Sleep and wake states are each maintained by activity in a corresponding neuronal network, with mutually inhibitory connections between the networks. In infant mammals, the durations of both states are exponentially distributed, whereas in adults, the wake states yield a heavy-tailed distribution. What drives this transformation of the wake distribution? Is it the altered network structure or a change in neuronal dynamics? What properties of the network are necessary for maintenance of neural activity on the network and what mechanisms are involved in transitioning between sleep and wake states? We explore these issues using probabilistic models and random graph theory, specifically looking at stochastic processes occurring on random graphs.

Cyanobacteriochrome (CBCR) is phytochrome-related photoreceptors with diverse photosensitivities spanning the entire visible spectrum with 5 different classes to date. Recent ultrafast characterization of the red/green NpR6012g4 (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.