Small-angle X-ray and small-angle neutron scattering (SAXS/SANS) provide unique structural information on biomolecules and their complexes in solution. SANS may provide multiple independent data sets by means of contrast variation experiments, that is, by measuring at different D 2 O concentrations and different perdeuteration conditions of the biomolecular complex. However, even the combined data from multiple SAXS/SANS sets is by far insufficient to define all degrees of freedom of a complex, leading to a significant risk of overfitting when refining biomolecular structures against SAXS/SANS data. Hence, to control against overfitting, the low-information SAXS/ SANS data must be complemented by accurate physical models, and, if possible, refined models should be cross-validated against independent data not used during the refinement. We present a method for refining atomic biomolecular structures against multiple sets of SAXS and SANS data using all-atom molecular dynamics simulations. Using the protein citrate synthase and the protein/RNA complex Sxl−Unr−msl2 mRNA as test cases, we demonstrate how multiple SAXS and SANS sets may be used for refinement and cross-validation, thereby excluding overfitting during refinement. For the Sxl−Unr−msl2 complex, we find that perdeuteration of the Unr domain leads to a unique, slightly compacted conformation, whereas other perdeuteration conditions lead to similar solution conformations compared to the nondeuterated state. In line with our previous method for predicting SAXS curves, SANS curves were predicted with explicit-solvent calculations, taking atomic models for both the hydration layer and the excluded solvent into account, thereby avoiding the use of solvent-related fitting parameters and solventreduced neutron scattering lengths. We expect the method to be useful for deriving and validating solution structures of biomolecules and soft-matter complexes, and for critically assessing whether multiple SAXS and SANS sets are mutually compatible.
Small-angle neutron scattering is a tool providing information on nanostructures of objects in the order of 1-300 nm. In this experiment a pouch bag lithium ion battery cell was investigated with SANS ex situ, in situ and in operando during charging and discharging. LiNi 0.33 Mn 0.33 Co 0.33 O 2 was used as cathode and graphite as anode material. The small-angle neutron scattering measurements were performed on the SANS-1 instrument at the FRM II neutron source of the Heinz Maier-Leibnitz Zentrum (MLZ) in Garching, Germany. Ex situ measurements of components of the cell as well as static in situ and dynamic in operando SANS experiments were performed with a complete Li-ion pouch bag cell. The cell was charged and discharged twice with C/3 and small-angle neutron scattering data were collected during the measurements. The observed intensity data were then evaluated and changes of the total scattering in the measured Q-range are correlated to the lithiation processes occurring inside the cell. Thus we can show that SANS can be used as a tool to monitor kinetic processes in Li-ion batteries in operando and non-destructively. Li-ion batteries have been used widely as power sources in transportable electronic devices and new markets such as hybrid and all battery electric vehicles are developing.1 This has also raised an enhanced interest in the development of analytical methods to study batteries during operation ("in operando"). Besides the improvement in energy and power density, researchers are trying to prolong the lifetime of lithium ion batteries. Cycle and storage life of Li-ion batteries are critical for electric vehicle or stationary power storage applications. A major degradation effect is the continuous decomposition of electrolyte -leading also to a growing solid electrolyte interface (SEI), a passivating layer on typically the anode active material (graphite), thus resulting in loss of conductivity and higher cell resistance. For the cathode phase transitions, structural disorder and metal dissolution are major aging effects. Corrosion of various materials in the battery and mechanical contact loss of active particles or current collectors are also an issue.2-4 One major goal is to understand the general reaction mechanism of the Li-intercalation process in the anode respectively cathode materials. The fundamental understanding of battery processes is a key for improving battery performance (e.g., in terms of energy density and power density) and lifetime.The small-angle scattering method is commonly used to gain information about the nanostructure of the investigated materials (i.e., size, volume and shape of particles). In combination with the special properties of neutrons, like the high penetration depth in materials, small-angle neutron scattering (SANS) can be used as a powerful tool for in situ investigations of Li-ion batteries. SANS can help to understand changes on the nanoscale of particles in cycled cells and during cell cycling. Our study's primary goal is to adapt, develop and extend this ...
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