Phenazines as model low-midpoint potential electron shuttles for photosynthetic bioelectrochemical systems

Bioelectrochemical approaches for energy conversion rely on efficient wiring of natural electron transport chains to electrodes. However, state-of-the-art exogenous electron mediators give rise to significant energy losses and, in the case of living systems, long-term cytotoxicity. Here, we explored new selection criteria for exogenous electron mediation by examining phenazines as novel low-midpoint potential molecules for wiring the photosynthetic electron transport chain of the cyanobacterium Synechocystis sp. PCC 6803 to electrodes. We identified pyocyanin (PYO) as an effective cell-permeable phenazine that can harvest electrons from highly reducing points of photosynthesis. PYO-mediated photocurrents were observed to be 4-fold higher than mediator-free systems with an energetic gain of 200 mV compared to the common high-midpoint potential mediator 2,6-dichloro-1,4-benzoquinone (DCBQ). The low-midpoint potential of PYO led to O₂ reduction side-reactions, which competed significantly against photocurrent generation; the tuning of mediator concentration was important for outcompeting the side-reactions whilst avoiding acute cytotoxicity. DCBQ-mediated photocurrents were generally much higher but also decayed rapidly and were non-recoverable with fresh mediator addition. This suggests that the cells can acquire DCBQ-resistance over time. In contrast, PYO gave rise to steadier current enhancement despite the co-generation of undesirable reactive oxygen species, and PYO-exposed cells did not develop acquired resistance. Moreover, we demonstrated that the cyanobacteria can be genetically engineered to produce PYO endogenously to improve long-term prospects. Overall, this study established that energetic gains can be achieved via the use of low-potential phenazines in photosynthetic bioelectrochemical systems, and quantifies the factors and trade-offs that determine efficacious mediation in living bioelectrochemical systems.

Phenazines were explored as novel low-midpoint potential molecules for wiring cyanobacteria to electrodes.     

1,2,7,8‐Tetra‐O‐acetyl­australine

The title compound, C₁₆H₂₃NO₈, was obtained during studies of australine derivatives. The molecular stereochemistry is similar to that reported for other australine derivatives even though there is no intermolecular hydrogen bonding in this case.     

The pasting properties of sonicated waxy rice starch suspensions

The effect of sonication on the pasting properties of waxy rice starch solutions (5 wt%) was investigated. It has been found that the functionality of starch granules was significantly influenced by the length of sonication and the solution temperature. A comparison of the pasting behaviour showed that the peak and final viscosities of the starch dispersions sonicated at temperatures near the onset temperature of gelatinisation were lower than those of the non-sonicated dispersions. The particle size measurements showed that the size of the heated and sonicated granules were smaller than that of the heated non-sonicated starch granules. Scanning electron microscopy (SEM) observations showed that the starch granule surface was not affected by sonication, and the size exclusion chromatography did not show any reduction in the size of the starch molecules. Based on these observations, the change in the pasting behaviour is explained in terms of the solubilisation of the swollen starch granules and starch aggregates induced by sonication.     

An efficient multi-scale Poisson solver for the incompressible Navier–Stokes equations with immersed boundaries

The iterative-multi-scale-finite-volume (IMSFV) procedure is applied as an efficient solver for the pressure Poisson equation arising in numerical methods for the simulation of incompressible flows with the immersed-interface method (IIM). Motivated by the requirements of the specific IIM implementation, a modified version of the IMSFV algorithm is presented to allow the solution of problems, in which the varying coefficient of the elliptic equation (e.g. the permeability of the medium in the context of the simulation of flows in porous media) varies over several orders of magnitude or even becomes zero within the integration domain. Furthermore, a strategy is proposed to incorporate the iterative procedure needed by the IIM to converge out constraints at immersed boundaries into the iterative IMSFV cycle. No significant deterioration of performance of the IMSFV method is observed with respect to cases, in which no iterative improvement of the boundary conditions is considered.     

An Approach to Model Partially Premixed Turbulent Combustion withProbability Density Function (PDF) Methods

In turbulent combustion one distinguishes between premixed, non-premixed and partially premixed combustion. While laminar flamelet models proved to be extremely valuable for a wide range of non-premixed flame simulations, similar approaches are more problematic in the partially premixed regime. Here the laminar flamelet concept for non-premixed turbulent combustion simulations is generalized for the partially premixed regime. Similar as in the unsteady flamelet approach, the joint statistics of a progress variable, mixture fraction and scalar dissipation rate is used to obtain the joint statistics of the compositions from pre-computed flame tables. The required distribution is computed with a joint PDF method and the main differences between the new approach and previous ones, are the pre-computed tables and the way the evolution of the progress variable is calculated. Instead of evolving 1D flamelets, steady 2D solutions of burning flamelets propagating into unburned mixtures with varying mixture fraction are considered. The location of a fluid particle in this 2D laminar flame is defined by its mixture fraction and a burning time, which are modeled for each computational particle used in the PDF method. Numerical experiments of a turbulent lifted diffusion flame and a premixed Bunsen flame demonstrate that this approach can be employed for a wide range of applications. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Stochastic Particle Method for Nonlinear Hyperbolic Problems

In this paper, we present a new method based on stochastic particles, which allows us to compute solutions of a system of nonlinear transport equations arising in the modeling of immiscible displacement in porous pedia. In this approach, we use different particles for different phases and move them according to the stochastic rules for which the probability density function depends on the spatial distribution of the particles. Our motivation for such a method is a Lagrangian modeling framework in which one can describe certain physical phenomena more naturally than in an Eulerian framework. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Mechanistic studies of Hangman salophen-mediated activation of O-O bonds

Stopped-flow kinetic studies of a HSX-Mn-SalophOMe (1) catalyst provide spectroscopic evidence for the direct generation of a manganese(V) oxo salophen from a manganese(III) perbenzoate. The O-O bond heterolysis reaction that produces the oxo is not facilitated by intramolecular proton transfer from the acid hanging group of the HSX platform. Instead, the hanging group stabilizes the catalyst against oxidative degradation and, consistent with recent predictions of theory, is geometrically matched to promote the end-on coordination of a H2O2 substrate prior to its oxidation at the manganese(V) oxo center.     

Joint PDF Closure of Turbulent Premixed Flames

In this paper, a novel model for turbulent premixed combustion in the corrugated flamelet regime is presented, which is based on transporting a joint probability density function (PDF) of velocity, turbulence frequency and a scalar vector. Due to the high dimensionality of the corresponding sample space, the PDF equation is solved with a Monte-Carlo method, where individual fluid elements are represented by computational particles. Unlike in most other PDF methods, the source term not only describes reaction rates, but accounts for “ignition” of reactive unburnt fluid elements due to propagating embedded quasi laminar flames within a turbulent flame brush. Unperturbed embedded flame structures and a constant laminar flame speed (as expected in the corrugated flamelet regime) are assumed. The probability for an individual particle to “ignite” during a time step is calculated based on an estimate of the mean flame surface density (FSD), latter gets transported by the PDF method. Whereas this model concept has recently been published [21], here, a new model to account for local production and dissipation of the FSD is proposed. The following particle properties are introduced: a flag indicating whether a particle represents the unburnt mixture; a flame residence time, which allows to resolve the embedded quasi laminar flame structure; and a flag indicating whether the flame residence time lies within a specified range. Latter is used to transport the FSD, but to account for flame stretching, curvature effects, collapse and cusp formation, a mixing model for the residence time is employed. The same mixing model also accounts for molecular mixing of the products with a co-flow. To validate the proposed PDF model, simulation results of three piloted methane-air Bunsen flames are compared with experimental data and very good agreement is observed.     

A Multigrid Algorithm for Hybrid Joint PDF Simulations in Complex Geometries

In hybrid joint probability density function (joint PDF) algorithms for turbulent reactive flows the equations for the mean flow discretized with a classical grid based method (e.g. finite volume methods (FVM)) are solved together with a Monte Carlo (particle) method for the joint velocity composition PDF. When applied for complex geometries, the solution strategy for such methods which aims at obtaining a converged solution of the coupled problem on a sufficiently fine grid becomes very important. This paper describes one important aspect of this solution strategy, i.e. multigrid computing, which is well known to be very efficient for computing numerical solutions on fine grids. Two sets of grid based variables are involved: cell-centered variables from the FVM and node-centered variables, which denote the moments of the PDF extracted from the particle fields. Starting from a given multiblock grid environment first a new (refined or coarsened) grid is defined retaining the grid quality. The projection and prolongation operators are defined for the two sets of variables. In this new grid environment the particles are redistributed. The effectiveness of the multigrid algorithm is demonstrated. Compared to solely solving on the finest grid, convergence can be reached about one order of magnitude faster when using the multigrid algorithm in three stages. Computation time used for projection or prolongation is negligible. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)