Riboswitches are structural RNA elements that are generally located in the 5′ untranslated region of messenger RNA. During regulation of gene expression, ligand binding to the aptamer domain of a riboswitch triggers a signal to the downstream expression platform1–3. A complete understanding of the structural basis of this mechanism requires the ability to study structural changes over time4. Here we use femtosecond X-ray free electron laser (XFEL) pulses5,6 to obtain structural measurements from crystals so small that diffusion of a ligand can be timed to initiate a reaction before diffraction. We demonstrate this approach by determining four structures of the adenine riboswitch aptamer domain during the course of a reaction, involving two unbound apo structures, one ligand-bound intermediate, and the final ligand-bound conformation. These structures support a reaction mechanism model with at least four states and illustrate the structural basis of signal transmission. The three-way junction and the P1 switch helix of the two apo conformers are notably different from those in the ligand-bound conformation. Our time-resolved crystallographic measurements with a 10-second delay captured the structure of an intermediate with changes in the binding pocket that accommodate the ligand. With at least a 10-minute delay, the RNA molecules were fully converted to the ligand-bound state, in which the substantial conformational changes resulted in conversion of the space group. Such notable changes in crystallo highlight the important opportunities that micro- and nanocrystals may offer in these and similar time-resolved diffraction studies. Together, these results demonstrate the potential of ‘mix-and-inject’ time-resolved serial crystallography to study biochemically important interactions between biomacromolecules and ligands, including those that involve large conformational changes.

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Solar thermal fuels (STFs) harvest and store solar energy in a closed cycle system through conformational change of molecules and can release the energy in the form of heat on demand. With the aim of developing tunable and optimized STFs for solid-state applications, we designed three azobenzene derivatives functionalized with bulky aromatic groups (phenyl, biphenyl, and tert-butyl phenyl groups). In contrast to pristine azobenzene, which crystallizes and makes nonuniform films, the bulky azobenzene derivatives formed uniform amorphous films that can be charged and discharged with light and heat for many cycles. Thermal stability of the films, a critical metric for thermally triggerable STFs, was greatly increased by the bulky functionalization (up to 180°C), and we were able to achieve record high energy density of 135 J/ g for solid-state STFs, over a 30% improvement compared to previous solid-state reports. Furthermore, the chargeability in the solid state was improved, up to 80% charged from 40% charged in previous solid-state reports. Our results point toward molecular engineering as an effective method to increase energy storage in STFs, improve chargeability, and improve the thermal stability of the thin film. KEYWORDS: solar thermal fuels heat storage, molecular thin films, solid-state applications, structural design, molecular engineering, photoswitching ■ INTRODUCTIONDespite the vast abundance of solar radiation, efficient conversion, storage, and distribution of this resource remains a challenge. One approach that uses the same material to both convert and store the sun's energy is the use of photoactive molecules. These molecules convert solar energy into strained or rearranged chemical bonds, with the amount of energy stored, ΔH, as the difference in energy between the ground and metastable states. Upon reversion back to the ground state, the energy is released in the form of heat with the molecules back in their ground state ready to be charged again. These materials for solar energy harvesting, referred to as solar thermal fuels (STF), operate in a closed cycle and ideally possess high energy density and cyclability with no degradation or emissions and easy distribution as "heat on demand".Previous work on candidate solar thermal fuels has shown both promise and many challenges in optimizing the required properties. For example, norbornadiene showed great potential with high energy density (89 kJ/mol). Although cyclability has been the biggest challenge, recent research by molecular modification of norbornedine using aryl substituents showed improvement of cyclability.1−3 Recent work showed that (Fulvalene) tetracarbonyl-diruthenium showed reasonable gravimetric energy density (30.6 Wh/kg) while showing high cyclability, but due to the use of expensive ruthenium, its potential is limited.4,5 Azobenzene possesses high cyclability but has been hindered by low energy density. 6 Recently, several templating methods of the solar thermal fuel molecule have gained attention and have been invest...

Static friction between amorphous silica surfaces with a varying number of interfacial siloxane (Si-O-Si) bridges was studied using molecular dynamic simulations. Static friction was found to increase linearly with the applied normal pressure, which can be explained in the framework of Prandlt-Tomlinson's model. Friction force was found to increase with concentration of siloxane bridges, but with a decreasing gradient, with the latter being due to interactions between neighboring siloxane bridges. In addition, we identified atomic-level wear mechanisms of silica. These mechanisms include both transfer of individual atoms accompanied by breaking interfacial siloxane bridges and transfer of atomic cluster initialized by rupturing of surface Si-O bonds. Our simulations showed that small clusters are continually formed and dissolved at the sliding interface, which plays an important role in wear at silica/silica interface.