Bleaching of the bacteriochlorophyll monomer band: Can absorption kinetics distinguish virtual from two-step electron transfer in bacterial photosynthesis?

The significance of absorption kinetics data in distinguishing between two-step and virtual mechanisms for the primary charge separation in reaction centers of photosynthetic bacteria is examined. A simple class of models is presented in which the transfer may occur predominantly by a virtual process and in which the intermediate state develops some small, but nonzero, transient population. The simplicity of the model allows both the virtual and two-step contributions to the intermediate state population to be easily calculated. We find that the peak intermediate state population can be either close to 1 or very small (<10^-2), depending on the system parameters such as electronic matrix elements, the energy denominator, and the lifetimes and dephasing rates in the electronic states. In particular, purely virtual transfer can yield a peak intermediate-state population of 10%. These calculations imply that observation of a small amount of bleaching in the bacteriochlorophyll monomer band does not necessarily rule out a virtual process.