Primary Photodynamics of the Green/Red-Absorbing Photoswitching Regulator of the Chromatic Adaptation E Domain from <i>Fremyella diplosiphon</i>

Phytochromes are red/far-red photosensory proteins that utilize the photoisomerization of a linear tetrapyrrole (bilin) chromophore to detect the red to far-red light ratio. Cyanobacteriochromes (CBCRs) are distantly related cyanobacterial photosensors with homologous bilin-binding GAF domains, but they exhibit greater spectral diversity. Different CBCR subfamilies have been described, with spectral sensitivity varying across the near-ultraviolet and throughout the visible spectrum, but all known CBCRs utilize photoisomerization of the bilin 15,16-double bond as the primary photochemical event. The first CBCR discovered was RcaE, responsible for tuning light harvesting to the incident color environment (complementary chromatic adaptation) in <i>Fremyella diplosiphon</i>. The green/red RcaE photocycle has recently been described in detail. We now extend this analysis by examining femtosecond photodynamics using ultrafast transient absorption techniques with broadband detection and multicomponent global analysis. Excited-state dynamics in both directions are significantly slower than those recently published for the red/green CBCR NpR6012g4. In the forward reaction, the primary Lumi-G photoproduct arises from the longer-lived excited-state populations, leading to a low photoproduct quantum yield. Using dual-excitation wavelength interleaved pump–probe spectroscopy, we observe multiphasic excited-state dynamics in the forward reaction (^15<i>Z</i>P_g → ^15<i>E</i>P_r), which we interpret as arising from ground-state inhomogeneity with different tautomers of the PCB chromophore. The reverse reaction (^15<i>E</i>P_r → ^15<i>Z</i>P_g) is characterized via pump–probe spectroscopy and also exhibits slow excited-state decay dynamics and a low photoproduct yield. These results provide the first description of excited-state dynamics for a green/red CBCR