Exact analytical solution of the Chemical Master Equation for the Finke-Watkzy model

Abstract

The Finke-Watkzy model is the reaction set consisting of autocatalysis, A + B --> 2B and the first order process A --> B. It has been widely used to describe phenomena as diverse as the formation of transition metal nanoparticles and protein misfolding and aggregation. It can also be regarded as a simple model for the spread of a non-fatal but incurable disease. The deterministic rate equations for this reaction set are easy to solve and the solution is used in the literature to fit experimental data. However, some applications of the Finke-Watkzy model may involve systems with a small number of molecules or individuals. In such cases, a stochastic description using a Chemical Master Equation or Gillespie's Stochastic Simulation Algorithm is more appropriate than a deterministic one. This is even more so because for this particular set of chemical reactions, the differences between deterministic and stochastic kinetics can be very significant. Here, we derive an analytical solution of the Chemical Master Equation for the Finke-Watkzy model. We consider both the original formulation of the model, where the reactions are assumed to be irreversible, and its generalization to the case of reversible reactions. For the former, we obtain analytical expressions for the time dependence of the probabilities of the number of A molecules. For the latter, we derive the corresponding steady-state probability distribution. Our findings may have implications for modeling the spread of epidemics and chemical reactions in living cells.