Search

We report that anion-π and cation-π interactions can occur on the same aromatic surface. Interactions of this type are referred to as ion pair-π interactions. Their existence, nature, and significance are elaborated in the context of spectral tuning, ion binding in solution, and activation of cell-penetrating peptides. The origin of spectral tuning by ion pair-π interactions is unraveled with energy-minimized excited-state structures: The solvent- and pH-independent red shift of absorption and emission of push-pull fluorophores originates from antiparallel ion pair-π attraction to their polarized excited state. In contrast, the complementary parallel ion pair-π repulsion is spectroscopically irrelevant, in part because of charge neutralization by intriguing proton and electron transfers on excited push-pull surfaces. With time-resolved fluorescence measurements, very important differences between antiparallel and parallel ion pair-π interactions are identified and quantitatively dissected from interference by aggregation and ion pair dissociation. Contributions from hydrogen bonding, proton transfer, π-π interactions, chromophore twisting, ion pairing, and self-assembly are systematically addressed and eliminated by concise structural modifications. Ion-exchange studies in solution, activation of cell-penetrating peptides in vesicles, and computational analysis all imply that the situation in the ground state is complementary to spectral tuning in the excited state; i.e., parallel rather than antiparallel ion pair-π interactions are preferred, despite repulsion from the push-pull dipole. The overall quite complete picture of ion pair-π interactions provided by these remarkably coherent yet complex results is expected to attract attention throughout the multiple disciplines of chemistry involved.

Backscattered Raman optical activity (ROA) spectra are measured for Δ- and Λ-tris-(ethylenediamine)rhodium(III) chloride in aqueous solution. In addition, the spectra of the four possible conformers in the Λ configuration are investigated by ab initio calculations. The Λ(δδδ) conformer is in best agreement with experimental spectra and examined in more details. The two most stable conformers according to the calculations are not compatible with the experimental ROA spectrum. Insights into the origin of observed band intensities are obtained by means of group coupling matrices. The influence of the first solvation shell is explored via an ab initio molecular dynamics simulation. Taking explicit solvent molecules into account further improves the agreement between calculation and experiment. Analysis of selected normal modes using group coupling matrices shows that solvent molecules lead to normal mode rotation and thus contribute to the ROA intensity, whereas the contribution of the Rh can be neglected.

Herein, we address the question whether anion-π and cation-π interactions can take place simultaneously on the same aromatic surface. Covalently positioned carboxylate-guanidinium pairs on the surface of 4-amino-1,8-naphthalimides are used as an example to explore push-pull chromophores as privileged platforms for such "ion pair-π" interactions. In antiparallel orientation with respect to the push-pull dipole, a bathochromic effect is observed. A red shift of 41 nm found in the least polar solvent is in good agreement with the 70 nm expected from theoretical calculations of ground and excited states. Decreasing shifts with solvent polarity, protonation, aggregation, and parallel carboxylate-guanidinium pairs imply that the intramolecular Stark effect from antiparallel ion pair-π interactions exceeds solvatochromic effects by far. Theoretical studies indicate that carboxylate-guanidinium pairs can also interact with the surfaces of π-acidic naphthalenediimides and π-basic pyrenes., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

An individual-based model describing the dynamics of one type of annual plants is presented. We use Monte Carlo simulations where each plant has its own history and the interactions among plants are between nearest neighbours. The character of the interaction (positive or negative) depends on local conditions. The plants compete for two external resources-water and light. The amount of water and/or light a plant receives depends on the external factor but also on local arrangement. Survival, growth and seed production of plants are determined by how well their demands for the resources are met. The survival and seeds production tests have a probabilistic character, which makes the dynamics more realistic than by using a deterministic approach. There is a non-linear coupling between the external supplies. Water evaporates from the soil at a rate depending on constant evaporation rate, local conditions and the amount of light. We examine the dynamics of the plant population along two environmental gradients, allowing also for surplus of water and/or light. We show that the largest number of plants is when the demands for both resources are equal to the supplies. We estimate also the role of evaporation and we find that it depends on the situation. It could be negative, but sometimes it has a positive character. We show that the link between the type of interaction (positive or negative) and external conditions has a complex character. In general in favourable environment plants have a stronger tendency for competitive interactions, leading to mostly isolated plants. When the conditions are getting more difficult, cooperation becomes the dominant type of interactions and the plants grow in clusters. The type of plants-sun-loving or shade tolerating, plays also an important role., (© 2013 Elsevier Ltd. All rights reserved.)

The polyaromatic terdentate ligand 6-(azaindol-1-yl)-2,2′-bipyridine (L7) combines one 5-membered chelate ring with a fused 6-membered chelate ring. It is designed to provide complexation properties intermediate between 2,2′;6′,2″-terpyridine (L1) (two fused 5-membered chelate rings) and 2,6-bis(azaindol-1-yl)pyridine (L6) (two fused 6-membered chelate rings). In polar organic solvents, L7 displays remarkable affinities for the successive fixation of two small univalent cations M = H+ or Li+, leading to stable [M(m)(L7)]m+ (m = 1–2) complexes. Upon reaction with M = Mg2+ or Zn2+ cations, the large positive charge densities borne by the metals result in the successive cooperative complexation of two ligands to give [M(L7)n]n+ (n = 1–2). For small Sc3+, unavoidable traces of water favor the formation of the protonated ligand at millimolar concentrations in acetonitrile, but the use of larger Y3+ cations leads to [Y(L7)n]n+ (n = 1, 2), for which stability constants of log(β(1,1)(Y,L7)) = 2.9(5) and log(β(1,2)(Y,L7)) = 5.3(4) are estimated. The complexation behaviors are supported by speciations in solution, thermodynamic analyses, and solution and solid-state structures.

Objectives: (1) To determine the surface roughness and material loss of a Y-TZP ceramic before and after tribochemical grit blasting and (2) to characterize the changes in elemental surface composition and the phase transformations after tribochemical treatment., Methods: Machined bar shaped specimens (Zeno, Wieland) were subdivided into three groups. After grit blasting for 10, 20 and 30s respectively, half of the specimens of each group were ultrasonically cleaned in ethanol for 10min. The other half was rinsed with a water spray. Surface roughness was measured using an electro-mechanical profilometer. The elemental composition of the samples was obtained by energy dispersive X-ray spectroscopy (EDS). X-ray diffraction (XRD) was used for phase transformations determination., Results: The median Ra increased significantly from 0.24 to 0.32-0.38μm after grit blasting. Augmentations were also noted for R(max), R(v) and R(p). The highest roughness parameters were, obtained for water sprayed specimens and samples abraded for 30s. Loss of material ranged between 1 and 3μm for 30s grit blasting. Tetragonal and cubic phases were identified in 'as machined' specimens. Grit blasting resulted in domain switching and lattice deformations. The elemental composition comprised Si and Al. The duration of grit blasting did not significantly, influence the atomic percentages of Si or Al. Significantly lower values for both Si and Al were noted, after ultrasonic cleaning., Significance: Grit blasting with CoJet™ Sand resulted in an increase of surface roughness, a removal of maximum 3μm of material and coated the surface with submicron silica and alumina particles., (Copyright © 2010 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.)

The definition of a nonequilibrium temperature through generalized fluctuation-dissipation relations relies on the independence of the fluctuation-dissipation temperature from the observable considered. We argue that this observable independence is deeply related to the uniformity of the phase-space probability distribution on the hypersurfaces of constant energy. This property is shown explicitly on three different stochastic models, where observable dependence of the fluctuation-dissipation temperature arises only when the uniformity of the phase-space distribution is broken. The first model is an energy transport model on a ring, with biased local transfer rules. In the second model, defined on a fully connected geometry, energy is exchanged with two heat baths at different temperatures, breaking the uniformity of the phase-space distribution. Finally, in the last model, the system is connected to a zero temperature reservoir, and preserves the uniformity of the phase-space distribution in the relaxation regime, leading to an observable-independent temperature.

On general grounds, a nonequilibrium temperature can be consistently defined from generalized fluctuation-dissipation relations only if it is independent of the observable considered. We argue that the dependence on the choice of observable generically occurs when the phase-space probability distribution is nonuniform on constant energy shells. We relate quantitatively this observable dependence to a fundamental characteristics of nonequilibrium systems, namely, the Shannon entropy difference with respect to the equilibrium state with the same energy. This relation is illustrated on a mean-field model in contact with two heat baths at different temperatures.

The role of the selection pressure and mutation amplitude on the behavior of a single-species population evolving on a two-dimensional lattice, in a periodically changing environment, is studied both analytically and numerically. The mean-field level of description allows one to highlight the delicate interplay between the different time-scale processes in the resulting complex dynamics of the system. We clarify the influence of the amplitude and period of the environmental changes on the critical value of the selection pressure corresponding to a phase-transition "extinct-alive" of the population. However, the intrinsic stochasticity and the dynamically-built in correlations among the individuals, as well as the role of the mutation-induced variety in population's evolution are not appropriately accounted for. A more refined level of description, which is an individual-based one, has to be considered. The inherent fluctuations do not destroy the phase transition "extinct-alive," and the mutation amplitude is strongly influencing the value of the critical selection pressure. The phase diagram in the plane of the population's parameters-selection and mutation are discussed as a function of the environmental variation characteristics. The differences between a smooth variation of the environment and an abrupt, catastrophic change are also addressed.

Considering a broad class of steady-state nonequilibrium systems for which some additive quantities are conserved by the dynamics, we introduce from a statistical approach intensive thermodynamic parameters (ITPs) conjugated to the conserved quantities. This definition does not require any detailed balance relation to be fulfilled. Rather, the system must satisfy a general additivity property, which holds in most of the models usually considered in the literature, including those described by a matrix product ansatz with finite matrices. The main property of these ITPs is to take equal values in two subsystems, making them a powerful tool to describe nonequilibrium phase coexistence, as illustrated on different models. We finally discuss the issue of the equalization of ITPs when two different systems are put into contact. This issue is closely related to the possibility of measuring the ITPs using a small auxiliary system, in the same way as temperature is measured with a thermometer, and points at one of the major difficulties of nonequilibrium statistical mechanics. In addition, an efficient alternative determination, based on the measure of fluctuations, is also proposed and illustrated.

We study analytically the emergence of spontaneous collective motion within large bidimensional groups of self-propelled particles with noisy local interactions, a schematic model for assemblies of biological organisms. As a central result, we derive from the individual dynamics the hydrodynamic equations for the density and velocity fields, thus giving a microscopic foundation to the phenomenological equations used in previous approaches. A homogeneous spontaneous motion emerges below a transition line in the noise-density plane. Yet, this state is shown to be unstable against spatial perturbations, suggesting that more complicated structures should eventually appear.

We show that intensive thermodynamic parameters associated to additive conserved quantities can be naturally defined from a statistical approach in far-from-equilibrium steady-state systems, under few assumptions, and without any detailed balance requirement. It may apply, e.g., to dissipative systems such as granular gases where volume or mass is still conserved or to systems with periodic boundary conditions where fluxes of conserved quantities are present. We emphasize the usefulness of this concept to characterize the coexistence of different nonequilibrium phases and discuss the influence of the contact between two different systems, in relation with measurement issues.

We present a lattice model of a system of predators of five kinds, competing for prey. The predators are grouped in packs and characterized by two parameters-the energy spent on hunting and energy gained by the kill. The success of hunting depends on the actual competition among predators found near a prey. We determine via Monte Carlo simulations the numbers of predators of each kind as a function of time and the distribution of the size of their packs. We show that the ratio of the energy spent by the competing predators determines their fate. The energy gain plays only a secondary role. We show also that the system self-organizes itself into groups of predators living in well defined packs, which size depends on the energy spent. The most preferred size dependence on the energy spent follows a very simple power law. We present also a mean-field-type approach to the problem and we discuss the differences in the results obtained by the two methods, showing in particular, that the simulation approach produces more reliable results.

The hydrodynamic description of probabilistic ballistic annihilation, for which no conservation laws hold, is an intricate problem with hard spherelike dynamics for which no exact solution exists. We consequently focus on simplified approaches, the Maxwell and very-hard-particle (VHP) models, which allows us to compute analytically upper and lower bounds for several quantities. The purpose is to test the possibility of describing such a far from equilibrium dynamics with simplified kinetic models. The motivation is also in turn to assess the relevance of some singular features appearing within the original model and the approximations invoked to study it. The scaling exponents are first obtained from the (simplified) Boltzmann equation, and are confronted against direct Monte Carlo simulations. Then, the Chapman-Enskog method is used to obtain constitutive relations and transport coefficients. The corresponding Navier-Stokes equations for the hydrodynamic fields are derived for both Maxwell and VHP models. We finally perform a linear stability analysis around the homogeneous solution, which illustrates the importance of dissipation in the possible development of spatial inhomogeneities.

We study a class of nonequilibrium lattice models describing local redistributions of a globally conserved quantity, which is interpreted as an energy. A particular subclass can be solved exactly, allowing us to define a statistical temperature T(th) along the same lines as in the equilibrium micro-canonical ensemble. We compute the response function and find that when the fluctuation-dissipation relation is linear, the slope T(FD)(-1) of this relation differs from the inverse temperature T(th)(-1). We argue that T(th) is physically more relevant than T(FD), since in the steady-state regime, it takes equal values in two subsystems of a large isolated system. Finally, a numerical renormalization group procedure suggests that all models within the class behave similarly at a coarse-grained level, leading to a parameter that describes the deviation from equilibrium. Quantitative predictions concerning this parameter are obtained within a mean-field framework.

Acquisition of tandem mass spectra from peptides or other analytes deposited on non-conductive membranes is inhibited on instruments combining matrix-assisted laser desorption/ionization with tandem time-of-flight analyzers (MALDI-TOF/TOF) due to a charging effect. A thin layer of gold renders the membrane conductive. This allows adequate data acquisition on MALDI-TOF/TOF systems. Therefore, this methodology extends the capacity of the molecular scanner concept to tandem mass spectrometry.

We study a class of nonequilibrium lattice models which describe local redistributions of a globally conserved energy. A particular subclass can be solved analytically, allowing us to define a temperature T(th) along the same lines as in the equilibrium microcanonical ensemble. The fluctuation-dissipation relation is explicitly found to be linear, but its slope differs from the inverse temperature T(-1)(th). A numerical renormalization group procedure suggests that, at a coarse-grained level, all models behave similarly, leading to a two-parameter description of their macroscopic properties.

We consider a dilute gas of hard spheres in dimension d> or =2 that upon collision either annihilate with probability p or undergo an elastic scattering with probability 1-p . For such a system neither mass, momentum, nor kinetic energy is a conserved quantity. We establish the hydrodynamic equations from the Boltzmann equation description. Within the Chapman-Enskog scheme, we determine the transport coefficients up to Navier-Stokes order, and give the closed set of equations for the hydrodynamic fields chosen for the above coarse-grained description (density, momentum, and kinetic temperature). Linear stability analysis is performed, and the conditions of stability for the local fields are discussed.

We study a model of a multispecies ecosystem described by Lotka-Volterra-like equations. Interactions among species form a network whose evolution is determined by the dynamics of the model. Numerical simulations show power-law distribution of intervals between extinctions, but only for ecosystems with sufficient variability of species and with networks of connectivity above certain threshold that is very close to the percolation threshold of the network. The effect of slow environmental changes on extinction dynamics, degree distribution of the network of interspecies interactions, and some emergent properties of our model are also examined.

Using a Monte Carlo approach we study the role of inheritance and natural selection in the dynamics of populations. We show that a population subject to inheritance has a much better chance of survival in a given condition than a population where new generations do not inherit genomes of their parents. The dependence of the survival chance on such factors as selection pressure, fecundity, or carrying capacity of the system is much stronger when selection and inheritance are present. We demonstrate, in accordance with biological observations, that in certain conditions evolution can save a population which would perish without it.

We investigate the problem of ballistically controlled reactions where particles either annihilate upon collision with probability p, or undergo an elastic shock with probability 1-p. Restricting to homogeneous systems, we provide in the scaling regime that emerges in the long time limit, analytical expressions for the exponents describing the time decay of the density and the root-mean-square velocity, as continuous functions of the probability p and of a parameter related to the dissipation of energy. We work at the level of molecular chaos (nonlinear Boltzmann equation), and using a systematic Sonine polynomials expansion of the velocity distribution, we obtain in arbitrary dimension the first non-Gaussian correction and the corresponding expressions for the decay exponents. We implement Monte Carlo simulations in two dimensions, which are in excellent agreement with our analytical predictions. For p<1, numerical simulations lead to the conjecture that unlike for pure annihilation (p=1), the velocity distribution becomes universal, i.e., does not depend on the initial conditions.

We study a solid-on-solid (SOS) model whose dynamics is inspired by recent studies of the synchronization transition in coupled map lattices (CML). The synchronization of CML is thus related with a depinning of interface from a binding wall. Critical behavior of our SOS model depends on a specific form of binding (i.e., transition rates of the dynamics). For an exponentially decaying binding the depinning belongs to the directed percolation universality class. Other types of depinning, including the one with a line of critical points, are observed for a power-law binding.

Recently some two-dimensional models with double symmetric absorbing states were shown to share the same critical behavior that was called the voter universality class. We show that, for an absorbing-states Potts model with finite but further than nearest-neighbor range of interactions, the critical point is split into two critical points: one of the Ising type and the other of the directed percolation universality class. Similar splitting takes place in the three-dimensional nearest-neighbor model.

We use spreading dynamics to study the synchronization transition (ST) of one-dimensional coupled map lattices (CML's). Recently, Baroni et al. [Phys. Rev. E 63, 036226 (2001)] have shown that the ST belongs to the directed percolation (DP) universality class for discontinuous CML's. This was confirmed by accurate numerical simulations for the Bernoulli map by Ahlers and Pikovsky [Phys. Rev. Lett. 88, 254101 (2002)]. Spreading dynamics confirms such an identification only for random synchronized states. For homogeneous synchronized states the spreading exponents eta and delta are different from the DP exponents but their sum equals the corresponding sum of the DP exponents. Such a relation is typical of models with infinitely many absorbing states. Moreover, we calculate the spreading exponents for the tent map for which the ST belongs to the bounded Kardar-Parisi-Zhang (BKPZ) universality class. The estimation of spreading exponents for random synchronized states is consistent with the hyperscaling relation, while it is inconsistent for the homogeneous ones. Finally, we examine the asymmetric tent map. For small asymmetry the ST remains of the BKPZ type. However, for large asymmetry a different critical behavior appears, with exponents being relatively close to those for DP.

The problem of ballistic annihilation for a spatially homogeneous system is revisited within Boltzmann's kinetic theory in two and three dimensions. Analytical results are derived for the time evolution of the particle density for some isotropic discrete bimodal velocity modulus distributions. According to the allowed values of the velocity modulus, different behaviors are obtained: power law decay with nonuniversal exponents depending continuously upon the ratio of the two velocities, or exponential decay. When one of the two velocities is equal to zero, the model describes the problem of ballistic annihilation in the presence of static traps. The analytical predictions are shown to be in agreement with the results of two-dimensional molecular dynamics simulations.

The problem of the steady-state velocity distribution in a driven inelastic Maxwell model of shaken granular material is revisited. Numerical solution of the master equation and analytical arguments show that the model has bilateral exponential velocity tails [P(v) approximately e(-|v|/sqrt[D])], where D is the amplitude of the noise. Previous study of this model predicted Gaussian tails [P(v) approximately e(-av(2))].

The problem of ballistically controlled annihilation is revisited for general initial velocity distributions and an arbitrary dimension. An analytical derivation of the hierarchy equations obeyed by the reduced distributions is given, and a scaling analysis of the corresponding spatially homogeneous system is performed. This approach points to the relevance of the nonlinear Boltzmann equation for dimensions larger than 1 and provides expressions for the exponents describing the decay of the particle density n(t) proportional, variant t(-xi) and the root-mean-square velocity v proportional, variant t(-gamma) in terms of a parameter related to the dissipation of kinetic energy. The Boltzmann equation is then solved perturbatively within a systematic expansion in Sonine polynomials. Analytical expressions for the exponents xi and gamma are obtained in arbitrary dimension as a function of the parameter mu characterizing the small velocity behavior of the initial velocity distribution. Moreover, the leading non-Gaussian corrections to the scaled velocity distribution are computed. These expressions for the scaling exponents are in good agreement with the values reported in the literature for continuous velocity distributions in d=1. For the two-dimensional case, we implement Monte Carlo and molecular dynamics simulations that turn out to be in excellent agreement with the analytical predictions.

Recently van der Meer et al. studied the breakdown of a granular cluster [Phys. Rev. Lett. 88, 174302 (2002)]. We reexamine this problem using an urn model, which takes into account fluctuations and finite-size effects. General arguments are given for the absence of a continuous transition when the number of urns (compartments) is greater than two. Monte Carlo simulations show that the lifetime of a cluster tau diverges at the limits of stability as tau approximately N(1/3), where N is the number of balls. After the breakdown, depending on the dynamical rules of our urn model, either normal or anomalous diffusion of the cluster takes place.

We present an individual-based model of a population that lives in a changing environment. The individuals forming the population are subject to mutations and selection pressure. Using Monte Carlo simulations we have shown that, depending on the values of the mutation rate and selection, the population may reach either an active phase (it will survive) or an absorbing phase (it will become extinct). We have determined that the transition between the two states (phases) is continuous. We have shown that when the selection is weaker the population lives in all available space, while if the selection is stronger, it will move to the regions where the living conditions are better, avoiding those with more difficult conditions. The dependence of the mean time to extinction on the rate of mutations has been determined and discussed.

We introduce an urn model that describes spatial separation of sand. In this dynamical model, in a certain range of parameters spontaneous symmetry breaking takes place and equipartitioning of sand into two compartments is broken. The steady-state equation for an order parameter, a critical line, and the tricritical point on the phase diagram are found exactly. The master equation and the first-passage problem for the model are solved numerically and the results are used to locate first-order transitions. Exponential divergence of a certain characteristic time shows that the model can also exhibit very strong metastability. In certain cases characteristic time diverges as N(z), where N is the number of balls and z=1 / 2 (critical line), 2 / 3 (tricritical point), or 1 / 3 (limits of stability).

The critical properties of a simple prey-predator model are revisited. For some values of the control parameters, the model exhibits a line of directed percolationlike transitions to a single absorbing state. For other values of the control parameters one finds a second line of continuous transitions toward an infinite number of absorbing states, and the corresponding steady-state exponents are mean-field-like. The critical behavior of the special point T (bicritical point), where the two transition lines meet, belongs to a different universality class. A particular strategy for preparing the initial states used for the dynamical Monte Carlo method is devised to correctly describe the physics of the system near the second transition line. Relationships with a forest fire model with immunization are also discussed.

A coarse grained description of a two-dimensional prey-predator system is given in terms of a simple three-state lattice model containing two control parameters: the spreading rates of prey and predator. The properties of the model are investigated by dynamical mean-field approximations and extensive numerical simulations. It is shown that the stationary state phase diagram is divided into two phases: a pure prey phase and a coexistence phase of prey and predator in which temporal and spatial oscillations can be present. Besides the usual directed percolationlike transition, the system exhibits an unexpected, different type of transition to the prey absorbing phase. The passage from the oscillatory domain to the nonoscillatory domain of the coexistence phase is described as a crossover phenomena, which persists even in the infinite size limit. The importance of finite size effects are discussed, and scaling relations between different quantities are established. Finally, physical arguments, based on the spatial structure of the model, are given to explain the underlying mechanism leading to local and global oscillations.

We propose a lattice model of two populations, predators and prey. The model is solved via Monte Carlo simulations. Each species moves randomly on the lattice and can live only a certain time without eating. The lattice cells are either grass (eaten by prey) or tree (giving cover for prey). Each animal has a reserve of food that is increased by eating (prey or grass) and decreased after each Monte Carlo step. To breed, a pair of animals must be adjacent and have a certain minimum of food supply. The number of offspring produced depends on the number of available empty sites. We show that such a predator-prey system may finally reach one of the following three steady states: coexisting, with predators and prey; pure prey; or an empty one, in which both populations become extinct. We demonstrate that the probability of arriving at one of the above states depends on the initial densities of the prey and predator populations, the amount of cover, and the way it is spatially distributed.

The emergency of mutual cooperation is studied in a spatially extended evolutionary prisoner's dilemma game in which the players are located on the sites of cubic lattices for dimensions d=1, 2, and 3. Each player can choose one of the three following strategies: cooperation (C), defection (D) or "tit for tat" (T). During the evolutionary process the randomly chosen players adopt one of their neighboring strategies if the chosen neighbor has a higher payoff. Moreover, an external constraint imposes that the players always cooperate with probability p. The stationary state phase diagram is computed by both using generalized mean-field approximations and Monte Carlo simulations. Nonequilibrium second-order phase transitions associated with the extinction of one of the possible strategies are found and the corresponding critical exponents belong to the directed percolation universality class. It is shown that externally forcing the collaboration does not always produce the desired result.

Intraarticular therapy with corticosteroids can cause systemic effects such as decreased concentration of plasma cortisol, but whether this might place a patient at risk from stress induced acute adrenal failure is not known. We describe a patient who presented with lethargy, hyponatremia, and then with acute abdomen. The diagnosis of acute adrenal crisis was related to suppression of the hypothalamic-pituitary-adrenal axis by intraarticular use of corticosteroid. This was confirmed by a low basal cortisol concentration and by a short Synacthen test that elicited an increase in plasma cortisol concentration from 36 to 481 nmol/l. Within 24 h of receiving 37.5 mg of hydrocortisone, the patient rapidly improved.

We report a case of severe hypoglycaemia following co-trimoxazole therapy. An 88-year-old woman was admitted with urinary tract infection and treated with co-trimoxazole (960 mg bid). Seven days after initiation of the treatment she became comatose. Blood sugar was 1.3 mmol/l and C-peptide at the upper limit of normal range. Glucose infusion restored normal consciousness and no hypoglycaemia recurred after interruption of co-trimoxazole therapy. Advanced aged was the only risk factor identified. Other risk factors described in previous case reports are renal failure, poor nutritional state and high doses of co-trimoxazole.