Molecular finite-size effects in stochastic models of equilibrium chemical systems

The reaction-diffusion master equation (RDME) is a standard modelling approach for understanding stochastic and spatial chemical kinetics. An inherent assumption is that molecules are point-like. This assumption is fine for dilute systems however experiments show that reactions inside cells occur in non-dilute conditions, i.e., highly crowded conditions. I will here introduce the crowded reaction-diffusion master equation (cRDME) which takes into account volume exclusion effects on stochastic kinetics due to a finite molecular radius. I will discuss how its possible to obtain an exact closed form solution of the cRDME for a general chemical system in equilibrium conditions. This solution enables us to deduce that an increase in molecular crowding can (i) lead to deviations from the classical inverse square root law for the noise-strength; (ii) flip the skewness of the probability distribution from right to left-skewed; (iii) shift the equilibrium of bimolecular reactions so that more product molecules are formed; (iv) strongly modulate the Fano factors and coefficients of variation.