Decreasing Solvent Polarity Research Articles

Influence of Decreasing Solvent Polarity (1,4‐Dioxane/Water Mixtures) on the Acid–Base and Copper(II)‐Binding Properties of Guanosine 5′‐Diphosphate

AbstractThe acidity constants of twofold protonated guanosine 5′‐diphosphate, H2(GDP)−, and the stability constants of the [Cu(H;GDP)] and [Cu(GDP)]− complexes were determined in H2O as well as in 30 or 50% (v/v) 1,4‐dioxane/H2O by potentiometric pH titrations (25°; I=0.1M, NaNO3). The results showed that in H2O one of the two protons of H2(GDP)− is located mainly at the N(7) site and the other one at the terminal β‐phosphate group. In contrast, for 50% 1,4‐dioxane/H2O solutions, a micro acidity‐constant evaluation evidenced that ca. 75% of the H2(GDP)− species have both protons phosphate‐bound, because the basicity of pyridine‐type N sites decreases with decreasing solvent polarity whereas the one of phosphate groups increases. In the [Cu(H;GDP)] complex, the proton and the metal ion are in all three solvents overwhelmingly phosphate‐bound, and the release of this proton is inhibited by decreasing polarity of the solvent. Based on previously determined straight‐line plots of log K$\rm{_{Cu(R-DP)}^{Cu}}$ vs. pK$\rm{_{H(R-DP)}^{H}}$ (where R represents a non‐interacting residue in simple diphosphate monoesters ROP(O−)(O)OP(O)(O−)2, RDP3−), which were now extended to mixed solvents (based on analogies), it is concluded that, in all three solvents, the [Cu(GDP)]− complex is more stable than expected based on the basicity of the diphosphate residue. This increased stability is attributed to macrochelate formation of the phosphate‐coordinated Cu2+ with N(7) of the guanine residue. The formation degree of this macrochelate amounts in aqueous solution to ca. 75% (being thus higher than that of the Cu2+ complex of adenosine 5′‐diphosphate) and in 50% (v/v) 1,4‐dioxane/H2O to ca. 60%, i.e., the formation degree of the macrochelate is only relatively little affected by the change in solvent, though it needs to be emphasized that the overall stability of the [Cu(GDP)]− complex increases with decreasing solvent polarity. By including previously studied systems in the considerations, the biological implications are shortly discussed, and it is concluded that Nature has here a tool to alter the structure of complexes by shifting them on a protein surface from a polar to an apolar region and vice versa.

Kinetics and Mechanism of Oxidation of L-Proline by Heptavalent Manganese: A Free Radical Intervention and Decarboxylation

The kinetics of oxidation of L-proline by permanganate in alkaline medium was studied spectrophotometrically. The reaction is first order with respect to[MnO4-] and is an apparent less than unit order, each in [L-proline] and [alkali] under the experimental conditions. The reaction rate increases with increase in ionic strength and decrease in solvent polarity of the medium. Addition of reaction products has no effect on the reaction rate. A mechanism involving the formation of a complex between the oxidant and substrate has been proposed. The reaction constants involved in the mechanism were evaluated. There is a good agreement between the observed and calculated rate constants under varying experimental conditions. The activation parameters with respect to the slow step of the proposed reaction scheme were evaluated and discussed.

Kinetics and Mechanistic Study of the Ruthenium(III) Catalysed Oxidative Decarboxylation of L‐Proline by Alkaline Heptavalent Manganese (Stopped flow technique)

The kinetics of ruthenium(III) catalysed oxidation of L‐Proline by permanganate in alkaline medium at a constant ionic strength has been studied spectrophotometrically using a rapid kinetic accessory. The reaction between permanganate and L‐Proline in alkaline medium exhibits 2:1 stoichiometry (KMnO4: L‐Proline). The reaction shows first order dependence on [permanganate] and [ruthenium(III)] and apparent less than unit order dependence each in L‐Proline and alkali concentrations. Reaction rate increases with increase in ionic strength and decrease in solvent polarity of the medium. Initial addition of reaction products did not affect the rate significantly. A mechanism involving the formation of a complex between catalyst and substrate has been proposed. The activation parameters were computed with respect to the slow step of the mechanism and discussed

The Role of Water in the Oligomerization Equilibria Involving Bis(pentafluorophenyl)borinic Acid in Dichloromethane Solution

The 1H, 19F, and 11B NMR data indicated that in CD2Cl2 solution monomeric bis(pentafluorophenyl)borinic acid, (C6F5)2BOH (1m), is in equilibrium with the cyclic trimer (1t) observed in the solid state. The position of the association equilibrium shifted to the right on increasing the concentration, on decreasing the temperature, and on decreasing solvent polarity, in the series CD2Cl2, CDCl3, CCl4, in agreement with the higher polarity of the monomer (2.38 D for 1m and 0.65 D for 1t, according to PM3 computations). At temperatures lower than 210 K the 1H and 19F NMR spectra revealed the simultaneous (reversible) formation of two novel compounds, which have been formulated as the (C6F5)2BOB(C6F5)2 anhydride (2) and the trimeric species [(C6F5)2BOH]3·OH2 (3), of C2 symmetry, with a water molecule formally inserted into a B−O(H)−B bridge of 1t, to give a very strong BO(H)···HO(H)B hydrogen bond (δ 18.6). 1H and 19F EXSY experiments at 184 K revealed exchange between 3 and 1m, and not 1t. The data showed that the formation of 3, observed at temperatures where the monomer−trimer equilibrium is frozen, occurs by aggregation of monomeric units and not by cycle opening from 1t. The stabilization of the water molecule in 3 is strong enough to promote the dehydration of 1 to give the anhydride 2; for entropic reasons, the reaction occurs only at very low temperatures and is reversed on raising the temperature. At higher temperatures, the position of the monomer−trimer equilibrium is affected by the amount of water, which stabilizes the trimeric form, owing to the formation of a hydrogen-bond adduct 4 containing exocyclic water. At low temperatures, in the presence of the monomer, this species progressively dehydrated, due to the formation of 3. The amount of water present in solution also affected the rate of attainment of the 1m/1t equilibrium, the oligomerization being exceedingly slow in anhydrous conditions. The catalytic role of water can be attributed to the increased nucleophilicity of the BOH group upon water coordination, which allows alternative aggregation pathways. Semiempirical computations, at the PM3 level, provided a picture of the oligomerization in the presence and in the absence of water that well agrees with the experimental findings.

Spectral and Mechanistic Study of the Ruthenium(III) Catalysed Oxidation of Gabapentin (Neurontin) by Heptavalent Manganese: A Free Radical Intervention

The kinetics of ruthenium(III) catalysed oxidation of Gabapentin by permanganate in alkaline medium at a constant ionic strength has been studied spectrophotometrically. The reaction between permanganate and gabapentin in alkaline medium exhibits 2:1 stoichiometry (KMnO4: gabapentin). The reaction shows first order dependence on [permanganate] and [ruthenium(III)] and apparent less than unit order dependence each in gabapentin and alkali concentrations. Reaction rate decreases with increase in ionic strength and decrease in solvent polarity of the medium. Initial addition of reaction products did not affect the rate significantly. A mechanism involving the formation of a complex between catalyst and substrate has been proposed. The activation parameters were computed with respect to the slow step of the mechanism and discussed.

Substituent and solvent effects on the N‐2—N‐3 hindered rotation of cis‐1,3‐diphenyltriazenes

AbstractLaser‐flash photolysis techniques were applied to investigate the dependence of the rotational barrier about the N‐2—N‐3 bond of symmetrically 4,4′‐disubstituted cis‐1,3‐diphenyltriazenes on substituents and solvents. The increase in the rotational barrier with increasing ability of the 4‐substituent to withdraw electrons implies the intramolecular process to be more susceptible to the electronic character of the aryl group attached to N‐1 than of that bonded to N‐3. Furthermore, the increase in the rotational barrier with decreasing solvent polarity implies an increase in dipole moment on rotation from the ground state to the transition state. Copyright © 2004 John Wiley & Sons, Ltd.

Kinetics and Mechanism of Oxidation of L‐proline by Trivalent Copper: A free radical intervention and decarboxylation

The kinetics of oxidation of L‐proline by diperiodatocuprate(III) (DPC) in aqueous alkaline medium at a constant ionic strength of 0.10 mol dm‐3 was studied spectrophotometrically. The reaction between DPC and L‐proline in alkaline medium exhibits 2:1 stoichiometry (DPC: L‐Proline). The reaction is of first order in [DPC], less than unit order in [L‐proline] and [alkali]. Periodate has no effect on the rate of reaction. The reaction rate increases with increase in ionic strength and decrease in solvent polarity of the medium. Effect of added products and ionic strength of the reaction medium have been investigated. The main products were identified by spot test and I.R spectra. A mechanism involving the DPC as the reactive species of the oxidant and a complex formation with L‐proline has been proposed. The reaction constants involved in the different steps of mechanism are calculated. The activation parameters with respect to slow step of the mechanism are computed and discussed and thermodynamic quantities are also calculated.

Programmed assembly of rigid-rod beta-barrel pores: thermal inversion of chirality.

The programmed assembly of p-octiphenyl rods carrying six complementary tripeptide strands was studied in the presence of bilayer membranes using circular dichroism (CD) spectroscopy. Thermal CD experiments demonstrated programmed assembly of anionic and cationic rods into supramolecules at low temperature that irreversibly transform into more stable supramolecules at intermediate and high temperature. Higher activation energies for programmed assembly with rods containing multiple guanidinium rather than ammonium cations was consistent with stabilization by guanidinium-anion complexes. Qualitative thermal inversion of supramolecular chirality during programmed assembly was detected continuously. Inversion of supramolecular chirality occurred with decreasing solvent polarity as well.

Effects of solvent composition and ionic strength on the interaction of quinoline antimalarials with ferriprotoporphyrin IX

Enthalpy–entropy compensation in the interaction of quinoline antimalarials with ferriprotoporphyrin IX (Fe(III)PPIX) in 40% aqueous dimethyl sulfoxide (DMSO) has been compared with that in pure aqueous solution. The data indicate that the degree of desolvation and loss of conformational freedom is virtually identical in both systems. Taken together with previous findings showing that the molar free energies of association of these drugs with Fe(III)PPIX in both solvent systems are very similar, this suggests that the recognition site on the metalloporphyrin is comparable in both cases. This is despite the fact that Fe(III)PPIX exists as a dimer in aqueous solution, but is monomeric in 40% DMSO. Free energies of association of chloroquine, quinine and quinidine with Fe(III)PPIX are largely insensitive to the concentration of sodium perchlorate in 40% DMSO. This demonstrates that electrostatic interactions play only a minor role in the overall stability of these complexes under these conditions. Increasing DMSO concentration greatly weakens the interactions of chloroquine, amodiaquine, quinine, quinidine and 9-epiquinine with Fe(III)PPIX. This suggests that hydrophobic interaction plays a major role in the stability of these complexes. Further investigation of chloroquine has revealed that the free energy of association with Fe(III)PPIX also weakens as a function of decreasing solvent polarity in pure organic solvents. However, the free energies of association are weaker in the mixed aqueous solvent than in pure organic solvents. This indicates that dispersion and electrostatic interactions are relatively strong in the non-aqueous environment. The results demonstrate that any successful model of antimalarial drug–Fe(III)PPIX interactions will need to take both solvation and electrostatic factors into account.

Effect of Temperature, Solvent Polarity, and Nature of Lewis Acid on the Rate Constants in the Carbocationic Polymerization of Isobutylene

The absolute rate constant of propagation for ion pairs (kp±) was determined by the diffusion clock method in the living carbocationic polymerization of isobutylene at different solvent polarity and temperature in conjunction with TiCl4, Me2AlCl, and BCl3 as Lewis acids. The kp± ((3.6−5.7) × 108 L mol-1 s-1 in hexanes/MeCl 60/40, v/v) was independent of temperature and nature of Lewis acid and increased moderately with increasing solvent polarity to a nearly diffusion-limited value (∼1.7 × 109 L mol-1 s-1) in pure MeCl. A similar kp± (∼(5−6) × 108 L mol-1 s-1) value was obtained in the nonliving polymerization of isobutylene in conjunction with EtAlCl2 in hexanes/MeCl 60/40 (v/v) at −80 °C, indicating that living and nonliving polymerizations proceed on identical propagating centers. The apparent equilibrium constant of ionization (activation) Kiapp (= KiKD0, where Ki is the absolute equilibrium constant of ionization and KD0 is the equilibrium constant of TiCl4 dimerization) was calculated from the apparent and absolute rate constant of propagation. The rate constant of deactivation, k-i was determined from the conversion vs polydispersity plots. From Kiapp and k-i, the apparent values of ki (kiapp = kiKD0, where ki is the absolute rate constant of ionization) were also calculated. On the basis of the results, the observed large differences in the overall polymerization rates with different solvent polarity and temperature can be attributed solely to the changes in the active center concentration, which decreases with decreasing solvent polarity and increasing temperature. From the temperature dependence of Kiapp, the apparent standard enthalpy (= ΔHi° + ΔHD0°) and entropy (= ΔSi° + ΔSD0°) of ionization, and from the temperature dependence of kiapp and k-i, the apparent activation enthalpy and entropy of the activation/deactivation process were calculated.

직접 메탄올 연료전지용 산화극 제조 변수가 성능에 미치는 영향

직접 메탄을 연료전지(DMFC)의 산화극 제조 변수들에 따른 단위전지의 성능변화 관찰 및 특성분석을 수행하였다. 촉매층에서의 이온 전도도에 영향을 주는 이오노머의 양과, 이오노머와 촉매의 결합구조를 결정하는 촉매 슬러리의 용매를 변수로 하였다 전체 이오노머의 비가 0.6일 때 최고 성능을 보였으며 분극 저항도 가장 작게 나타났다. 전기화학적 활성면적도 이오노머가 늘어날수록 증가하였다. 극성이 작은 용매일수록 이오노머가 잘 용해되지 않아 촉매 응집체의 크기가 커졌다 기존의 물이나 알코올 종류의 용매들에 비해 극성이 낮은 DPK <TEX>$(\varepsilon=12.60)$</TEX>를 사용하여 제조한 전극이 가장 높은 성능을 보였으며 낮은 분극 저항 값을 가졌다. Single cell performance has been investigated and characterized with variables in the fabrication of DMFC anode. The performance was checked as a function of ionomer content which affects ion conductivity in the catalyst layer, and catalyst slurry solvent which determines structure of agglomerates consisting of an ionomer and a catalyst. Anode with total ionomer to catalyst ratio of 0.6 showed the best performance and the lowest polarization resistance. Also, electrochemically effective surface area increased with ionomer content. As solubility of the ionomer decreases with decreasing solvent polarity, the size of agglomerates consisting of a catalyst and an ionomer became larger in the less polar solvent. The anode using DPK <TEX>$(\varepsilon=12.60)$</TEX> as a solvent, which is less polar than generally-used water or alcohol species, showed the maximum performance and the lowest polarization resistance.

Oxidation of Nicotinate Ion by Heptavalent Manganese: a Kinetic and Mechanistic Approach

Abstract The kinetics of oxidation of nicotinate ion by permanganate in alkaline medium at a constant ionic strength has been studied spectrophotometrically. The reaction between permanganate and nicotinate ion in alkaline medium exhibits 2 : 1 stoichiometry KMnO4 : Nicotinate ion). The reaction shows first order dependence of rate on permanganate concentration and less than unit order dependence each in nicotinate ion and alkali concentrations. Reaction rate increases with increase in ionic strength and decrease in solvent polarity of the medium. Initial addition of reaction products did not affect the rate significantly. A mechanism involving the formation of an intermediate complex between oxidant and substrate has been proposed. The rate and the activation parameters for the slow step have been evaluated.

Effect of solvent on an NMR chemical shift difference between glycyl geminal α-protons as a probe of β-turn formation of short peptides

Proton NMR spectra of short peptides with a glycyl (Gly) or N-methylglycyl (sarcosyl, Sar) residue were measured in various mixed solvents with a wide range of dielectric constants: 78.3–2.3. From analyses of the octet and quartet signals of the geminal α-protons of Gly and Sar residues, respectively, we have estimated chemical shift differences between the two α-protons, Δ δ α/α′. It is found that the Δ δ α/α′ values increase with decreasing solvent polarity and the increasing rates depend significantly on amino acid sequences. By referring to infrared spectra and chemical shift of the terminal NH protons, δ NH, of the peptides, the Δ δ α/α′ values were found to be a good probe of β-turn formation. From solvent-dependent change of Δ δ α/α′, we estimated the free energies for the β-turn formation and compared the results with those estimated from δ NH. Using the resulting free energies, we have discussed effects of solvent on the β-turn formation.

Substituent effects on the thermal cis-to-trans isomerization of 1,3-diphenyltriazenes in aqueous solution.

The thermal cis-to-trans isomerization of some symmetrically p,p'-disubstituted 1,3-diphenyltriazenes has been studied by means of laser-flash photolysis techniques. The geometric isomerization is catalyzed by general acids and general bases as a result of acid/base-promoted 1,3-prototropic rearrangements. Acid catalysis becomes more prominent as the electron-donating character of the para substituent increases, while base catalysis becomes more important as the electron-withdrawing character of the para substituent increases. In addition, the rate ascribed to the interconversion of neutral cis rotamers through hindered rotation around the nitrogen-nitrogen single bond is found to decrease as the electron-withdrawing character of the para substituent increases. Rates of interconversion of neutral cis rotamers are also found to decrease with decreasing solvent polarity, which is indicative of the involvement of a polar transition state. On the other hand, kinetic investigations of the acid-catalyzed decomposition of target triazenes are consistent with an A1 mechanism.

CIDEP spin probing of the nanopores in cotton

Time-resolved electron paramagnetic resonance (TREPR) experiments are reported on an α-hydroxy acetophenone in waterg-glycerol and ethanol-ethanediol mixtures over a viscosity range of 1–32 cP. The magnitude and viscosity dependence of the radical pair mechanism polarization is in good agreement with theory. The triplet mechanism polarization is found to increase with decreasing solvent polarity and this is ascribed to an increase in the α-cleavage rate constant. The liquid spectra are used to interpret TREPR spectra of the acetophenone in dry and wet cotton. In dry cotton anti-phase structure is seen and is due to some of the radical pairs being trapped in cages. The triplet and radical pair mechanism polarization shows that the acetophenone adsorbs to the apolar crystallite surface and releases radicals into the polar amorphous regions where they experience a microviscosity of approximately 30 cP. In wet cotton there are no cages and the viscosity is greater than in ethanol alone.

Kinetics and Mechanism of Oxidation of Bromate by Diperiodatonickelate(IV) in Aqueous Alkaline Medium--A Simple Method for Formation of Perbromate

The kinetics of oxidation of bromate by diperiodatonickelate(IV) (DPN) in alkaline medium at a constant ionic strength has been studied spectrophotometrically. The reaction between DPN and bromate in alkaline medium exhibits 1:1 stoichiometry (DPN:bromate). The reaction shows first order dependence on [DPN] and zero order dependence in [bromate] and less than unit order dependence in alkali concentrations. Periodate has a retarding effect on the rate of reaction. Reaction rate increases with increase in ionic strength and decrease in solvent polarity of the medium. Initial addition of reaction products did not affect the rate significantly. A mechanism involving the monoperiodatonickelate(IV) (MPN) as the reactive species of the oxidant has been proposed. The reaction constants involved in the mechanism are evaluated. The activation parameters were computed with respect to the slow step of the mechanism.

Solvent Effect of the Hole Migration along a Poly(N-vinylcarbazole) Chain as Revealed by Picosecond Transient Absorption and Dichroism Measurements

Photoinduced electron-transfer dynamics (charge separation, charge recombination, and hole transfer reactions) in poly(N-vinylcarbazole) (PVCz) and solvent effects on the reaction profiles were studied by means of picosecond transient absorption spectroscopy and transient dichroism measurements. Electron-transfer dynamics in PVCz in rather polar solutions was well-described by the simple scheme that the cation state of the carbazolyl (Cz+) moiety continuously migrates along pendant Cz moieties in the polymer chain with the charge recombination at the initial position of the charge separation. The rate constant of the hole transfer (HT) from the Cz+ to neighboring ones was (230 ps)-1 in pyridine. With a decrease in the solvent polarity, the HT rate constant drastically decreased, and it was ≤107 s-1 in chloroform solution. This solvent polarity effect was accounted for not only by the increase in the attractive Coulombic interaction with decreasing solvent polarity but also by the delocalization of the cationic state over Cz units. By integrating the present results with those in other aromatic vinyl polymers, the factors regulating the hole-transfer process in aromatic vinyl polymers were discussed.

Kinetics and mechanistic study of the ruthenium(III) catalyzed oxidative deamination and decarboxylation of L-valine by alkaline permanganate

The kinetics of ruthenium(III) catalyzed oxidation of L-valine by permanganate in alkaline medium at a constant ionic strength has been studied spectrophotometrically. The reaction between permanganate and L-valine in alkaline medium exhibits 2:1 stoichiometry (KMnO4:L-valine). The reaction shows first-order dependence on the concentration of permanganate and ruthenium(III) and less than unit-order dependence on the concentrations of L-valine and alkali. The reaction rate increases both with an increase in ionic strength and a decrease in solvent polarity of the medium. Initial addition of reaction products did not significantly affect the rate. A mechanism involving the formation of a complex between catalyst and substrate has been proposed. The activation parameters were computed with respect to the slowest step of the mechanism.Key words: oxidation, L-valine, catalysis, ruthenium(III), kinetics.

Solvent-Induced Crystal Morphology Transformation in a Ternary Soap System: Sodium Stearate Crystalline Fibers and Platelets

A ternary model system, sodium stearate (NaSt) crystallites dispersed in mixtures of water and propylene glycol (PG) at 1−5 wt % NaSt concentrations, has been studied. Two different morphologies, fibers and platelets, were observed with light and electron microscopy. At the molecular level, however, these two morphologies share the same layered crystalline structure as revealed by X-ray diffraction. NaSt fibers occur in water-rich mixtures, and by cryogenic electron microscopy they are shown as lamellar ribbons. With increasing PG/H2O ratios in the NaSt/H2O/PG system, platelike crystallites become the preferred morphology. The rheological properties of these suspensions were studied and correlated with the observed morphologies. Decreasing solvent polarity induced morphological transformation from fibers to plates, as further demonstrated by changing solvents from water, to glycerin, to ethylene glycol, and to propylene glycol. The delineation of these trends can be useful in the design of cleaning produc...

Cooling dynamics of an optically excited molecular probe in solution from femtosecond broadband transient absorption spectroscopy

The cooling of p-nitroaniline (PNA), dimethylamino-p-nitroaniline (DPNA) and trans-stilbene (t-stilbene) in solution is studied experimentally and theoretically. Using the pump–supercontinuum probe (PSCP) technique we observed the complete spectral evolution of hot absorption induced by femtosecond optical pumping. In t-stilbene the hot S1 state results from Sn→S1 internal conversion with 50 fs characteristic time. The time constant of intramolecular thermalization or intramolecular vibrational redistribution (IVR) in S1 is estimated as τIVR≪100 fs. In PNA and DPNA the hot ground state is prepared by S1→S0 relaxation with characteristic time 0.3–1.0 ps. The initial molecular temperature is 1300 K for PNA and 860 K for t-stilbene. The subsequent cooling dynamics (vibrational cooling) is deduced from the transient spectra by assuming: (i) a Gaussian shape for the hot absorption band, (ii) a linear dependence of its peak frequency νm and width square Γ2 on molecular temperature T. Within this framework we derive analytic expressions for the differential absorption signal ΔOD(T(t),ν). After calibration with stationary absorption spectra in a low temperature range, the solute temperature T(t) may be evaluated from a transient absorption experiment. For highly polar PNA and DPNA, T(t) is well described by a biexponential decay which reflects local heating effects, while for nonpolar t-stilbene the local heating is negligible and the cooling proceeds monoexponentially. To rationalize this behavior, an analytic model is developed, which considers energy flow from the hot solute to a first solvent shell and then to the bulk solvent. Fastest cooling is found for PNA in water: a time constant of 0.64 ps (68%) corresponds to solute–solvent energy transfer while 2.0 ps (32%) characterizes the cooling of the first shell. In aprotic solvents cooling is slower than in alcohols and slows down further with decreasing solvent polarity. This contrasts with nonpolar t-stilbene which cools down with 8.5 ps both in acetonitrile and cyclohexane. Comparison of the cooling kinetics for PNA in water with those for DPNA in water-acetonitrile mixtures suggests that the solute–solvent energy transfer proceeds mainly through hydrogen bonds.