Increasing Solvent Viscosity Research Articles

Effect of protein dynamics upon reactions that occur in the heme pocket of horseradish peroxidase.

Free base and Pd porphyrin derivatives of horseradish peroxidase show long-lived excited states that are quenched by the presence of the peroxidase inhibitor, benzhydroxamic acid. The relaxation times of the excited-state luminescence and the rates of the quenching reaction for these derivatives of peroxidase were monitored as a function of pH, temperature, and viscosity with the view of examining how protein dynamics affect the quenching reaction. As solvent viscosity increases, the rate decreases, but at the limit of very high viscosity (i.e., high glycerol or sugar glass) the quenching still occurs. A model is presented that is consistent with the known structure of the enzyme-inhibitor complex. It is considered that the inhibitor is held at an established position but that solvent-dependent and independent motions allow a limited diffusion of the two reactants. Since there is a steep dependence upon distance and orientation, the diffusion toward the favorable position for reaction enhances the reaction rate. The solvent viscosity dependent and independent effects were separated and analyzed. The importance of internal reaction dynamics is demonstrated in the observation that rigidity of solvent imposed by incorporating the protein into glass at room temperature allows the reaction to occur, while the reaction is inhibited at low temperature. The results emphasize that protein dynamics plays a role in determining reaction rates.

Solvent Viscosity Effects on Photochromic Reactions of a Diarylethene Derivative As Revealed by Picosecond Laser Spectroscopy

The solvent viscosity effect on photochromic reaction processes such as cyclization and cycloreversion of a diarylethene derivative, 2-bis(2,4,5-trimethyl-3-thienyl)maleic anhydride (TMTMA), was investigated by means of steady-state spectroscopy, picosecond transient absorption, and fluorescence measurements. The cyclization reaction yields from the excited open-ring isomer decreased with an increase in solvent viscosity. Picosecond transient absorption spectroscopy revealed that the cyclization reaction took place via two pathways; very rapid reaction immediately after the excitation in competition with the relaxation to the fluorescent state and the rather slow reaction from the relaxed fluorescent state with the time constant of several hundred picoseconds. The latter slow process was strongly dependent on the solvent viscosity, whereas the former rapid process was almost independent of the viscosity. This viscosity dependence of the reaction from the fluorescent state was attributed to the solvent mol...

Photochemical studies as a function of solvent viscosity. A new photochemical pathway in the reaction of (eta5-C5H4Me)2Mo2(CO)6 with CCl4.

This paper reports the results of a study that used systematic changes in the solvent viscosity to probe the photochemical reactivity of the Cp'2Mo2(CO)6 (Cp' = eta5-C5H4CH3) molecule. The quantum yields for photolysis of Cp'2Mo2(CO)6 in the presence of CCl4 were studied as a function of solvent viscosity. The quantum yields did not decrease to zero with increasing solvent viscosity but rather leveled off at a constant, non-zero value. This result cannot be explained by any of the previously reported radical or Mo-CO dissociation photochemical pathways for this molecule, and therefore an additional photochemical pathway is suggested to be operating in the reaction. The new pathway may involve a reactive isomer of Cp'2Mo2(CO)6 or possibly electron transfer between the excited state of Cp'2Mo2(CO)6 and CCl4.

Effect of viscosity on the deamidation rate of a model Asn-hexapeptide.

The effect of viscosity on the deamidation rate of a model Asn-containing hexapeptide (l-Val-l-Tyr-Pro-l-Asn-Gly-l-Ala) was assessed in aqueous solution and in solids containing varying amounts of poly(vinyl pyrrolidone) (PVP) and water. Stability studies were conducted at 0.1 mg/mL peptide and 0-50% PVP (w/w) in aqueous solution, and at 5% (w/w) peptide and different relative humidities (31.6, 53.1, 74.4 and 96%) in the solid state. The parent peptide and its deamidation products were analysed by reverse-phase high-performance liquid chromatography. Deamidation rates decreased with increasing solvent viscosity in a manner described by a semi-empirical mathematical model developed to describe this relationship. The results suggest that the motion of the Asn side-chain along the reaction coordinate is a function of the macroscopic solvent viscosity. However, the apparent energy barrier for the diffusive movement of the side-chain appears to be less than the energy barrier for that associated with macroscopic viscosity. The dependence of the deamidation rate on viscosity in both viscous solution and hydrated solids further demonstrates the importance of mobility in peptide deamidation.

Effect of ethanol addition on the elongational flow behavior of aqueous solutions of poly(ethylene oxide)

In this work, we explore the effect of ethanol addition on the extension thickening behavior of aqueous solutions of poly(ethylene oxide) (PEO) in porous media and opposed-jets flow. The main aspect analyzed is the formation of transient entanglements of polymer molecules, which are responsible for the sudden increase in pressure drops and apparent extensional viscosities with strain rates observed in elongational flows of PEO solutions. The results show that changing the solvent by adding ethanol modifies the solution behavior because of two effects. First, the increase in the solvent viscosity increases the disentanglement time of the transient entanglements so that the onset of extension thickening occurs at lower strain rates in the presence of ethanol. Second, ethanol solutions are poorer solvents than water for PEO. This promotes intermolecular interactions between PEO coils when the solutions are semi-dilute. The net result is a lowering of the onset strain rate for extension thickening to values that are much smaller than those expected for the same PEO chains dissolved in a higher viscosity and poorer solvent. The results also show that coil-coil interactions in solution can be detected at lower concentrations in porous media flow than in opposed jets flow.

Segmental Mobility in Phosphorus-Containing Dendrimers. Studies by Fluorescent Spectroscopy

Steady-state fluorescence spectra and decays of excitations of a phosphorus-containing dendrimer labeled with 12 internal labels and of an iminophosphorane model compound bearing two labels dissolved in acetonitrile, diglyme, 1,4-dioxane, triethylene glycol, and cyclohexanol revealed that the interior of dendrimers contained many solvent molecules, and movements of internally located pyrene labels were not reduced by interactions with the dendrimer core. This conclusion was based on the following findings. A ratio of the intensities of pyrene−pyrene excimer and pyrene monomer emissions (IE/IM) decreased with increasing solvent viscosity but, with exception of acetonitrile solutions, was very close for the dendrimers and for the model. Monomer and excimer emission intensity decays were fitted with two-exponential functions with time constants τ1 and τ2 characterizing formation of pyrene−pyrene excimers and recovery of pyrene moieties in their ground state, respectively. For solvent viscosities varying from...

Viscosity dependence of protein dynamics

The influence of solvent viscosity on protein dynamics was investigated with molecular dynamics simulations of factor Xa in two solvents differing only in viscosity, by a factor of 10. We obtained this viscosity change by changing the masses of the solvent atoms by a factor of 100. Equilibrium properties of the protein, that is, the average structure, its fluctuations, and the secondary structure, show no significant dependence on the solvent viscosity. The dynamic properties of the protein, that is, the atom-positional correlation times and torsional angle transitions, however, depend on the solvent viscosity. The protein appears to be much more mobile in the solvent of lower viscosity. It feels the influence of the solvent not only on the surface but even in its core. With increasing solvent viscosity, the positional relaxation times of atoms in the protein core increase as much as those of atoms on the protein surface, and the relative increase in the core is even larger than on the surface. Proteins 2001;42:414–421. © 2001 Wiley-Liss, Inc.

Viscosity dependence of protein dynamics

The influence of solvent viscosity on protein dynamics was investigated with molecular dynamics simulations of factor Xa in two solvents differing only in viscosity, by a factor of 10. We obtained this viscosity change by changing the masses of the solvent atoms by a factor of 100. Equilibrium properties of the protein, that is, the average structure, its fluctuations, and the secondary structure, show no significant dependence on the solvent viscosity. The dynamic properties of the protein, that is, the atom-positional correlation times and torsional angle transitions, however, depend on the solvent viscosity. The protein appears to be much more mobile in the solvent of lower viscosity. It feels the influence of the solvent not only on the surface but even in its core. With increasing solvent viscosity, the positional relaxation times of atoms in the protein core increase as much as those of atoms on the protein surface, and the relative increase in the core is even larger than on the surface.

The Application of Diffusing-Wave Spectroscopy to Monitor the Phase Behavior of Emulsion–Polysaccharide Systems

Droplet aggregation is an important cause of instability in emulsions because it may, on one hand, lead to an increased creaming rate, resulting in fast separation of a concentrated emulsion phase (creamed layer). On the other hand, it may also lead to the formation of a stabilizing, droplet-based network. Early detection of instability is often difficult due to the high turbidity and viscosity of more concentrated food emulsions. The applicability of diffusing-wave spectroscopy (DWS) for monitoring droplet aggregation and creaming was studied using a model system consisting of a protein-stabilized emulsion, to which a soluble polymer (“thickener”) was added. This addition leads to an increased solvent viscosity and may induce droplet aggregation. In addition, the redistribution process of emulsion droplets in aggregating concentrated emulsions was directly observed by confocal scanning laser microscopy (CSLM). By DWS the decrease of the droplet mobility caused by the viscosity increase of the continuous phase could be separated from the effect of droplet aggregation. Moreover, a distinction could be made between aggregation, leading to increased creaming rates and that leading to the formation of a stabilizing droplet network. The potential of DWS for in situ measurement of the stability of concentrated emulsions is discussed.

Pressure effect on the dynamics of an isolated alpha-helix studied by 15N-1H NMR relaxation.

Dynamics and structure of (1-36)bacteriorhodopsin solubilized in chloroform/methanol mixture (1:1) were investigated by 1H-15N NMR spectroscopy under a hydrostatic pressure of 2000 bar. It was shown that the peptide retains its spatial structure at high pressure. 15N transverse and longitudinal relaxation times, 15N[1H] nuclear Overhauser effects, chemical shifts and the translation diffusion rate of the peptide at 2000 bar were compared with the respective data at ambient pressure [Orekhov et al. (1999) J. Biomol. NMR, 14, 345-356]. The model free analysis of the relaxation data for the helical 9-31 fragment revealed that the high pressure decreases the overall rotation and translation diffusion, as well as apparent order parameters of fast picosecond internal motions (S2) but has no effect on internal nanosecond motions (S2 and taus) of the peptide. The decrease of translation and overall rotation diffusion was attributed to the increase in solvent viscosity and the decrease of apparent order parameters S2f to a compression of hydrogen bonds. It is suggested that this compression causes an elongation of H-N bonds and a decrease of absolute values of chemical shift anisotropy (CSA). In particular, the observed decrease of S2f at 2000 bar can be explained by 0.001 nm increase of N-H bond lengths and 10 ppm decrease of 15N CSA values.

Diffusional barrier crossing in a two-state protein folding reaction.

There has been some debate as to whether protein folding involves diffusive chain motions and thus depends on solvent viscosity. The interpretation of folding kinetics in viscous solvents has remained difficult and controversial, in that viscogenic agents affect folding rates not only by increasing solvent viscosity but also by increasing protein stability. By carefully choosing experimental conditions, we can now eliminate the effect on stability and show that the folding dynamics of the cold shock protein CspB are viscosity dependent. Thus Kramers' theory of reaction rates rather than transition state theory should be used to describe this folding reaction.

Acceleration of Bimolecular Reactions by Solvent Viscosity

We report the first example of acceleration of a bimolecular reaction by increased solvent viscosity. In a series of hydrocarbon solvents of graded viscosity, the rates of two cycloadditions, the Diels−Alder dimerization of cyclopentadiene and the 1,3-dipolar cycloaddition of diphenyldiazomethane with ethyl phenylpropiolate, rise with increasing viscosity to about 1 cP and then fall thereafter. We interpret this rise as viscosity-induced acceleration up to ∼1 cP, which cannot be accounted for by current kinetic theory. Several examples from the literature are reinterpreted in these terms, providing further support for our concept.

A Consistance Model for Anion Exclusion and Surface Diffusion

AbstractA decomposition of the diffusion flux equation for an electrostatically bound and mobile hydrated ion into two components is proposed. The first component includes the effects arising solely from the irregular pore shape and increase in solvent viscosity in the proximity of negatively charged pore walls. Apart from these effects, the second flux component includes an additional contribution from an increased (decreased) concentration for cations (anions) close to the pore walls. Defining the distribution coefficient, Kd, in a fashion that allows negative values for co-ions readily accounts for their exclusion without the need to introduce somewhat artificial quantities like the “effective co-ion porosity”. In this study, it is thus possible to retain the purely volumetric meaning of the porosity and to maintain consistency throughout the conceptualization for anions, cations and electrically neutral species. Furthermore, the decomposition of the flux equation provides support for surface diffusion, a subject of great controversy and lively debate in the literature. In this connection, the role of concentration to regulate the diffusive flux for ions in relation to neutral species is emphasized. Implications for the theoretical apparent and effective diffusivities in compacted montmorillonite clay are also discussed and a modified form of the macroscopic theory is proposed.

Axisymmetric entry flow of semi-dilute xanthan gum solutions: prediction and experiment

Experimental observations of the flow patterns in axisymmetric entry for rigid rod molecules are presented. The features of the flow are a large secondary flow vortex whose size is relatively independent of the flowrate, a vortex which increases in size with the rigid rod polymer concentration, decreases in size as the solvent viscosity increases, and increases as the contraction ratio increases. The secondary flow vortex sizes predicted and observed for dilute and semi-dilute rigid fibre suspensions are similar to those observed for the semi-dilute rigid rod xanthan gum solutions. Also, rounding a corner in the contraction had no effect on the observed asymmetric entry flow of the rigid rod molecular solutions. Good agreement was observed between the dimensionless re-attachment length for a 0.04% w/w xanthan gum solution and the predicted secondary re-attachment length from Binding [J. Non-Newtonian Fluid Mech. 27 (1988) 173–189] which used the independently measured extensional viscosity in the prediction. The agreement between experiment and observation confirmed Binding's assertion that the fluid will show vortex enhancement if its steady shear viscosity undergoes shear thinning more rapidly than its extensional viscosity undergoes elongational thinning. Vortex enhancement for fluids with constant extensional viscosity and steady shear viscosity, as observed, agreed with predictions by Debbaut et al. [J. Non-Newtonian Fluid Mech. 29 (1988) 119–146] and Debbaut and Crochet [J. Non-Newtonian Fluid Mech. 30 (1988) 169–184]. From the data presented it would be fair to speculate that normal stresses impede vortex growth while elongational thickening increases vortex growth.

The role of the solvent viscosity on the spatiotemporal instabilities of large aperture dye lasers

Local intensity fluctuations in a large aperture dye laser have been measured for several solvent viscosities. A decrease on the relative fluctuations amplitude as the solvent viscosity increases is found. From a theoretical approach, we show that the intensity fluctuations follow the same behavior as the molecular polarization orientation driven by the laser field.

Fluorescence quenching of 2,5-bis-2-(2-naphthyl)vinyl pyrazine(B2NVP) by cobalt ions and oxygen molecules

The fluorescence quenching of 2,5-bis-2-(2-naphthyl) vinyl pyrazine B2NVP by cobalt ions and molecular oxygen was investigated in different media by using steady state emission and lifetime measurements, respectively. The quenching rate constant, k q, and the critical transfer distance, R 0, for quenching of B2NVP by Co +2 ions indicates a Förster type long-range energy transfer is obeyed. The activation energy associated with fluorescence quenching of B2NVP by Co +2 ions was calculated to be E a=5.53 kJ mol −1 in methanol. The quenching rate constant, k q, by molecular oxygen decreases with increasing solvent viscosity and followed a function of Aη − α with α=0.73.

High Pressure Studies on the Excited Intramolecular Charge-Transfer State in 4-(9-Anthrylmethyl)-N,N-dimethylaniline.

The dynamic solvent effect on the intramolecular charge-transfer (ICT) -state formation of 4- (9-anthrylmethyl) -N, Ndimethylaniline (AMDMA) in the excited state was examined by using picosecond time-resolved fluorescence spectroscopy at high pressures. The non-exponential population decay of the local excited (LE) -state emission is analyzed by fitting to a double exponential function. As solvent viscosity increases, the rate of the ICT-state formation becomes noticeably faster than the solvent relaxation time, which is gauged by the longest component of longitudinal relaxation time of the solvent (τL). The effect on the ICT reaction of AMDMA in the excited state can be accounted for in terms of the pressure tuning effect of the solvent viscosity.

High‐Pressure Picosecond Time‐Resolved Fluorescence Studies on the Excited Intramolecular Charge‐Transfer Kinetics of 4‐(9‐Anthrylmethyl)‐N,N‐Dimethylaniline in Alcohol

AbstractPicosecond time‐resolved fluorescence spectroscopy has been used to examine the dynamic solvent effect on the excited intramolecular charge‐transfer (ICT)‐state formation in 4‐(9‐anthrylmethyl)‐N,N‐dimethylaniline (AMDMA) in linear alcohol solvents as a function of pressure. The non‐exponential population decay of the local excited (LE)‐state emission is analyzed by fitting to a double exponential function. As solvent viscosity increases, the rate of the ICT‐state formation of AMDMA becomes noticeably faster than the solvent relaxation time, which is gauged by the longitudinal relaxation time of the solvent (τL). The pressure effect on the ICT reaction of AMDMA in the excited state can be accounted for in terms of the pressure‐tuning effect of the solvent viscosity.In addition, the time‐dependent Stokes shift (TDSS) of the ICT emission is measured at high pressures.

Tris(1‐naphthyl)borane Studied by Laser Flash Photolysis, Matrix Isolation, and Low‐Temperature NMR Spectroscopy: Two Isomers with Significantly Different Triplet Excited State Properties

AbstractLaser flash photolysis (308 nm) of tris(1‐naphthyl)borane (TNB, 1) in cyclohexane solution yields a transient spectrum that is best interpreted as the superposition of the spectra of two transient species with λmax = 460 nm (transient 1, τ = 10 μs) and λmax = 640 nm (transient 2, τ = 20 μs). Transient 2 also shows some absorption at λ > 800 nm. Both transient species exhibit behaviour typical of triplet excited states. The seemingly paradoxical situation of two long‐lived triplet excited states being generated by excitation of a single compound can be resolved in terms of two isomeric structures of TNB. The activation energy for isomerization is very low and has been calculated (AM1/RHF) to be of the order of 2.7 kcal/mol. Further evidence comes from low‐temperature 1H‐NMR spectroscopy. At T = 183 K, some NMR bands of 1 are significantly broadened, while the shape of other bands remains nearly constant. This indicates that the broadening effect observed is due to the slowing‐down of an intramolecular rearrangement, and not to a general increase in solvent viscosity. The phosphorescence spectrum of 1, matrix‐isolated in Ar at 10 K, consists of three bands, which can be attributed to two different emitting isomers. The corresponding triplet energies differ significantly and are 57.8 and ca. 48 kcal/mol, respectively. The LFP experiments performed with 1 gave no evidence for the formation of the hypovalent boron species 1‐naphthylborene or di‐1‐naphthylboryl radical.

Determination of a fraction of spin-correlated radical ion pairs in irradiated alkanes by quantum oscillation technique

Amplitudes of quantum oscillations observed in the recombination fluorescence of the (diphenylsulfide- d 10) +/( p-terphenyll- d 14) − radical ion pairs in alkanene solutions have been used to estimate a fraction of spin-correlated singlet pairs Θ in the radiation track. Θ values of 0.36 – 0.54 for the solvent series studied were found. Values for a fraction of singlet recombinations f s (0.52–0.66), calculated from Θ values, are in satisfactory agreement with those available from literature. It has been observed that Θ and f s values decrease with increasing solvent viscosity (from n-hexane to cis-decalin).