Cyanobacteriochrome (CBCR) are phytochrome-related photoreceptors with diverse photosensitivities spanning the entire visible spectrum with 5 different classes to date. The primary ultrafast Z-to-E and E-to-Z isomerization photodynamics of the red/green CBCR NpR6012g4 from Nostoc punctiforme was studied by transient absorption pump-probe and pump-dump-probe spectroscopy. A 2 ps dump pulse resonant with the stimulated emission band depleted 21% of the excited-state population, while the initial photoproduct Lumi-R was depleted by only 11%. We observed a red-shifted ground-state intermediate (GSI) that we assign to a metastable state that failed to isomerize fully. Multicomponent global analysis implicates the generation of additional Lumi-R from the GSI via crossing over the ground-state thermal barrier for full isomerization, explaining the discrepancy between excited-state and Lumi-R depletion by the dump pulse. This second-chance ground-state dynamics provides a plausible explanation for the unusually high quantum yield of 40% for the primary isomerization step in the forward reaction of NpR6012g4.

Second-chance forward isomerization dynamics of the red/green cyanobacteriochrome NpR6012g4 from Nostoc punctiforme

244th National Meeting (2012)

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