Theory of the NO+CO surface reaction model

We derive a pair approximation (PA) for the NO+CO model with instantaneous reactions. For both the triangular and square lattices, the PA, derived here using a simpler approach, yields a phase diagram with an active state for CO-fractions y in the interval y_1 < y < y_2, with a continuous (discontinuous) phase transition to a poisoned state at y_1 (y_2). This is in qualitative agreement with simulation for the triangular lattice, where our theory gives a rather accurate prediction for y_2. To obtain the correct phase diagram for the square lattice, i.e., no active state, we reformulate the PA using sublattices. The (formerly) active regime is then replaced by a poisoned state with broken symmetry (unequal sub- lattice coverages), as observed recently by Kortluke et al. [Chem. Phys. Lett. 275, 85 (1997)]. In contrast with their approach, in which the active state persists, although reduced in extent, we report here the first qualitatively correct theory of the NO+CO model on the square lattice. Surface diffusion of nitrogen can lead to an active state in this case. In one dimension, the PA predicts that diffusion is required for the existence of an active state.Comment: 15 pages, 9 figure

Oscillations and dynamics in a two-dimensional prey-predator system

Using Monte Carlo simulations we study two-dimensional prey-predator systems. Measuring the variance of densities of prey and predators on the triangular lattice and on the lattice with eight neighbours, we conclude that temporal oscillations of these densities vanish in the thermodynamic limit. This result suggests that such oscillations do not exist in two-dimensional models, at least when driven by local dynamics. Depending on the control parameter, the model could be either in an active or in an absorbing phase, which are separated by the critical point. The critical behaviour of this model is studied using the dynamical Monte Carlo method. This model has two dynamically nonsymmetric absorbing states. In principle both absorbing states can be used for the analysis of the critical point. However, dynamical simulations which start from the unstable absorbing state suffer from metastable-like effects, which sometimes renders the method inefficient.Comment: 7 eps figures, Phys.Rev.E - in pres

Maturation of membrane-bound hydrogenase of Alcaligenes eutrophus H16.

The formation of the catalytically active membrane-bound hydrogenase (MBH) of Alcaligenes eutrophus H16 requires the genes for the small and large subunits of the enzyme (hoxK and hoxG, respectively) and an accompanying set of accessory genes (C. Kortl ke, K. Horstmann, E. Schwartz, M. Rohde, R. Binsack, and B. Friedrich, J. Bacteriol. 174:6277-6289, 1992). Other genes located in the adjacent pleiotropic region are also required. In the absence of these genes, MBH is synthesized but is catalytically inactive. Immunological analyses revealed that cells containing active MBH produced the small and large subunits of the enzyme in two distinct conformations each; only one of each, presumably the immature form, occurred in cells devoid of MBH activity. The results suggest that the conversion of the two subunits into the catalytically active membrane-associated heterodimer depends on specific maturation processes mediated by hox genes