Kinetics Of Breakage Research Articles

On the rheological modeling of associative polymers

The viscoelastic behavior of a commercial hydrophobic alkali-soluble emulsion (HASE) associative polymer in small-amplitude oscillatory, steady, and unsteady simple-shear flows is analyzed with a model that couples the upper-convected Maxwell constitutive equation with a kinetic equation that accounts for structural changes induced by the flows. A spectrum of relaxation times is considered in the prediction of rheological properties to account for the association dynamics between hydrophobic groups along the HASE backbone and physical entanglements. Viscoelastic response is similar to that of a transient network assembled through hydrophobic associations, where the kinetics of chain breakage and reformation is consistent with classical descriptions of transient network formulations. The model accounts for deviations from the Cox–Merz rule and predicts observed limiting behaviors at high strain rates in stress relaxation and inception of shear flow.

Kinetics of Loop Formation and Breakage in the Denatured State of Iso-1-cytochrome c

The earliest events in protein folding involve the formation of simple loops. Observing the rates of loop closure under denaturing conditions can provide direct insight into the relative probability and sequence determinants for formation of loops of different sizes. The persistence of these initial contacts is equally important for efficient folding, so measurement of rates of loop breakage under denaturing conditions is also essential. We have used stopped-flow and continuous-flow methods to measure the rates of histidine-heme loop formation and breakage in the denatured state of iso-1-cytochrome c (in the presence of 3 M guanidine HCl). The data indicate that the mechanism for forming loops is a two-step process, the first step being the deprotonation of the histidine, and the second step being the binding of the histidine to the heme. This mechanism makes it possible to extract both the rate constants of formation, k(f), and breakage, k(b), of loops from the pH dependence of the observed rate constant, k(obs). To determine the dependence of k(f) and k(b) on loop size, we have carried out kinetic measurements for seven single surface histidine variants of iso-1-cytochrome c. A scaling factor (the dependence of k(f) on log[loop size]) of approximately -1.8 is observed for loop formation, similar to that observed in other systems. The magnitude of k(b) varies from 30 s(-1) to 300 s(-1), indicating that the stability of different loops varies considerably. The implications of the kinetics of loop formation and breakage in the denatured state for the mechanism of protein folding are discussed.

Kinetics of Drop Breakage in Stirred Vessels under Turbulent Conditions

The emulsification of dodecane oil was studied in a standard stirred vessel filled with distilled water. The in situ size distribution of the drops was measured at certain time intervals by using a particle size analyzer while stirring. The results showed that an increase in the stirring time and stirring rate resulted in a decrease in the mean drop size of drops and shifted the drop size distribution toward smaller sizes. The minimum droplet size in the turbulent flow was found to be proportional to amin∼(WD)−1.75. Normalization of data exhibited that the size distribution of drops is self‐preserving. The distribution of mean drop size and distribution functions of drops were determined by using models developed from the Focker‐Plank equation regarding size distribution of particles in turbulent flows. The evaluation of droplet breakage with time in isotropic turbulent flow showed that the size distribution can be represented by the summation of log‐normal distribution function in series. It was found that the experimental data could be represented well by the empirical models.

Fluidised-bed jet milling of pharmaceutical powders

Jet milling is often employed to produce very fine product or to tackle materials that are difficult to mill. This paper analyses particle breakage in a single jet region in a fluidised bed with a view to identify the role of jet hydrodynamics and material properties. By making a number of simplifying assumptions, the particle breakage in the jet can be estimated by coupling a hydrodynamic model of the jet with the kinetics of single-particle impact breakage. This approach works satisfactorily for a number of particulate solids, e.g., α-lactose monohydrate, but poorly for some others, e.g., paracetamol. The underlying cause of this feature is unclear, although it is likely to be due to the hydrodynamic mechanism. In this work, a sensitivity analysis has been carried out which highlights the importance of some of the hydrodynamic parameters on particle flow and breakage in the jet region. A more general model of jet hydrodynamics is needed to predict reliably the particle behaviour in this region.

The radiation hypersensitivity of cells at mitosis

Purpose : Mitotic cells are hypersensitive to ionizing radiation, exhibiting single-hit inactivation coefficients near to those of repair deficient cell lines and lymphocytes. To elucidate possible mechanisms for this hypersensitivity, the kinetics of oxygen radiosensitization, the proportion of indirect effect by OH radicals and the kinetics of radiation-induced DNA strand breakage in the chromatin of mitotic cells were investigated. Materials and methods : Synchronized populations of >90% mitotic HT-29 cells were obtained by the mitotic shake-off method. Cells were irradiated at ≤4°C with 137 Cs γ-rays. Cellular oxygen concentration was varied by gassing cell suspensions prior to and during irradiation with mixtures of pure N 2 that contained 5% CO 2 and measured quantities of O 2. The indirect effect of OH radicals was investigated with the radical scavenger, DMSO. DNA strand breakage was measured by the comet assay. Results : Mitotic HT-29 cell inactivation is well described by a single-hit inactivation coefficient (α) of 1.14 ±0.06 Gy -1. The oxygen enhancement ratio of mitotic cells (at 10% survival) was found to be ~2.0, significantly lower than the value of 2.8 measured for interphase (asynchronous) cells. More than 60% of mitotic cell killing was eliminated when the media contained 2 M DMSO, indicating that indirect effect is as important in the killing of mitotic cells as it is for interphase cells. The chromatin in mitotic cells was found to be ~2.8 times more sensitive to radiation-induced DNA single-strand breakage than the chromatin of interphase cells. Conclusions : The α -inactivation coefficient of mitotic HT-29 cells was ~30 times larger than that of interphase cells. Mitotic cell chromatin appears to contain intrinsic DNA breaks that are not lethal. In addition, chromatin in mitotic cells was found to be more susceptible to radiation-induced DNA strand-breakage than the dispersed chromatin of interphase cells. How the enhanced production of these simple DNA lesions (that are usually reparable) translates into the lethal (non-reparable) events associated with α -inactivation is not known. The compaction/dispersion status of DNA throughout the cell cycle appears to be an important factor for determining intrinsic cell radiosensitivity and might be manipulated for radiotherapeutic advantage.

Pulsed electric field breakage of cellular tissues: visualisation of percolative properties

Results of numerical and experimental investigation of the electric breakage of a cellular material in pulsed electric fields (PEF) are presented. The numerical model simulates the conductive properties of a cellular material by a two-dimensional array of biological cells. The application of an external field in the form of an idealised square pulse sequence with a pulse duration t i , and a pulse repetition time Δ t is assumed. The simulation model includes the known mechanisms of temporal and spatial evolution of the conductive properties of different microstructural elements in a tissue. The kinetics of breakage at different values of electric field strength E, t i and Δ t was studied in the experimental investigation. A 5×55-mm cylindrical slab of apple was taken as a sample. The results of the experimental and numerical studies were compared. A hypothesis for the nature of tissue properties evolution after PEF treatment was proposed. This phenomenon was considered as a correlated percolation governed by two key processes: resealing of cells and moisture transfer processes inside the cellular structure. The breakage kinetics was shown to be very sensitive to the repetition times Δ t of the PEF treatment. Correlated percolation patterns were observed in a case when Δ t exceeded the characteristic time of the moisture transfer processes while random percolation patterns were observed in other cases. The long-term mode of the pulse repetition times in PEF treatment allows the experimental visualisation of macroscopic percolation channels in the sample. Considerable differences were observed between the damage kinetics at long and short repetition times both for experimental and simulation data.

Shear flocculation—kinetics of floc coalescence and breakage

The flocculation of ultrafine (size<10 μm) hydrophobic particles in the presence of a high shear field is known as shear flocculation. Most of the studies carried out to date are related to surface chemistry of the process. However, study of the shear flocculation process using mathematical models is quite limited. The present study uses a lumped population balance model (PBM) to understand the effect of different parameters affecting the kinetics of floc coalescence and breakage processes in shear flocculation. Results show that the flocculation rate decreases with increase in shear rate and the breakage rate is approximately proportional to the 0.70 power of the volume of the floc. High concentrations of sodium oleate reduce the floc strength. Modifiers like sodium carbonate and sodium silicate decrease the flocculation rate.

Simulation and experimental investigation of food material breakage using pulsed electric field treatment

We consider the simplified dielectric breakage model used for simulation of the kinetics of food material breakage under pulsed electric field (PEF) treatment. The model is based on an effective media approximation, which includes equations with the same morphology parameters as in percolation theory. The probability of a whole cell breakage by a pulse of t i duration is estimated on the basis of electroporation theory. We account for the bridging effect resulting from the deviations of the local conductivity near the selected cell from the average effective media conductivity. The most important feature of the proposed model is the existence of the “jamming” behaviour occurring sometimes in experimental observations of the biological tissue breakage. The different transitions corresponding to the “jamming” steps are identified. Experimental results were obtained for thin apple slices treated with electric pulses at field strengths E=0.2–2.2 kV cm −1, pulse durations t i=10–100 μs, pulse repetition times t=10–100 ms and the number of pulses n=1–100 000. The model gives results consistent in general with the experimental observations. We discuss the correlation between the degree of food material destruction, field strength, time of PEF treatment and power consumption.

Modeling of the particle breakage kinetics in the wet mills for the paper industry

To develop the physical properties of paper, it is necessary to prepare the cellulose fibres. This is done in wet mills or refiners. In these machines, the dilute fibre suspension is forced through a gap between two surfaces fitted with bars that move relatively to each other. Their fibre collection efficiency, together with the cyclic stresses, lead to particle breakage (fibre shortening), fibrillation and hydration. In this article, a model for the kinetics of particle breakage is proposed that links the weight-average fibre length to the energy per unit mass consumed by the solid phase and the average impact intensity. The experimental evidence shows the validity of the assumptions and supports this theory of physical comminution. The raw material–mill combination can be optimized and the shortening effect can be predicted under given mill conditions.

Kinetics of hydrogen bond breakage in the process of unfolding of ribonuclease A measured by pulsed hydrogen exchange.

A sensitive test for kinetic unfolding intermediates in ribonuclease A (EC 3.1.27.5) is performed under conditions where the enzyme unfolds slowly (10 degrees C, pH 8.0, 4.5 M guanidinium chloride). Exchange of peptide NH protons (2H-1H) is used to monitor structural opening of individual hydrogen bonds during unfolding, and kinetic models are developed for hydrogen exchange during the process of protein unfolding. The analysis indicates that the kinetic process of unfolding can be monitored by EX1 exchange (limited by the rate of opening) for ribonuclease A in these conditions. Of the 49 protons whose unfolding/exchange kinetics was measured, 47 have known hydrogen bond acceptor groups. To test whether exchange during unfolding follows the EX2 (base-catalyzed) or the EX1 (uncatalyzed) mechanism, unfolding/exchange was measured both at pH 8.0 and at pH 9.0. A few faster-exchanging protons were found that undergo exchange by both EX1 and EX2 processes, but the 43 slower-exchanging protons at pH 8 undergo exchange only by the EX1 mechanism, and they have closely similar rates. Thus, it is likely that all 49 protons undergo EX1 exchange at the same rate. The results indicate that a single rate-limiting step in unfolding breaks the entire network of peptide hydrogen bonds and causes the overall unfolding of ribonuclease A. The additional exchange observed for some protons that follows the EX2 mechanism probably results from equilibrium unfolding intermediates and will be discussed elsewhere.

Dissecting the Disulphide-coupled Folding Pathway of Bovine Pancreatic Trypsin Inhibitor: Forming the First Disulphide Bonds in Analogues of the Reduced Protein

The kinetics of disulphide bond formation and breakage have been measured in five analogues of the single-disulphide intermediates that occur in folding of bovine pancreatic trypsin inhibitor (BPTI), in which the cysteine residues not involved in disulphide bonds have been replaced by serine residues. Only a single disulphide bond can be made in each analogue, allowing the rates and equilibrium constants of the corresponding microscopic steps to be determined. These steps cannot be characterized in normal BPTI with six cysteine residues, as only the total overall rate of forming all 15 possible disulphide bonds can be measured.The intramolecular rate of forming each disulphide bond was found in the dithiol forms of reduced BPTI to be approximately proportional inversely to the size of the disulphide loop formed. On this basis, the 30-51 disulphide bond is not formed preferentially in the fully reduced protein, even though it predominates amongst the one-disulphide intermediates as a result of rapid intramolecular disulphide rearrangements. It is found to be more stable than the other one-disulphide intermediates because its disulphide is more stable to reduction than the others. This is undoubtedly due to the stable partially folded structure of (30-51), as the other intermediates are much less structured under folding conditions.Urea (8 M) uniformly decreased by a factor of 5 the effective concentrations of cysteine thiol groups in the analogues and in reduced BPTI, suggesting that reduced BPTI under normal refolding conditions is not an ideal random coil; any non-random conformations do not, however, favour formation of any specific disulphide bonds.Findings from these studies have led to further refinement and development of various approaches to measuring and interpreting the intramolecular rates of disulphide bond formation and breakage in proteins.

Influence of the nature of an organic solvent on the kinetic parameters of the thermal homolysis of five-coordinate complexes of alkylcobalt

By the spectrophotometric method in solvents of different natures a study was made of the kinetics of the homolytic breakage of the axial cobalt-carbon bond in five-coordinate complexes with dianionic tetradentate equatorial ligands. The negative values obtained for the activation entropies of the processes investigated are explained by the existence of interaction between the components of the primary radical pair and the solvent molecules. The kinetic parameters of the redox-interaction reactions of some macrocyclic complexes of cobalt(II) with chloroform and carbon tetrachloride were determined. A scheme in which the autocatalytic homolysis of the Co-CH3 bond in the presence of chlorine-containing solvents is proposed.

The Kinetics of Hydroxyl-Radical-Induced Strand Breakage of Hyaluronic Acid. A Pulse Radiolysis Study Using Conductometry and Laser-Light-Scattering

Hydroxyl radicals were generated radiolytically in N2O- and N2O/O2(4:1)-saturated aqueous solutions of hyaluronic acid. The hydroxyl radicals react rapidly with hyaluronic acid mainly by abstracting carbon-bound H atoms. As a consequence of subsequent free-radical reactions, chain breakage occurs the kinetics of which has been followed using the pulse radiolysis technique. In the absence of oxygen, strand breakage was followed by the change in conductivity induced by the release of cationic counterions condensed at the surface of hyaluronic acid which is a polyanion consisting of subunits of glucuronic acid alternating with N-acetyl-glucosamine. It appears that strand breakage is not due to one single first-order process, however, the contributions of the different components cannot be adequately resolved. At pH 7 the overall half-life is 1.4 ms, in both acid and basic solutions the rate of free-radical induced strand breakage is accelerated (at pH 4.8, t1/2 = 0.6 ms; at pH 10, t1/2 = 0.18 ms). In the absence of oxygen there is no effect of dose rate on the kinetics of strand breakage. In the presence of oxygen in addition to conductometric detection, strand breakage was also followed by changes in low-angle laser light-scattering. These two techniques are complementary in that in this system the conductometry requires high doses per pulse while the light-scattering technique is best operated in the low-dose range. In the presence of oxygen a pronounced dose-rate effect is observed, e.g. at pH 9.7 after a dose of 9.4 Gy the overall half-time is approx. 0.5 s, while after a dose of 6.6 Gy the half-time is approx. 0.23 s. Both the yield and the rate of strand breakage increase with increasing pH, e.g. at pH 7 G(strand breaks) = 0.7 x 10(-7) mol J-1 and at pH 10.4, 4.8 x 10(-7) mol J-7. The radiolytic yields of CO2, H2O2, organic hydroperoxides, O2.- and oxygen consumption have been determined in gamma-irradiated N2O/O2(4:1)-saturated solutions of both hyaluronic acid and beta-cyclodextrin.

The Kinetics of Radiation-induced Strand Breakage in Polynucleotides in the Presence of Oxygen: A Time-resolved Light-scattering Study

The time-resolved light-scattering changes of aqueous, aerated solutions of poly-C, poly-U and poly-A at pH 7.8, following pulse irradiation, have been studied as indices of strand break formation. With doses of 4-24 Gy/pulse a number of kinetically distinct components have been detected. For the poly-pyrimidines an immediate and fast reduction (tau 1/2 less than or equal to 50 microseconds) in light-scattering intensity (LSI), accounting for approximately 20% of the total LSI change, is followed by a much slower loss (k1 approximately 1.6 s-1) which constitutes their major LSI change. For poly-A a similar fast component is observed, present to an extent equivalent to the one noted with poly-C; it constitutes, however, over 50% of the purine polynucleotide's total response, with the remainder of the change being a slower loss (tau 1/2 approximately 0.09 s). Optical pulse radiolysis studies of poly-C and poly-U, in support of the LSI investigations, show that transient absorbances in a region assigned to base peroxyl radicals decay in a complex fashion, with some at a rate equivalent to that for the slow (major) component of LSI loss. These observations support a proposal that the rate-limiting step of major strand breakage for these polynucleotides, in the presence of oxygen, is a base peroxyl radical-mediated abstraction of a H-atom from an adjacent sugar moiety (Bothe et al. 1986), with the resulting sugar peroxyl radicals then leading to strand break formation at a rate equivalent to that for loss of the initial, fast LSI components. These latter processes are attributed to strand breaks arising from the direct interaction of .OH with the polynucleotide sugar phosphate backbone.

Mechanism of Cleavage of Apurinic Sites by 9-Aminoellipticine

We have studied the kinetics of breakage of apurinic (AP) sites by the intercalating agent 9-aminoellipticine using fluorimetric methods with single (ss)- and double (ds)-stranded apurinic DNA. In order to understand the chemical process, high performance liquid chromatography was used to follow the reaction kinetics with the apurinic oligonucleotide model T(AP)T. The unstable intermediate, which is responsible for the beta-elimination step, is a Schiff base resulting from the interaction of the amino group of the aromatic amine with the aldehyde function of the deoxyribose moiety (AP site). Fluorescence occurs simultaneously with the breakage of both ss and ds DNA and of the oligonucleotide and arises from the formation of a conjugated double bond on the Schiff base through the beta-elimination reaction. In optimal conditions, the second order rate constant for the fluorescence build up is 15 x 10(3) min-1 M-1 for ds DNA and 0.105 x 10(3) min-1 M-1 for T(AP)T. The ability of 9-aminoellipticine to induce fluorescence and breakage of ss DNA and T(AP)T shows that intercalation is not essential for this reaction to occur. Nevertheless, the greater rate constant with DNA suggests that stacking is an important parameter for the reaction of the aromatic amine with the AP site.

Investigation of the kinetics of breakage as a first step towards the assessment of the economics of ultra-fine grinding of a British low-rank coal : Woodburn, E T; Kalligeris-Skentzus, A Powder TechnolV53, N2, Dec 1987, p137–143

Investigation of the kinetics of breakage as a first step towards the assessment of the economics of ultra-fine grinding of a British low-rank coal : Woodburn, E T; Kalligeris-Skentzus, A Powder TechnolV53, N2, Dec 1987, p137–143

The investigation of the kinetics of breakage as a first step towards the assessment of the economics of ultra-fine grinding of a British low-rank coal

The characteristics of breakage of a low-rank, semi-bituminous British coal were investigated from batch grinding tests in a laboratory ball mill. For short periods of grinding, first-order breakage occurred, but as fine material built up in the mill, a slowing down of the rates of breakage was observed. The specific rate of breakage of particles smaller than 1400 μm was found to be a simple power function of size. The breakage distribution parameters were found to be a normalised function of size. The reduction in the rates of breakage was correlated in terms of the size distribution of particles in the mill with reasonable accuracy. Finally the removal of fine particles at regular intervals during the grinding restored the first-order kinetics.

Effect of cell-cycle position and dose on the kinetics of DNA double-strand breakage repair in X-irradiated Chinese hamster cells.

The kinetics of DNA double-strand breakage (d.s.b.) repair in X-irradiated Chinese hamster V79 cells were found to be affected by cell-cycle position. In mitotic cells, the repair kinetics were monophasic with a half-time value of about 32 min, whilst in G1, S, or asynchronous cultures, the kinetics were biphasic with half-time values of around 2.7 and 27 min. The repair of DNA single-strand breakage (s.s.b) was also shown to be slower in mitotic than in interphase cells. The DNA d.s.b. repair system, in both mitotic and interphase cells, showed no evidence of saturation within the X-ray dose range covered. The implications of these findings for the mechanism of DNA d.s.b. repair and for models of ionizing radiation action are discussed.

Some solutions for the kinetics of combined fracture and abrasion breakage

A phenomenological description of autogenous breakage of ores is accomplished by considering the simultaneous occurrence of fracture- and abrasion-like processes. A general differential equation is derived and solved for the batch abrasion phenomenon with and without fracture breakage of lumps within a screen size interval. For short grinding times, that is, for less than 50% of breakage, pure abrasion produces an almost first-order disappearance of the top size class material. A strong increase in the kinetic of disappearance occurs when the larger lumps reduce to the lower sieve size of the interval. Approximate solutions for the complete batch and continuous steady-state equations are developed by using a first-order fracture and the limiting first-order abrasion kinetics. In accordance with experimental findings, non-normalized B values with regard to particle size and bimodal distributions of progeny fragments are predicted by the model.

Analysis of the kinetics of coal breakage by wet grinding in the szego mill

This paper deals with the mechanism of breakage of Pittsburgh coal in the Szego mill, for the preparation of coal-grade 2 oil and coal-water slurries. Following a brief introduction to the mill and its operation, the kinetics of coal breakage is discussed. The non-first-order breakage in the mill is due to material and environment effects.A two-component mechanistic model, considering the coal in a size interval to be composed of crushed feed particles and flaky particles, is used to explain the breakage. The selection functions for both components in various ▪ size intervals are estimated from experimental data using an optimization technique. Finally, the variation of the selection functions with particle size is discussed.