed with both oxalylglycine and alpha-ketoglutarate must be ascribed to formation of specific alpha-carbonyl hydrogen bonding, whereas any effects observed with alpha-ketoglutarate alone must be due to an alpha-carbonyl addition reaction. We have used this logic to test the source of the four phenomena listed above. In each case, oxalylglycine and alpha-ketoglutarate showed the same effect. Therefore, we conclude that all four phenomena are in fact due to the formation of a specific alpha-carbonyl hydrogen bond and that the specific carbonyl addition reaction between alpha-ketoglutarate and an enzyme lysine group, postulated in one proposed catalytic mechanism, does not occur.

Effect of solvent on the rate constant for the radiative deactivation of singlet molecular oxygen (1.DELTA.gO2)

Relative rate constants for the radiative deactivation (k/sub r/) of singlet molecular oxygen (/sup 3/Sigma/sub g//sup -/O/sub 2/ reverse arrow /sup 1/..delta../sub g/O/sub 2/) have been determined in 15 solvents. A substantial solvent effect is observed. Changes in the value of k/sub r/ can exceed a factor of 20. A reasonably good correlation exists between the solvent polarizability, defined as a function of the solvent refractive index, and the radiative rate constant. We suggest that our data support a model in which /sup 1/..delta../sub g/O/sub 2/ is perturbed through the formation of a discrete oxygen-solvent collision complex.

ChemInform Abstract: A Volumetric Study on the Thermal cis‐to‐trans Isomerization of 4‐(Dimethylamino)‐4′‐nitroazobenzene and 4,4′‐Bis(dialkylamino)azobenzenes: Evidence of an Inversion Mechanism

The reaction volume (ΔV) for the thermal cis-to-trans isomerization of 4-(dimethylamino)-4′-nitroazobenzene in various solvents has been obtained by means of the photostationary state and capillary method. It has been shown that the activation volume (ΔV‡) is closely related to the reaction volume in each solvent. Substantial kinetic pressure and solvent effects have been observed for 4,4′-bis(dimethylamino)azobenzene, 4,4′-bis(diethylamino)azobenzene, and 4,4′-(dipyrrolidin-1-yl)azobenzene. The π–π* conjugation band of the trans-isomers of these compounds shows a remarkable solvatochromic shift, and logarithms of kinetic constants in various solvents are satisfactorily correlated with the Taft π* scale, which represents solvent polarity–polarizability interactions. From McRae's formula, it has been deduced that the interaction between the solvent dipole and the solvent-induced dipole has an important influence on the behaviour of these ‘push–pull’ and ‘push–push’ azobenzenes. The results afford unequivocal evidence that the isomerization proceeds via a coplanar inversion transition state which is electronically similar to the trans-isomer.

Activation parameters for the carbonic anhydrase II-catalyzed hydration of CO2.

We have determined the activation parameters of kcat and kcat/Km for the carbonic anhydrase II-catalyzed hydration of CO2. The enthalpy and entropy of activation for kcat is 7860 +/- 120 cal mol-1 and -3.99 +/- 0.42 cal mol-1 K-1, respectively, for the human enzyme. Results for the bovine enzyme were statistically indistinguishable from those of the human enzyme. The entropy of activation of kcat for the human enzyme was further decomposed into partially compensating electrostatic(es) (delta S*es = +15.1 cal mol-1 K-1) and nonelectrostatic(nes) (delta S*nes = -19.1 cal mol-1 K-1) terms. Computer simulations of a formal kinetic mechanism for carbonic anhydrase II-catalyzed CO2 hydration show that 82% of the temperature effect on kcat can be attributed to the temperature effect on the intramolecular proton transfer step. The reported activation parameters are consistent with a substantial enzyme or active site solvent conformational change in the transition state of the intramolecular proton transfer step, and is consistent with the mechanism of proton transfer proposed by Venkatasubban and Silverman (Venkatasubban, K. S., and Silverman, D. N. (1980) Biochemistry 19, 4984-4989).

A volumetric study on the thermal cis-to-trans isomerization of 4-(dimethylamino)-4′-nitroazobenzene and 4,4′-bis(dialkylamino)azobenzenes: evidence of an inversion mechanism

The reaction volume (ΔV) for the thermal cis-to-trans isomerization of 4-(dimethylamino)-4′-nitroazobenzene in various solvents has been obtained by means of the photostationary state and capillary method. It has been shown that the activation volume (ΔV‡) is closely related to the reaction volume in each solvent. Substantial kinetic pressure and solvent effects have been observed for 4,4′-bis(dimethylamino)azobenzene, 4,4′-bis(diethylamino)azobenzene, and 4,4′-(dipyrrolidin-1-yl)azobenzene. The π–π* conjugation band of the trans-isomers of these compounds shows a remarkable solvatochromic shift, and logarithms of kinetic constants in various solvents are satisfactorily correlated with the Taft π* scale, which represents solvent polarity–polarizability interactions. From McRae's formula, it has been deduced that the interaction between the solvent dipole and the solvent-induced dipole has an important influence on the behaviour of these ‘push–pull’ and ‘push–push’ azobenzenes. The results afford unequivocal evidence that the isomerization proceeds via a coplanar inversion transition state which is electronically similar to the trans-isomer.

Production of Solvents by Clostridium acetobutylicum Cultures Maintained at Neutral pH

The formation of acetone and n-butanol by Clostridium acetobutylicum NCIB 8052 (ATCC 824) was monitored in batch culture at 35 degrees C in a glucose (2% [wt/vol]) minimal medium maintained throughout at either pH 5.0 or 7.0. At pH 5, good solvent production was obtained in the unsupplemented medium, although addition of acetate plus butyrate (10 mM each) caused solvent production to be initiated at a lower biomass concentration. At pH 7, although a purely acidogenic fermentation was maintained in the unsupplemented medium, low concentrations of acetone and n-butanol were produced when the glucose content of the medium was increased (to 4% [wt/vol]). Substantial solvent concentrations were, however, obtained at pH 7 in the 2% glucose medium supplemented with high concentrations of acetate plus butyrate (100 mM each, supplied as their potassium salts). Thus, C. acetobutylicum NCIB 8052, like C. beijerinckii VPI 13436, is able to produce solvents at neutral pH, although good yields are obtained only when adequately high concentrations of acetate and butyrate are supplied. Supplementation of the glucose minimal medium with propionate (20 mM) at pH 5 led to the production of some n-propanol as well as acetone and n-butanol; the final culture medium was virtually acid free. At pH 7, supplementation with propionate (150 mM) again led to the formation of n-propanol but also provoked production of some acetone and n-butanol, although in considerably smaller amounts than were obtained when the same basal medium had been fortified with acetate and butyrate at pH 7.

Solvent isotope effects on equilibriums of mono- and dihydration of neutral pteridine

In a thorough kinetic study of the nonenzymatic hydration of pteridine, Pocker et al. reported results in which no substantial solvent isotope effect on the equilibrium of monohydration of pteridine was evident at pH values at and above neutrality. Reinvestigating this reaction, it was confirmed that the solvent isotope effect on the monohydration of pteridine is close to unity. In addition it was found that pteridine dihydrate is formed in neutral solution more slowly than the monohydrate. The dihydrate had been believed to be formed only as the cation in acid solution. Like monohydration, the equilibrium of dihydration appears to be almost completely insensitive to the replacement of water by D/sub 2/O.