Elucidation of the Ultrafast Origin of Multiphasic Dynamics in a Far-Red-Sensing Cyanobacteriochrome.

Cyanobacteriochromes are photoreceptors that constitute a significant subset of phycocyanobilin-bound proteins, yet the details of their excited-state photochemical and structural dynamics have not been fully elucidated. Here, we investigate the photoisomerization dynamics of a newly identified far-red/orange light-absorbing cyanobacteriochrome using femtosecond-resolved fluorescence and absorption methods. We observed active-site relaxations ranging from a few to hundreds of picoseconds for both far-red and orange-absorbing states. As such relaxations modulate the potential energy landscape of the chromophore, we also observed a unique dynamic spectral tuning in the far-red-absorbing state and an apparent dynamic Stokes shift in the orange-absorbing state in the femtosecond-resolved fluorescence spectra. We found that the isomerization reactions in both states occur within 320-400 ps. The observed correlation of the local relaxation and the phycocyanobilin twisting can be critical to the subsequent conformational changes after isomerization through the conical intersection to reach the final biological functions. Understanding of the time scales of the local relaxations and isomerization reactions is important to guide the design and engineering of phycocyanobilin-based light-sensitive systems of desired optical properties via synthetic biology.