Some diffusion influenced reactions are better described by a distance dependent reactivity term rather than a boundary condition. Important examples are Forster quenching, electron transfer reactions and protein conformation dynamics effect on ligand binding to heme proteins. As the resulting equation is very difficult to solve analytically, we have suggested a perturbation expansion for the survival probability of this problem in terms of the ratio of diffusion to reaction rate (""""""""viscosity expansions""""""""). Hence good approximations are obtained in the limit where (in the above example) protein conformation relaxes fast compared to ligand binding.
