The internal conversion and hot ground-state dynamics of trans- and cis-azomethane starting in the S(1) state have been investigated by nonadiabatic ab initio surface hopping dynamics using MCSCF-GVB-CAS and MRCISD methods and by determining energy minima and saddle points, minima on the crossing seam, and minimum energy pathways on the ground and first excited-state surfaces. The lifetimes and photoproducts from the dynamics simulations, geometric properties, excitation energies of selected stationary points and minimum energy pathways between them are reported. Our results favor a statistical model with trans-AZM moving to the ground-state minima before the first CN dissociation takes place. A detailed discussion in comparison to recent experimental and theoretical data is presented.

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Devising analysis tools for elucidating the regulatory mechanism of complex enzymes has been a challenging task for many decades. It generally requires the determination of the structural-dynamical information of protein solvent systems far from equilibrium over multiple length and time scales, which is still difficult both theoretically and experimentally. To cope with the problem, we introduce a full-residue space multiscale simulation method based on a combination of the kinetic Monte Carlo and molecular dynamics techniques, in which the rates of the rate-determining processes are evaluated from a biomolecular forcefield on the fly during the simulation run by taking into account the full space of residues. To demonstrate its reliability and efficiency, we explore the light-induced functional behavior of the full-length phototropin1 from Chlamydomonas reinhardtii (Cr-phot1) and its various subdomains. Our results demonstrate that in the dark state the light oxygen voltage-2-Jα (LOV2-Jα) photoswitch inhibits the enzymatic activity of the kinase, whereas the LOV1-Jα photoswitch controls the dimerization with the LOV2 domain. This leads to the repulsion of the LOV1-LOV2 linker out of the interface region between both LOV domains, which results in a positively charged surface suitable for cell-membrane interaction. By contrast, in the light state, we observe that the distance between both LOV domains is increased and the LOV1-LOV2 linker forms a helix-turn-helix (HTH) motif, which enables gene control through nucleotide binding. Finally, we find that the kinase is activated through the disruption of the Jα-helix from the LOV2 domain, which is followed by a stretching of the activation loop (A-loop) and broadening of the catalytic cleft of the kinase.

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LOV (light-, oxygen- or voltage-sensitive) domains act as photosensory units of many prokaryotic and eukaryotic proteins. Upon blue light excitation they undergo a photocycle via the excited triplet state of their flavin chromophore yielding the flavin-cysteinyl adduct. Adduct formation is highly conserved among all LOV domains and constitutes the primary step of LOV domain signaling. But recently, it has been shown that signal propagation can also be triggered by flavin photoreduction to the neutral semiquinone offering new prospects for protein engineering. This, however, requires mutation of the photo-active Cys. Here, we report on LOV1 mutants of C. reinhardtii phototropin in which adduct formation is suppressed although the photo-active Cys is present. Introduction of a Tyr into the LOV core induces a proton coupled electron transfer towards the flavin chromophore. Flavin radical species are formed via either the excited flavin singlet or triplet state depending on the geometry of donor and acceptor. This photoreductive pathway resembles the photoreaction observed in other blue light photoreceptors, e.g. blue-light sensors using flavin adenine dinucleotide (BLUF) domains or cryptochromes. The ability to tune the photoreactivity of the flavin chromophore inside the LOV core has implications for the mechanism of adduct formation in the wild type and may be of use for protein engineering.