In this report, we compare the femtosecond
to nanosecond primary
reverse photodynamics (15E
Pg → 15Z
Pr) 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 higher-energy-absorbing representatives exhibiting
both faster excited-state 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-Go intermediates, with class II and III representatives
evolving to a secondary Meta-G photointermediate. Class II Meta-GR intermediates were orange absorbing, whereas class III Meta-G
had structurally relaxed, red-absorbing chromophores that resemble
their dark-adapted 15Z
Pr states.
Differences in the reverse and forward reaction mechanisms are discussed
within the context of structural constraints.