TY - JOUR AB - The three-dimensional (3D) crystal structures of the GAF3 domain of cyanobacteriochrome Slr1393 (Synechocystis PCC6803) carrying a phycocyanobilin chromophore could be solved in both 15-Z dark-adapted state, Pr, λmax = 649 nm, and 15-E photoproduct, Pg, λmax = 536 nm (resolution, 1.6 and 1.86 Å, respectively). The structural data allowed identifying the large spectral shift of the Pr-to-Pg conversion as resulting from an out-of-plane rotation of the chromophore’s peripheral rings and an outward movement of a short helix formed from a formerly unstructured loop. In addition, a third structure (2.1-Å resolution) starting from the photoproduct crystals allowed identification of elements that regulate the absorption maxima. In this peculiar form, generated during X-ray exposition, protein and chromophore conformation still resemble the photoproduct state, except for the D-ring already in 15-Z configuration and tilted out of plane akin the dark state. Due to its formation from the photoproduct, it might be considered an early conformational change initiating the parental state-recovering photocycle. The high quality and the distinct features of the three forms allowed for applying quantum-chemical calculations in the framework of multiscale modeling to rationalize the absorption maxima changes. A systematic analysis of the PCB chromophore in the presence and absence of the protein environment showed that the direct electrostatic effect is negligible on the spectral tuning. However, the protein forces the outer pyrrole rings of the chromophore to deviate from coplanarity, which is identified as the dominating factor for the color regulation. AU - Xu, Xiuling AU - Port, Astrid AU - Wiebeler, Christian AU - Zhao, Kai-Hong AU - Schapiro, Igor AU - Gärtner, Wolfgang ID - 15628 JF - Proceedings of the National Academy of Sciences SN - 0027-8424 TI - Structural Elements Regulating the Photochromicity in a Cyanobacteriochrome ER - TY - JOUR AB - Interfaces of room temperature ionic liquids (RTILs) are important for both applications and basic science and are therefore intensely studied. However, the evolution of their interface structure with the cation’s alkyl chain length n from Coulomb to van der Waals interaction domination has not yet been studied for even a single broad homologous RTIL series. We present here such a study of the liquid–air interface for n=2to22, using angstrom-resolution X-ray methods. For n<6, a typical “simple liquid” monotonic surface-normal electron density profile ρe(z) is obtained, like those of water and organic solvents. For n>6, increasingly more pronounced nanoscale self-segregation of the molecules’ charged moieties and apolar chains yields surface layering with alternating regions of headgroups and chains. The layering decays into the bulk over a few, to a few tens, of nanometers. The layering periods and decay lengths, their linear n dependence, and slopes are discussed within two models, one with partial-chain interdigitation and the other with liquid-like chains. No surface-parallel long-range order is found within the surface layer. For n=22, a different surface phase is observed above melting. Our results also impact general liquid-phase issues like supramolecular self-aggregation and bulk–surface structure relations. AU - Haddad, Julia AU - Pontoni, Diego AU - Murphy, Bridget M. AU - Festersen, Sven AU - Runge, Benjamin AU - Magnussen, Olaf M. AU - Steinrück, Hans-Georg AU - Reichert, Harald AU - Ocko, Benjamin M. AU - Deutsch, Moshe ID - 23626 JF - Proceedings of the National Academy of Sciences SN - 0027-8424 TI - Surface structure evolution in a homologous series of ionic liquids VL - 115 ER - TY - JOUR AB - Significance The hierarchical assembly pathway of the cytoskeletal protein vimentin may be responsible for the astonishing mechanical properties of the emerging filaments, such as high flexibility and extensibility, and thus play a key role in cellular mechanics. A two-step assembly mechanism, involving a lateral and a subsequent elongational step, has been established; however, the elongational step could not be followed in solution. Here, we show direct in situ observation and modeling of the elongation reaction of the filaments on the relevant length and time scales, using time-resolved, multiangle static and dynamic light scattering. We thus achieve sufficient spatio-temporal resolution without the need of labeling, staining, or adsorption to substrates. AU - Lopez, Carlos G. AU - Saldanha, Oliva AU - Huber, Klaus AU - Köster, Sarah ID - 41838 IS - 40 JF - Proceedings of the National Academy of Sciences KW - Multidisciplinary SN - 0027-8424 TI - Lateral association and elongation of vimentin intermediate filament proteins: A time-resolved light-scattering study VL - 113 ER - TY - JOUR AB - Optical 2D Fourier transform spectroscopy (2DFTS) provides insight into the many-body interactions in direct gap semiconductors by separating the contributions to the coherent nonlinear optical response. We demonstrate these features of optical 2DFTS by studying the heavy-hole and light-hole excitonic resonances in a gallium arsenide quantum well at low temperature. Varying the polarization of the incident beams exploits selection rules to achieve further separation. Calculations using a full many-body theory agree well with experimental results and unambiguously demonstrate the dominance of many-body physics. AU - Zhang, T. AU - Kuznetsova, I. AU - Meier, Torsten AU - Li, X. AU - Mirin, R. P. AU - Thomas, P. AU - Cundiff, S. T. ID - 23487 IS - 36 JF - Proceedings of the National Academy of Sciences (PNAS) SN - 0027-8424 TI - Polarization-dependent optical 2D Fourier transform spectroscopy of semiconductors VL - 104 ER -