The role of fixed and mobile buffers in the kinetics of proton movement.

We derive a simple expression for the effective diffusion coefficient of protons in Fick's second law, Deff, when both spatially fixed, HF, and mobile, HM, buffers are present. These buffers are present at moderately high concentrations ([Ftot], [Mtot] greater than 1 mM) in most biological systems. We consider only the case where the protonation reactions remain at equilibrium during the diffusion process. When the pH is to the alkaline side of the pK values of the fixed and mobile buffers ([H+] less than KF, KM), the effective diffusion coefficient of protons in Ficks second law is: (Formula: see text) where DH is the diffusion coefficient of the protons free in the aqueous phase and DHM is the diffusion coefficient of the mobile buffer. The equation illustrates three features of diffusion in a buffered system. Firstly, the effective diffusion coefficient of protons is always lower than the diffusion coefficient of free protons. Secondly, increasing the concentration of fixed buffers always decreases Deff. Thirdly, increasing the concentration of mobile buffer can increase Deff when fixed buffers are present.