Femtosecond Photodynamics of the Red/Green Cyanobacteriochrome NpR6012g4 from <i>Nostoc punctiforme</i>. 1. Forward Dynamics

Abstract

Phytochromes are well-known red/far-red photosensory proteins that utilize the photoisomerization of a linear tetrapyrrole (bilin) chromophore to detect the ratio of red to far-red light. Cyanobacteriochromes (CBCRs) are related photosensory proteins with a bilin-binding GAF domain, but much more diverse spectral sensitivity, with five recognized subfamilies of CBCRs described to date. The mechanisms that underlie this spectral diversity have not yet been fully elucidated. One of the main CBCR subfamilies photoconverts between a red-absorbing ground state, like the familiar P_r state of phytochromes, and a green-absorbing photoproduct (P_g). Here, we examine the ultrafast forward photodynamics of the red/green CBCR NpR6012g4 from the <i>NpR6012</i> locus of the nitrogen-fixing cyanobacterium <i>Nostoc punctiforme</i>. Using transient absorption spectroscopy with broadband detection and multicomponent global analysis, we observed multiphasic excited-state dynamics that induces the forward reaction (red-absorbing to green-absorbing), which we interpret as arising from ground-state heterogeneity. Excited-state decays with lifetimes of 55 and 345 ps generate the primary photoproduct (Lumi-R), and the fastest decay (5 ps) did not produce Lumi-R. Although the photoinduced kinetics of Npr6012g4 is comparable with that of the Cph1 phytochrome isolated from <i>Synechocystis</i> cyanobacteria, NpR6012g4 exhibits a ≥2–3-fold higher photochemical quantum yield. Understanding the structural basis of this enhanced quantum yield may prove to be useful in increasing the photochemical efficiency of other bilin-based photosensors