Effects of Molecular Crowding and Obstacles on a Target Search

We are reviewing diffusion-controlled target reactions, where an immobile target is surrounded by diffusing reactants. The reaction occurs between the target and one of the reactants. We summarize the conventional results for the decay of the target survival probability in the dilute limit. In the dilute limit, the diffusion of each reactant is considered as an independent event. Subsequently, we present recent results obtained at high reactant concentrations, where the migration of each reactant is influenced by the configurations of the other reactants, and the diffusion is correlated. Using lattice models, we demonstrate that the search for a target by surrounding reactants can be expedited by prohibiting simultaneous double occupancy of a site. This is achieved by blocking the migration of a reactant to a site that is already occupied by another reactant, resulting in a site-blocking effect. This effect proves advantageous for efficiently searching the target. We present a detailed analytical method to account for site-blocking effects in a simple case, instead of providing numerous examples.