Sonolysis of high macroviscosity systems: hen albumen hydrogel.

To understand molecular effects of ultrasound on protein gels (cross-linked, hydrated macromolecular systems of immeasurably high macroviscosity, but low microviscosity), the thick fraction of hen albumen was sonicated. The immeasurably high viscosity of the intact thick fraction decreased to 2.5-4.0 mPa·s (depending on the sample) after a 12 min sonication (0.14 mM of radicals were produced and 19 J g(-1) of thermal energy absorbed) indicating that the 3D protein network was degraded. SDS-PAGE analysis indicated the breaking of intermolecular S-S bridges holding together the protein network rather than the primary structure of constituent proteins. Despite the relatively large concentration of OH radical produced in the sonication time range applied, no protein cross-linking was observed which can be attributed to the high degree of protein glycosylation and protein immobility. Differential scanning calorimetry (DSC) showed that both the amount of bound water and the enthalpy of denaturation of the constituent proteins are not affected by sonication, which is consistent with the SDS-PAGE results. A small increase in sample turbidity can be attributed to the small extent of thermal denaturation occurring in the vicinity of cavitation sites.

Appropriate excitation wavelength removes obfuscations from pyrene excimer kinetics and mechanism studies.

The experimental procedures of time-resolved laser photolysis studies of pyrene excimer formation in solution have been scrutinized and the appropriate modifications have been implemented. Contrary to the experimental methods applied in all related previous work, the selection of a suitable excitation wavelength (such that the corresponding pyrene absorbance is less than 0.5 absorbance units) utilized in our study results in simple homogeneous kinetics. Consequently, the rate parameters obtained and the mechanism proposed differ significantly from those published previously. The rate constant values of the unimolecular decay of the pyrene monomer, the unimolecular decay of the pyrene excimer, and the excimer formation in decane solution (η = 0.860 mPa s) at 25 °C are (2.38 ± 0.01) × 10(6) s(-1), (2.78 ± 0.02) × 10(7) s(-1), and (3.11 ± 0.06) × 10(9) M(-1) s(-1), respectively. The dissociation of the excimer to form a singlet excited state pyrene and a ground state pyrene was shown to be negligible. The energies of activation corresponding to the monomer and excimer unimolecular decays were found to be 2.51 ± 0.07 and 25.7 ± 0.7 kJ mol(-1), respectively. Also, our temperature resolved laser photolysis data revealed that the excimer formation has a negative energy of activation equal to -11.2 ± 0.5 kJ mol(-1). This unique phenomenon may be attributed to steric effects in the collision of the reactants. The current findings are important for the correct data analysis and interpretation in many applications of the pyrene excimer.

Effects of microviscosity, dry electron scavenging, and protein mobility on the radiolysis of albumen hydrogel.

Nanosecond pulse radiolysis experiments performed on the oxygen and nitrogen saturated thick fraction of egg white (which has an immeasurably high macroviscosity) produced a rate constant for hydrated electron reaction with oxygen equal to 1.7 × 10(10) M(-1) s(-1) at 18 °C, indicating that, due to the very low microviscosity, hydrated electrons are as mobile and reactive in the albumen hydrogel as in neat water. Also, the radiolytic yield for the hydrated electron (G-value) in the thick fraction of egg white (measured at the end of a 14 ns electron pulse) was found to be 86% of that determined in neat water, which can be attributed to the reaction of dry electrons with the protein constituents. Steady-state γ radiolysis studies on air-saturated thick and thin fraction (true solution) revealed that the immobility of egg white proteins disfavors reactions that result in molecular mass change.

Mobility of molecules and ions solubilized in protein gels: diffusion in the thick fraction of hen egg white.

The thick fraction of hen egg white is a protein hydrogel with an immeasurably high viscosity composed of ∼90% water that can serve as a model system for mammalian mucous membrane. Measurements of the rate constants of diffusion-controlled reactions occurring within the gel (and corresponding activation energies) and electric conductivity revealed that the thick fraction of egg white can be envisioned as a 3D network comprising hydrated protein molecules (held by intermolecular S-S bridges) surrounded by water pools and channels (of nonuniform diameters) that have a microviscosity that is very similar to that of bulk water. This was corroborated by differential scanning calorimetry measurements that revealed that 16% of water is bound to proteins. The melting kinetics of ice crystallites (produced from the freezable water) indicates nonhomogeneous water pool size.

Macro- and microscale rheological properties of poly(vinyl alcohol) aqueous solutions.

Electric conductivity measurements indicated that microviscosities of poly(vinyl alcohol), PVA, solutions [up to 10% (w/w)] are comparable to that of pure water (contrary to Walden's rule), but are different (25%) from those determined from diffusion-controlled reaction measurements that coincide with the values obtained using pure water. Energies of activation of fluidity of PVA solutions are found to increase with PVA concentration, whereas those of the diffusion-controlled reactions are independent of PVA concentration. By comparing the macro- and microviscosities, it was concluded that PVA aqueous solutions can be envisioned as dynamic systems comprising hydrated PVA molecules (that affect the macroviscosity) and "interconnected water pools" (located between macromolecules), the rheological properties of which are very similar to that of pure water. Fluorescence depolarization measurements indicated that pool sizes are relatively large, which was corroborated by DSC measurements that showed that each PVA hydroxyl group interacts with no more than two water molecules.

Analysis and improvement of rate constant determination of reactions involving charged reactants.

Kinetics of tris(2,2'-bipyridine)ruthenium(ii), Ru(bpy)(3)(2+), luminescence quenching by copper(ii) (in the form of chloride, nitrate, sulfate and perchlorate salt) was studied using pulse laser photolysis technique. The pseudo-first order rate constant versus quencher concentration plots obtained were found to be nonlinear, bending upward. The ionic strength effect contribution was evaluated by applying the Debye-Hückel extended law and was found to be as important as other effects such as cation-counter anion complex and ion-pairing complex formation which were all found to be dependent on the counter anion. It is shown that the slope of the tangent line to the pseudo-first order curve at zero quencher concentration is equal to the quenching rate constants at zero ionic strength. Also, this value corresponds to quenching solely by Cu(2+) and is free from contributions from other species that are present at higher concentrations. This method produced a value, (1.6 +/- 0.2) x 10(7) M(-1) s(-1), (lower than previously published ones) which is in agreement with the quenching rate constant measurement analysis presented. Comparison between Stern-Volmer plots obtained using steady-state fluorimetry data and laser photolysis data showed that in 50 mM CuCl(2) and CuSO(4) aqueous solutions about 5% of Ru(bpy)(3)(2+) is in the form of ion-pairing complexes. Our method was also applied to quenching by another divalent cation, methyl viologen, where it was found that charge transfer complexation effect contribution was about 50% of that of ionic strength effect, while ion-pairing complexation was not significant in the concentration range used. The quenching rate constant at zero ionic strength was found to be (2.3 +/- 0.2) x 10(8) M(-1) s(-1). The method proposed is also applicable to pulse radiolysis and stopped flow measurements.

Radiolysis of aqueous solutions of 1,1- and 1,2-dichloroethane.

The yields of chloride ion and molecular hydrogen were determined in the gamma, the fast electron, and the 5 MeV helium ion radiolysis of deaerated and aerated aqueous solutions of 1,1- and 1,2-dichloroethane. In deaerated solutions irradiated with gamma-rays or fast electrons, the yield of chloride ion increases while the yield of molecular hydrogen decreases with increasing dichloroethane concentration. These results are due to the quantitative reaction of both the hydrated electron and the hydrogen atom with the dichloroethane to produce chloride ions. The yield of chloride ions is significantly larger in aerobic than in anaerobic conditions and is dependent upon the dose rate. Formation of peroxyl radicals by the reaction of molecular oxygen with chlorinated hydrocarbon radicals and their subsequent chemistry are responsible for the observed increase in chloride ions. The yield of chloride ion with 5 MeV helium ions is smaller than with gamma irradiation, while the yield of molecular hydrogen is larger reflecting the higher density of reactive species and consequent increase in intratrack reactions in a helium ion track compared to a gamma-ray track.

Rate coefficient measurements of hydrated electrons and hydroxyl radicals with chlorinated ethanes in aqueous solutions.

Rate coefficients for the reactions of hydrated electrons and hydroxyl radicals with various chloroethanes were determined in aqueous solutions using pulse radiolysis techniques. The rate coefficients for the hydrated electron increase from 0.17 x 10(9) to 16.3 x 10(9) M(-1) s(-1) with increasing number of chlorine atoms from monochloroethane to hexachloroethane. Very little difference in rates is found between the isomers. Rate coefficients for the OH radicals range from 1 to 5 x 10(8) M(-1)s(-1) and have very little variation with the number of chlorine atoms except when no H atom is available on a carbon atom. The use of competition kinetics with low concentrations of SCN(-) as a reference is reviewed and suitable model simulations proposed. Possible explanations for the discrepancies between the previously published rate coefficients and the present values are offered.

Pulse radiolysis of aqueous thiocyanate solution.

The pulse radiolysis of N(2)O saturated aqueous solutions of KSCN was studied under neutral pH conditions. The observed optical absorption spectrum of the SCN(*) radical in solution is more complex than previously reported, but it is in good agreement with that measured in the gas phase. Kinetic traces at 330 and 472 nm corresponding to SCN(*) and (SCN)(2)(*-), respectively, were fit using a Monte Carlo simulation kinetic model. The rate coefficient for the oxidation of SCN(-) ions by OH radicals, an important reaction used in competition kinetics measurements, was found to be (1.4 +/- 0.1) x 10(10) M(-1) s(-1), about 30% higher than the normally accepted value. A detailed discussion of the reaction mechanism is presented.