Elucidating Ultrafast Multiphasic Dynamics in the Photoisomerization of Cyanobacteriochrome.

Abstract

Understanding photoisomerization dynamics in cyanobacteriochromes is important to the development of optical agents in near-infrared biological imaging and optogenetics. Here, by integrating femtosecond spectroscopy and site-directed mutagenesis, we investigate the photoinduced Pr-state isomerization dynamics and mechanism of a unique red/green cyanobacteriochrome from Leptolyngbya sp. JSC-1. We observed multiphasic dynamics in the Pr state, a widespread phenomenon for photoreceptors in the phytochrome superfamily, and revealed their origins; the initial dynamics over a few to tens and hundreds of picoseconds arises from the local active-site relaxations followed by the slow double-bond isomerization in several hundreds of picoseconds. Such continuous active-site evolution results in a unique spectral tuning effect that favors the blue-side emission and suppresses the red-side emission. We also observed the faster dynamics in both relaxation and isomerization with critical mutants at the active site that render a looser active site. These results clearly distinguish the multiphasic dynamics between relaxation and isomerization and reveal a novel molecular mechanism for better biological applications.