Competing excited-state deactivation processes in bacteriophytochromes

Bacteriophytochromes are photoreceptor proteins of widespread use as templates for the engineering of fluorescent proteins with emission maxima in the near-infrared regime beyond 650 nm ideally suited for deep-tissue imaging of living cells. The main challenge for such engineering is that native bacteriophytochromes have very low fluorescence quantum yields because of competing excited-state deactivation processes, which include both the well-known photoisomerization reaction of their linear tetrapyrrole chromophore and excited-state proton transfer reactions from the chromophore to the surrounding protein. Here, we describe how hybrid quantum mechanics/molecular mechanics modeling of the photochemistry of these proteins have provided valuable guidelines for strengthening the fluorescence through inhibition of the competing non-radiative processes. Specifically, based on the results of such modeling, we present a strategy to inhibit the photoisomerization on steric grounds and identify the most probable proton transfer reaction to exert a negative influence on the fluorescence quantum yields. It is our hope that these results will help stimulate further contributions from quantum chemistry toward realizing the potential for entirely new bioimaging applications commonly attributed to brightly near-infrared fluorescent bacteriophytochromes.