Increasing Solvent Viscosity Research Articles

Role of cytoarchitecture in cytoplasmic transport

Cytoplasm is a thick viscoelastic gel consisting of a highly concentrated protein solution interspersed through a matrix of cytoskeletal filaments and organellar structures. Diffusion of soluble low-molecular-weight molecules through cytoplasm is slowed by a factor of 5–10 relative to a dilute aqueous solution, reflecting increased solvent viscosity, molecular crowding, tortuosity imposed by cytoplasmic membranes and organelles, and transient binding to immobile structures. Larger molecules are further slowed by cytoskeletal sieving. Messenger RNA and ribosomes may be prevented from entering certain parts of cytoplasm due to their size and may follow preferential channels as they exit the nucleus. Mechanisms also exist for energy-dependent transport of larger molecules and organelles

The Mechanism of Rabbit Muscle Enolase: Identification of the Rate-Limiting Steps and the Site of Li+Inhibition

Steady-state and non-steady-state techniques have been used to identify the rate-limiting steps for ββ enolase (rabbit muscle enolase), at pH 7.1, with Mn2+as the required cation. A minimum mechanism for enolase includes eight steps,[formula]where S is phosphoglycerate, P is phosphoenolpyruvate (PEP), I is the carbanion intermediate, M is Me2+, and EM is the holoenolase (i.e., the first Me2+is bound). Asterisks represent a different conformation of the quaternary complexes. At pH 7.1, the primary kinetic isotope effect = 1, andkcatdecreases as solvent viscosity increases. The changes in protein fluorescence that occur upon substrate binding and product release [EMSM[formula](EMSM)* and (EMPM)*[formula]EMPM] were followed by stopped-flow fluorimetry; the viscosity dependence of the observed rates was also determined. The data support the following mechanism. Product formation is fast and precedes the slow steps of the reaction, consistent with the observation of a pre-steady-state burst of PEP. The rate-limiting steps arek+6, the conformational change associated with product release, andk+8, the dissociation of PEP. Li+inhibits the activity of enolase by increasingk+6andk−3, thus decreasing the steady-state concentration of (EMSM)*.

Electrocatalytic reduction of halothane

Cyclic and rotating-disc voltammetry were used to investigate the electrochemical reduction of halothane (2-bromo-2-chloro-1,1,1-trifluoroethane) on various metal electrodes (Cu, Ag, Au, Zn, Cd, Hg, Ga, In, Tl, Sn, Sb, W, Ni, Pd and Pt) in several solvents (water, propylene carbonate, acetonitrile, methanol, acetone and tetrahydrofuran). In agreement with previous mechanistic studies, the overall reaction was found to involve two electrons yielding 2-chloro-1,1,1-trifluoroethane and bromide as the main products. The transfer of the first electron is likely to be the rate-determining step. Irrespective of the metal, solvent or electrolyte used, no reaction intermediates were detected by fast scan cyclic voltammetry, i.e. all electrode and/or chemical reactions following this rate-determining step are too fast. The effect of the nature of the solvent is manifested mainly in the decreasing rate of the halothane transport as the solvent viscosity increases, and in the decreasing solvation of the bromide anion with the decreasing polarity of the solvent, which makes the reduction less favourable energetically. In contrast with a previous mechanistic study, a strong effect of the nature of metal on the halothane reduction was observed. First, the halothane reduction is quite inhibited when an oxide film is formed at the metal surface. Second, in the absence of the metal oxide film, the half-wave potentials of the halothane reduction on various metals can differ by several hundreds of millivolts. A simple theory of this effect was outlined, which assumes formation of an intermediate bond between the surface metal atom and the Br moiety of the halothane molecule.

Mechanisms of the Photochemical Rearrangement of Diphenyl Ethers.

The mechanism of the photochemical rearrangement of diphenyl ether (1a) was studied. Irradiation of 1a in ethanol gave 2-phenylphenol (2, 42%) and 4-phenylphenol (3, 11%) as rearrangement products, in addition to phenol (4, 30%) and benzene (5, 25%) as diffusion products. Cross-coupling experiments employing [(2)H(10)]1a demonstrated that the formation of 2- and 4-phenylphenol was an intramolecular process. Irradiation of 1a in benzene or in toluene gave biphenyls in good yields. The combined yields of rearrangement products (2and 3) increased with increase of solvent viscosity, with a concomitant decrease in the formation of 4. All the results can be rationalized in terms of excitation of 1a to the singlet state and dissociation to a radical pair intermediate involving phenoxy and phenyl radicals. Intramolecular recombination of these radicals gives rearrangement products, and escape followed by hydrogen abstraction from the solvent gives diffusion products. When position 4 of 1a was occupied by an electron-donating substituent (1b-e), aryloxy-phenyl bond cleavage to give the corresponding rearrangement products prevailed over phenoxy-aryl bond cleavage. The opposite was the case for substrates with an electron-withdrawing substituent at position 4 (1h,i).

Photoinduced Radical Cleavage of Bromophenyl Ketones

Several bromophenyl alkyl ketones undergo photoinduced radical cleavage of bromine atoms; the resulting acylphenyl radicals are trapped as the dehalogenated ketones either by alkane solvents or by added thiols. Quantum yields for this process decrease with increasing solvent viscosity; this behavior indicates that recoupling of the phenyl/bromine radical pairs competes with their diffusion apart. In some cases Norrish type II elimination competes with the cleavage reaction. Steady state studies indicate room temperature rate constants for triplet state C−Br radical cleavage of 2 and 1 × 108 s-1, respectively, for m- and p-bromoacetophenone. Flash kinetics measurements of the temperature dependence for their triplet decay provided activation parameters for triplet state cleavage: ΔS⧧ = −5 eu for the para isomer and −3 eu for the meta; ΔH⧧ = 5.3 kcal/mol for both. The values for the meta isomer are the same in methanol and toluene and extrapolate to the room temperature decay rate measured by steady state ...

Spectral and Photophysical Properties of Ethylene-Bridged Side-to-Side Porphyrin Dimers. 1. Ground-State Absorption and Fluorescence Study and Calculation of Electronic Structure oftrans-1,2-Bis(meso-octaethylporphyrinyl)ethene

We have studied a double-bond bridged porphyrin dimer, trans-1,2-bis(meso-octaethylporphyrinyl)ethene (tbisOEP), which in solutions exhibits new spectral properties: (i) pronounced absorption bands in addition to the monomer ones are observed in the 480−500 and 600−900 nm regions; (ii) a broad-band fluorescence with a viscosity-dependent intensity and spectral position is detected in the near-IR region (750−1100 nm). An investigation of fluorescence excitation spectra, combined with semiempirical quantum chemical calculations and geometry optimizations suggest that in solution tbisOEP exists in two main conformations, which we name conformers P and U. The ratio of their concentrations was estimated to be approximately 5:1 in toluene at room temperature. The P conformer was found to be responsible for the “usual” monomer-type absorption bands (Soret and Q) in the tbisOEP absorption spectrum, whereas the U conformer is responsible for the additional absorptions in the 480−500 and 600−900 nm regions. The unusual near-IR fluorescence originates from the U conformer and its quantum yield (Φ) is 6 × 10-4 in toluene. Increase of solvent viscosity results in a strong blue-shift of the near-IR emission and increase of its intensity (Φ = 4 × 10-3 in paraffin oil). Both conformers P and U were found to exhibit very short exited-state lifetimes (<10 ps in toluene) which become significantly longer in more viscous solvents. The calculations suggest that the peculiar ground-state spectral properties of the U conformer result from its particular geometrical structure favoring a common conjugation between the π orbitals of the porphyrin rings and the ethylene bond, whereas only excitonic interactions exist in the P conformer. Therefore, the U conformer can be considered as a real supermolecule rather than two interacting separate porphyrin macrocycles. Our results point to the key role of the connecting bridge in the formation of the optical properties of the ethylene-bridged porphyrin dimers.

Effect of polyhydroxyalcohols on COS absorption in aqueous methyldiethanolamine

The removal of COS from natural gas is an area of increasing interest in natural gas sweetening, in view of tightening sulfur specifications. In an attempt to improve the absorption of COS in aqueous methyldiethanolamine (MDEA) solutions, the addition of polyhydroxyalcohols (PHA) was studied in small-scale laboratory tests. The addition of 25—30 wt.% ethylene glycol or glycerol increased the reaction rate by a factor up to 5–10, depending on the solvent loading and MDEA concentration. However, pilot plant studies revealed that the COS absorption could not be improved, or even worsened, under practical absorbing conditions. This is attributed to increased solvent viscosity which counteracts the increased reaction rate. The selectivity with respect to CO 2 absorption improved, however, in some cases by up to a factor of two.

Net sugar transport is a multistep process. Evidence for cytosolic sugar binding sites in erythrocytes.

Human erythrocyte net sugar transport is hypothesized to be rate-limited by reduced cytosolic diffusion of sugars and/or by reversible sugar association with intracellular macromolecules [Naftalin, R.J., Smith, P.M., & Roselaar, S.E. (1985) Biochim. Biophys. Acta 820, 235-249]. The present study examines these hypotheses. Protein-mediated 3-O-methylglucose uptake at 4 degrees C by human erythrocytes and by resealed, hypotonically lysed erythrocytes (ghosts) is inhibited by increasing solvent viscosity. Protein-mediated transport and transbilayer diffusion of the slowly transported substrate 6-NBD glucosamine are unaffected by increasing solvent viscosity. These findings suggest that protein-mediated 3-O-methylglucose transport is diffusion-limited in erythrocytes. More detailed analyses of red cell 3-O-methylglucose uptake (at 4 degrees C and at limiting extracellular sugar levels) reveal that net influx is a biexponential process characterized by rapid filling of a small compartment (C1 = 29 +/- 6% total cell volume; k1 = 7.4 +/- 1.7 min-1) and slow filling of a larger compartment (C2 = 71 +/- 6% total cell volume k2 = 0.56 +/- 0.11 min-1). Erythrocyte D-glucose net uptake at 4 degrees C is also a biphasic process. Transmembrane sugar leakage is a monoexponential process indicating that multicomponent, protein-mediated uptake does not result from sugar uptake by two cell populations of differing cellular volume. Sugar exit at limiting 3-O-methylglucose concentrations is described by single exponential kinetics. This demonstrates that multicomponent sugar uptake does not result from influx into two populations of cells with widely different sugar transporter content. We conclude that biexponential sugar uptake results from slow (relative to transport) exchange of sugars between serial, intracellular sugar compartments. Biexponential sugar uptake is observed under equilibrium exchange conditions (intracellular sugar concentration = extracellular sugar concentration) but only at 3-O-methylglucose concentrations of less than 1 mM. Above this sugar concentration, exchange uptake is a monoexponential process. Because diffusion rates are independent of diffusant concentration, this suggests that multicomponent uptake results from high-affinity sugar binding within the cell. The concentration of cytosolic binding sites (30 microM, Kd(app) = 400 microM) was estimated from the equilibrium cellular 3-O-methylglucose space. Biexponential net 3-O-methylglucose uptake is also observed in human erythrocyte ghosts, in control human K562 cells, and in K562 cells induced to synthesize hemoglobin by prolonged exposure to hemin. This demonstrates that neither membrane-bound nor free cytosolic hemoglobin forms the sugar-binding complex.(ABSTRACT TRUNCATED AT 400 WORDS)

Suspension NMR Spectroscopy of Phosphines and Carbonylnickel Complexes Immobilized on Silica

The 31P suspension NMR spectra o f silica-immobilized bifunctional phosphines and their di- and tricarbonylnickel complexes were recorded using conventional high resolution NMR equipment. The practical advantages of suspension NMR spectroscopy, as compared to 31P CP/MAS NMR spectroscopy, are discussed. General tendencies are demonstrated: The suspension NMR signals become broader with increasing solvent viscosity and decreasing solvent polarity. Phenyl groups as spacers lead to broader resonances than alkyl chains. The linewidths increase in the case of chelation.

Fluorescence quenching of N, N′-bis(2,5-di-tert-butylphenyl)-3,4:9,10-perylenebis(dicarboximide) (DBPI) by molecular oxygen

The effect of the solvent viscosity on the fluorescence quenching of N, N′-bis(2,5-di-tert-butylphenyl)-3,4:9,10-perylenebis (dicarboximide) (DBPI) by molecular oxygen was investigated by measuring the fluorescence decay. The solvent viscosity was changed by using different solvents at 0.1 MPa and by application of hydrostatic pressure. The quenching rate constant k q in non-polar solvents increased with increasing solvent viscosity η, whereas in polar alcoholic solvents it decreased with increasing solvent viscosity and followed a function of Aη − α with α = 0.48±0.12. When the viscosity was changed by application of hydrostatic pressure, k q of DBPI was also found to follow a function of Aη − α with α values of 0.71±0.02 and 0.58±0.08 in methanol and methylcyclohexane respectively. However, in n-hexane, the k q values exhibited a convex shape vs. viscosity. The activation energies associated with the fluorescence quenching of DBPI were calculated to be E a = 9.87±1.2, 14.1±0.24 and −3.96±0.6 kJ mol −1 in methanol, methylcyclohexane and n-hexane respectively. The negative activation energy and the convex shape of the viscosity dependence of k q obtained in n-hexane indicate complex formation between DBPI ∗ and molecular oxygen prior to fluorescence quenching. The large molecular size of DBPI does not increase the magnitude of k q or influence its dependence on η.

A microscopic theory of chemical reactions under high pressures

A new approach to the microscopic description of high-pressure processes is formulated where the pressure effect on a chemical reaction system is represented by ah effective pressure-dependent potential of a compressed medium. The method is applied to discuss reaction kinetics and chemical equilibria under pressures from 0 to 45 kbar (1 bar = 105 Pa). It was found that under pressures exceeding 50 kbar the deformation of the original potential-energy surface of a chemical reaction caused by pressure can be so strong that the reaction becomes barrierless. It is also shown that at high pressures reaction kinetics can demonstrate considerably nonequilibrium (not obeying transition-state theory) behavior as a result of increased solvent viscosity.

Viscosity Effects on the Intramolecular Fluorescence Quenching in Dipeptide Model Compounds

AbstractThe finding that the increased solvent viscosity decreases the rate for intramolecular fluorescence quenching in the title compounds [N(n)D], giving a singlet‐exciplex intermediate, but increases the rate for the reverse process, regenerating the locally excited singlet‐state N(n)D, is explained in terms of the viscosity‐dependent free‐energy change for this fluorescence quenching process (ΔGet). A linear correlation between the ΔGet and the logarithm of solvent viscosity is found to provide a good criterion for a singlet‐exciplex mechanism in polar solvents.

Rotational motion of Brownian particles with surface charge

A theory of rotational Brownian motion of particles with surface charges is developed. It is based on an exact solution of the generalized Hubbard-Onsager equations. Dielectric friction in a polar solvent as well as a local increase of solvent viscosity due to electrostriction are taken into account. It is shown that for irregular surface charge distributions only two parameters are important for rotational Brownian dynamics: the total charge Q = Σ iqi , and the sum of squares of the surface charges, J B = Σ iqi 2. Calculations show that the rotational diffusion coefficient depends strongly on the total surface charge, and may differ significantly from the ordinary Stokes value.

Viscous cosolvent effect on the ultrasonic absorption of bovine serum albumin

Protein-ligand binding and enzyme activity have been shown to be regulated by solvent viscosity, induced by the addition of viscous cosolvents. This was indirectly interpreted as an effect on protein dynamics. However, viscous cosolvents might affect dynamic, e.g., viscosity, as well as thermodynamic properties of the solution, e.g., activity of solution components. This work was undertaken to examine the effect of viscous cosolvent on the structural dynamics of proteins and its correlation with dynamic and thermodynamic solution properties. For this purpose we studied the effect of viscous cosolvent on the specific ultrasonic absorption, delta mu, of bovine serum albumin, at pH = 7.0 and at 21 degrees C, and frequency range of 3-4 MHz. Ultrasonic absorption (UA) directly probes protein dynamics related to energy dissipation processes. It was found that the addition of sucrose, glycerol, or ethylene glycol increased the BSA delta mu. This increase correlates well with the solvent viscosity, but not with the cosolvent mass concentration, activity of the solvent components, dielectric constant, or the hydration of charged groups. On the grounds of these results and previously reported findings, as well as theoretical considerations, we propose the following mechanism for the solvent viscosity effect on the protein structural fluctuations, reflected in the UA: increased solvent viscosity alters the frequency spectrum of the polypeptide chain movements; attenuating the fast (small amplitude) movements, and enhancing the slow (large amplitude) ones. This modulates the interaction strength between the polypeptide and water species that "lubricates" the chain's movements, leading to larger protein-volume fluctuation and higher ultrasonic absorption. This study demonstrates that solvent viscosity is a regulator of protein structural fluctuations.

Application of the rotating disk method to the study of bitumen dissolution into organic solvents

AbstractThe rotating disk system is demonstrated as a tool for performing studies of the dissolution rate of bitumen into organic solvents. Mass transfer coefficients to the rotating disk are constant in position and time can be made comparable to coefficients for sand particles in agitated liquids by adjusting the rotation rate. Data is presented for bitumen dissolution into pentane, hexane, heptane, toluene and oleic acid. Increasing the level of dissolved bitumen in the solvent decreases the bitumen dissolution flux primarily due to the increase in solvent viscosity.

The effect of the solvent viscosity on the migration of small molecules through the structure of myoglobin

The migration rate of small molecules through the structure of proteins can be monitored by quenching the light emitted from an excited optical probe located within the protein. In the present study we examined the influence of the solvent viscosity on the migration rate of the quencher anthraquinone sulfonate through myoglobin towards an excited Zn protoporphyrin molecule at the binding site of the protein. The solvent viscosity was increased by adding dextrans of different molecular weight but forming isoviscous solutions. The results demonstrate that the migration rate in the protein decreases with increasing solvent viscosity. This suggests that the fluctuations on the protein structure, which make the above migration possible, are affected by the solvent macroviscosity.

Quenching of positronium atoms by nitrobenzene in various solutions

A systematic study has been carried out on the quenching of positronium (Ps) atoms by nitrobenzene in various solvents. In order to see the effect of the energy level of Ps, in the localized bubble state, on the quenching capability of nitrobenzene, overall second-order rate constants were plotted against solvent surface tensions. It was found that the rate constant increased with increasing surface tension up to a critical value of ca. 26 dyn cm–1, corresponding to a Ps bubble energy of 1.24 eV. In liquids where the Ps energy is higher than this, the rate constant decreased with increase in solvent viscosity, as is expected for a diffusion-controlled reaction.

Study of aqueous dextran solutions under high pressures and different temperatures by dynamic light scattering

The behaviour of dextrans of different molar masses in aqueous solutions was investigated by dynamic light scattering at different temperatures, ranging from 30 to 100°C and different pressures, in the range 1–3000 bar. It has been shown that concentration dependence of the apparent diffusion coefficient, D app, of all samples changes the behaviour about the overlap concentration C∗. It has also been shown, that the diffusion coefficient increases with temperature (as a consequence of faster fluctuations of the molecules) and decreases with pressure. This could be accounted for by increase of solvent (water) viscosity with pressure. The activation energy of diffusion is independent of molar mass of dextran for the samples of molar masses from 7.4–21.1 × 10 6 g mol −1, but it is considerably lower for the sample of molar mass of 1.0 × 10 6 g mol −1. Pressure has a very small effect on activation energy of diffusion. The characteristic exponent for diffusion coefficient D 0 dependence on molar mass was found to be 0.32.

ChemInform Abstract: The Temperature‐ and Viscosity‐Dependent Photostationary E/Z Ratio in Triplet‐Sensitized Photoisomerization of Cyclooctene.

AbstractIn the triplet‐sensitized photoisomerization with benzene, toluene, or p‐xylene, the photostationary E/Z ratio of cyclooctene, (II)/(I), dramatically increases with lowered irradiation temp. or with increased solvent viscosity, whereas the E/Z ratio of oct‐2‐ene, (III)/(IV), stays unchanged over the temp. range indicated.

The temperature- and viscosity-dependent photostationary E/Z ratio in triplet-sensitized photoisomerization of cyclo-octene

In the triplet-sensitized photoisomerization with benzene, toluene, or p-xylene, the photostationary E/Z ratio of cyclo-octene, (E/Z)pss, dramatically increases with lowered irradiation temperature (+ 75 to –123 °C) or with increased solvent viscosity, whereas the (E/Z)pss ratio of oct-2-ene stayed unchanged over the temperature range +25 to –78 °C. Theoretical examinations indicate that the temperature-dependent (E/Z) value is not a simple function of solvent viscosity but also depends upon the solvent structure. The Stern–Volmer studies further revealed that the variable quenching rate constants for Z and E isomers are responsible for the temperature- and viscosity-dependent photostationary state.