A new method of calculating the spectral function of system-bath interaction during elementary biochemical reaction is proposed. This method was applied to the primary electron transfer in the photosynthetic reaction center of purple bacteria Rh. Sphaeroides. The calculated spectral functions differ significantly from the commonly used ohmic function. It is shown that the unidirectionality of the electron transfer along the A-branch in the reaction center of Rh. Sphaeroides can be caused by the asymmetry or reaction system interaction with the protein environment.