Conservation and Diversity in the Primary Reverse Photodynamics of the Canonical Red/Green Cyanobacteriochrome Family.

In this report, we compare the . femtosecond to nanosecond primary reverse photodynamics (P-15E(g) -> P-15(Z)r) of eight tetrapyrrole binding photoswitching cyanobacteriochromes in the canonical red/green family from the cyanobacterium Nostoc punctiforme. Three characteristic classes were identified on the basis of the diversity of excited-state and ground-state properties, including the lifetime, photocycle initiation quantum yield, photointermediate stability, spectra, and temporal properties. We observed a correlation between the excited-state lifetime and peak wavelength of the electronic absorption spectrum with high-erenergy-absorbing representatives exhibiting both faster excitedstate decay times and higher photoisomerization quantum yields. The latter was attributed to both an increased number of structural restraints and differences in H-bonding networks that facilitate photoisomerization. All three classes exhibited primary Lumi-G(o) intermediates, with class II and III representatives evolving to a secondary Meta-G photointermediate. Class II Meta-G(R) intermediates were orange absorbing, whereas class III Meta-G had structurally relaxed, red-absorbing chromophores that resemble their dark-adapted P-15Z(r) states. Differences in the reverse and forward reaction mechanisms are discussed within the context of structural constraints.