Responsive behavior of 4-(N-maleimido)azobenzene in polymers with aromatic main chain and side chain linked units

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Abstract

Polymer matrices with aromatic main chain and side chain linked units (polystyrene, polysulfone) were employed to construct thin films containing maleimidoazobenzene molecules in order to develop new materials with responsive properties. The photochromic behavior of the active compound was studied both in the solid and solution state for comparison. The thermal cis-trans isomerization of azobenzene chromophores in polymer films follows two different kinetic pathways. A double-exponential relaxation process (fast and slow) took place in polystyrene matrix under 70 ºC while the thermal isomerization at 70 ºC and higher follows a first-order kinetics. The cis isomer content corresponding to the photostationary state was found to be lower in polymer films than in solution.

Translated abstract

Matrizes poliméricas com cadeia principal aromática e cadeias laterais formadas por unidades de poliestireno e polissulfona foram empregadas para construir filmes finos contendo moléculas de maleimidoazobenzeno, a fim de desenvolver novos materiais com propriedades responsivas. O comportamento fotocrômico do composto ativo foi estudado no estado sólido e em solução, para comparação. A isomerisação térmica cis-trans de cromóforos de azobenzeno em filmes finos de polímeros segue duas etapas cinéticas diferentes. Um processo de relaxação de segunda ordem (rápido e lento) ocorre em matriz de poliestireno abaixo de 70 ºC, enquanto a isomerização térmica a 70 ºC ou em temperatura mais alta segue cinética de primeira ordem. A quantidade de isômero cis correspondente ao estado fotoestacionário foi mais baixa em filmes do polímero do que em solução.

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Structure and dynamics of an unfolded protein examined by molecular dynamics simulation.

Kresten Lindorff-Larsen, Nikola Trbovic, Paul Maragakis … (2012)

The accurate characterization of the structure and dynamics of proteins in disordered states is a difficult problem at the frontier of structural biology whose solution promises to further our understanding of protein folding and intrinsically disordered proteins. Molecular dynamics (MD) simulations have added considerably to our understanding of folded proteins, but the accuracy with which the force fields used in such simulations can describe disordered proteins is unclear. In this work, using a modern force field, we performed a 200 μs unrestrained MD simulation of the acid-unfolded state of an experimentally well-characterized protein, ACBP, to explore the extent to which state-of-the-art simulation can describe the structural and dynamical features of a disordered protein. By comparing the simulation results with the results of NMR experiments, we demonstrate that the simulation successfully captures important aspects of both the local and global structure. Our simulation was ~2 orders of magnitude longer than those in previous studies of unfolded proteins, a length sufficient to observe repeated formation and breaking of helical structure, which we found to occur on a multimicrosecond time scale. We observed one structural feature that formed but did not break during the simulation, highlighting the difficulty in sampling disordered states. Overall, however, our simulation results are in reasonable agreement with the experimental data, demonstrating that MD simulations can already be useful in describing disordered proteins. Finally, our direct calculation of certain NMR observables from the simulation provides new insight into the general relationship between structural features of disordered proteins and experimental NMR relaxation properties. © 2012 American Chemical Society

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Spectators Control Selectivity in Surface Chemistry: Acrolein Partial Hydrogenation Over Pd

Karl-Heinz Dostert, Casey O'Brien, Francisco Ivars-Barceló … (2015)

We present a mechanistic study on selective hydrogenation of acrolein over model Pd surfaces—both single crystal Pd(111) and Pd nanoparticles supported on a model oxide support. We show for the first time that selective hydrogenation of the C=O bond in acrolein to form an unsaturated alcohol is possible over Pd(111) with nearly 100% selectivity. However, this process requires a very distinct modification of the Pd(111) surface with an overlayer of oxopropyl spectator species that are formed from acrolein during the initial stages of reaction and turn the metal surface selective toward propenol formation. By applying pulsed multimolecular beam experiments and in situ infrared reflection–absorption spectroscopy, we identified the chemical nature of the spectator and the reactive surface intermediate (propenoxy species) and experimentally followed the simultaneous evolution of the reactive intermediate on the surface and formation of the product in the gas phase.