31 research outputs found
Switches induced by quorum sensing in a model of enzyme-loaded microparticles
Quorum sensing refers to the ability of bacteria and other single-celled organisms to respond to changes in cell density or number with population-wide changes in behaviour. Here, simulations were performed to investigate quorum sensing in groups of diffusively coupled enzyme microparticles using a well-characterized autocatalytic reaction which raises the pH of the medium: hydrolysis of urea by urease. The enzyme urease is found in both plants and microorganisms, and has been widely exploited in engineering processes. We demonstrate how increases in group size can be used to achieve a sigmoidal switch in pH at high enzyme loading, oscillations in pH at intermediate enzyme loading and a bistable, hysteretic switch at low enzyme loading. Thus, quorum sensing can be exploited to obtain different types of response in the same system, depending on the enzyme concentration. The implications for microorganisms in colonies are discussed, and the results could help in the design of synthetic quorum sensing for biotechnology applications such as drug delivery
Temporal Control of Gelation and Polymerization Fronts Driven by an Autocatalytic Enzyme Reaction
Chemical systems that remain kinetically dormant until activated have numerous applications in materials science. Herein we present a method for the control of gelation that exploits an inbuilt switch: the increase in pH after an induction period in the urease-catalyzed hydrolysis of urea was used to trigger the base-catalyzed Michael addition of a water-soluble trithiol to a polyethylene glycol diacrylate. The time to gelation (minutes to hours) was either preset through the initial concentrations or the reaction was initiated locally by a base, thus resulting in polymerization fronts that converted the mixture from a liquid into a gel (ca. 0.1â
mmâminâ1). The rate of hydrolytic degradation of the hydrogel depended on the initial concentrations, thus resulting in a gel lifetime of hours to months. In this way, temporal programming of gelation was possible under mild conditions by using the output of an autocatalytic enzyme reaction to drive both the polymerization and subsequent degradation of a hydrogel
Periodic nucleation of calcium phosphate in a stirred biocatalytic reaction
Highly ordered superstructures composed of inorganic nanoparticles appear in natural and synthetic systems, however the mechanisms of nonâequilibrium selfâorganization that may be involved are still poorly understood. Herein, we performed a kinetic investigation of the precipitation of calcium phosphate using a process widely found in microorganisms: the hydrolysis of urea by enzyme urease. With high initial ratio of calcium ion to phosphate, periodic precipitation was obtained accompanied by pH oscillations in a wellâstirred, closed reactor. We propose that an internal pHâregulated change in the concentration of phosphate ion is the driving force for periodicity. A simple model involving the biocatalytic reaction network coupled with burst nucleation of nanoparticles above a critical supersaturation reproduced key features of the experiments. These findings may provide insight to the selfâorganization of nanoparticles in biomineralization and improve design strategies of biomaterials for medical applications
Helical Turing patterns in the Lengyel-Epstein model in thin cylindrical layers
The formation of Turing patterns was investigated in thin cylindrical layers using the Lengyel-Epstein
model of the chlorine dioxide-iodine-malonic acid reaction. The influence of the width of the layer
W and the diameter D of the inner cylinder on the pattern with intrinsic wavelength l were
determined in simulations with initial random noise perturbations to the uniform state for W< l/2
and D l or lower. We show that the geometric constraints of the reaction domain may result in the
formation of helical Turing patterns with parameters that give stripes (b Âź 0.2) or spots (b Âź 0.37) in
two dimensions. For b Âź 0.2, the helices were composed of lamellae and defects were likely as the
diameter of the cylinder increased. With b Âź 0.37, the helices consisted of semi-cylinders and
the orientation of stripes on the outer surface (and hence winding number) increased with increasing
diameter until a new stripe appeared
Kinetics of the ureaâurease clock reaction with urease immobilized in hydrogel beads
Feedback driven by enzyme catalyzed reactions occurs widely in biology and has been well characterized in single celled organisms such as yeast. There are still few examples of robust enzyme oscillators in vitro that might be used to study nonlinear dynamical behavior. One of the simplest is the ureaâurease reaction that displays autocatalysis driven by the increase in pH accompanying the production of ammonia. A clock reaction was obtained from low to high pH in batch reactor and bistability and oscillations were reported in a continuous flow rector. However, the oscillations were found to be irreproducible and one contributing factor may be the lack of stability of the enzyme in solution at room temperature. Here, we investigated the effect of immobilizing urease in thiol-poly(ethylene glycol) acrylate (PEGDA) hydrogel beads, prepared using emulsion polymerization, on the ureaâurease reaction. The resultant mm-sized beads were found to reproduce the pH clock and, under the conditions employed here, the stability of the enzyme was increased from hours to days
Evolution of spiral and scroll waves of excitation in a mathematical model of ischaemic border zone
Abnormal electrical activity from the boundaries of ischemic cardiac tissue
is recognized as one of the major causes in generation of ischemia-reperfusion
arrhythmias. Here we present theoretical analysis of the waves of electrical
activity that can rise on the boundary of cardiac cell network upon its
recovery from ischaemia-like conditions. The main factors included in our
analysis are macroscopic gradients of the cell-to-cell coupling and cell
excitability and microscopic heterogeneity of individual cells. The interplay
between these factors allows one to explain how spirals form, drift together
with the moving boundary, get transiently pinned to local inhomogeneities, and
finally penetrate into the bulk of the well-coupled tissue where they reach
macroscopic scale. The asymptotic theory of the drift of spiral and scroll
waves based on response functions provides explanation of the drifts involved
in this mechanism, with the exception of effects due to the discreteness of
cardiac tissue. In particular, this asymptotic theory allows an extrapolation
of 2D events into 3D, which has shown that cells within the border zone can
give rise to 3D analogues of spirals, the scroll waves. When and if such scroll
waves escape into a better coupled tissue, they are likely to collapse due to
the positive filament tension. However, our simulations have shown that such
collapse of newly generated scrolls is not inevitable and that under certain
conditions filament tension becomes negative, leading to scroll filaments to
expand and multiply leading to a fibrillation-like state within small areas of
cardiac tissue.Comment: 26 pages, 13 figures, appendix and 2 movies, as accepted to PLoS ONE
2011/08/0
Evidence for Burgers' equation describing the untwisting of scroll rings
We study the dynamics of rotating scroll waves in three-dimensional excitable systems. Experiments are carried out with the 1,4-cyclohexanedione Belousov-Zhabotinsky reaction and wave patterns are measured using optical tomography. We create twisted scroll rings for which the rotation phase varies along their circular rotation backbone and measure the untwisting dynamics of the collapsing structures. Experimental data reveal the formation of an asymmetric, plateau-like phase profile with a growing region of leading phase and a shrinking region of lagging phase. The experimental data support a quantitative description in terms of a nonlinear diffusion equation
Reaction-diffusion hydrogels from urease enzyme particles for patterned coatings
The reaction and diffusion of small molecules is used to initiate the formation of protective polymeric layers, or biofilms, that attach cells to surfaces. Here, inspired by biofilm formation, we present a general method for the growth of hydrogels from urease enzyme-particles by combining production of ammonia with a pH-regulated polymerization reaction in solution. We show through experiments and simulations how the propagating basic front and thiol-acrylate polymerization were continuously maintained by the localized urease reaction in the presence of urea, resulting in hydrogel layers around the enzyme particles at surfaces, interfaces or in motion. The hydrogels adhere the enzyme-particles to surfaces and have a tunable growth rate of the order of 10âÂľmâminâ1 that depends on the size and spatial distribution of particles. This approach can be exploited to create enzyme-hydrogels or chemically patterned coatings for applications in biocatalytic flow reactors
Dispersion relations for the convective instability of an acidity front in Hele-Shaw cells
info:eu-repo/semantics/publishe