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Ultrafast structural changes direct the first molecular events of vision

The visual pigment rhodopsin plays a critical role in the process of low-light vision in vertebrates. It is present in the disk membranes of rod cells in the retina and is responsible for transforming the absorption of light into a physiological signal. Rhodopsin has a unique structure that consists of seven transmembrane (TM) α-helices with an 11-cis retinal chromophore covalently bound to the Lysine sidechain of 7th TM helix. A negatively charged amino acid (glutamate) forms a salt bridge with the protonated Schiff base (PSB) of the chromophore to stabilize the receptor in the resting state.

Rhodopsin transforms the absorption of light into a physiological signal through conformational changes that activate the intracellular G protein transducin—a member of the Gi/o/t family—initiating a signaling cascade, resulting in electrical impulses sent to the brain and ultimately leading to visual perception. Although previous studies have provided valuable insights into the mechanism of signal transduction in rhodopsin, methods that provide both a high spatial and temporal resolution are necessary to fully understand the activation mechanism at the atomic scale from femtoseconds to milliseconds. This study presents the first experimentally-derived picture of the rhodopsin activation mechanism at the atomic scale using time-resolved serial femtosecond crystallography in association with hybrid quantum mechanics/molecular mechanics (QM/MM) simulations. The results show that light-induced structural changes in rhodopsin occur on a timescale of hundreds of femtoseconds, and they reveal new details about the conformational changes that occur during activation.

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Approximate Computing for Nuclear Reactor Simulations

During the last decades, computing power has been subject to tremendous progress due to the shrinking of transistor size as predicted by Moore’s law. However, as we approach the physical limits of this scaling, alternative techniques have to be deployed to increase computing performance. In this regard, the next big advance is envisioned to be the usage of approximate computing hardware based on field-programmable gate arrays and/or digital-analogue in-memory circuits. Such approximate computing can provide disproportional gain (x1000) in energy efficiency and/or execution time for acceptable loss of simulation accuracy. This could be highly beneficial in order to accelerate computational intensive simulations such as reactor core analyses with higher resolution multi-physics models. On the other hand, the execution of programming codes on low-precision hardware may result in inadequate outcomes due to quality degradation and/or algorithm divergence. To address these questions, studies on the stability and the performance of advanced reactor simulation algorithms as function of reduced floating-point arithmetic precision are being conducted at the laboratory for reactor physics and thermal-hydraulics. Results obtained so far indicate a large room for the acceleration of nuclear engineering applications using mixed-precision hardware. Therefore, research is now being enlarged towards assessing multiprecision computing methods for reactor core simulations with higher spatial resolution.

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