Reactive Ground-State Pathways Are Not Ubiquitous
in Red/Green Cyanobacteriochromes
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Abstract
Recent
characterization of the red/green cyanobacteriochrome (CBCR) NpR6012g4
revealed a high quantum yield for its forward photoreaction [J. Am. Chem. Soc. 2012, 134, 130−133] that was ascribed to the activity of hidden, productive
ground-state intermediates. The dynamics of the pathways involving
these ground-state intermediates was resolved with femtosecond dispersed
pump–dump–probe spectroscopy, the first such study reported
for any CBCR. To address the ubiquity of such second-chance initiation
dynamics (SCID) in CBCRs, we examined the closely related red/green
CBCR NpF2164g6 from <i>Nostoc punctiforme</i>. Both NpF2164g6
and NpR6012g4 use phycocyanobilin as the chromophore precursor and
exhibit similar excited-state dynamics. However, NpF2164g6 exhibits
a lower quantum yield of 32% for the generation of the isomerized
Lumi-R primary photoproduct, compared to 40% for NpR6012g4. This difference
arises from significantly different ground-state dynamics between
the two proteins, with the SCID mechanism deactivated in NpF2164g6.
We present an integrated inhomogeneous target model that self-consistently
fits the pump–probe and pump–dump–probe signals
for both forward and reverse photoreactions in both proteins. This
work demonstrates that reactive ground-state intermediates are not
ubiquitous phenomena in CBCRs