Recent ultrafast characterization of the red/green NpR6012g4 cyanobacteriochrome (CBCR) from Nostoc punctiforme revealed a high quantum yield for its forward photoreaction [J. Am. Chem. Soc. 134, 130�133 (2012)] 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 Nostoc punctiforme, which 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. The reactive ground state intermediate was also resolved in the green/red RcaE CBCR from Fremyella diplosiphon, but the dump pulse enhances total quantum yield in one of the resolved photoproducts. The SCID mechanism proposed for NpR6012g4 is unable to account for this enhancement and indicates that the dump pulse is capable of modulating the modality (photoactivity vs. fluorescence) of single domain CBCR photodynamics. This is similar to the originally proposed coherent control formalisms underlying pump-dump schemes, but involving direct evolution on both excited- and ground-state potential energy surfaces. Figure 1. <img src='https://acs-archive-assets2.s3.amazonaws.com/240nm/uploads/37963_abstract_boston_gnp.jpg' alt='' />

Populating reactive ground-state intermediates in cyanobacteriochromes with pump-dump pulse sequences

247th National Meeting (2014)

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