Calcium carbonate is a model system to investigate the mechanism of solid formation by precipitation from solutions, and it is often considered in the debated classical and nonclassical nucleation mechanism. Despite the great scientific relevance of calcium carbonate in different scientific areas, little is known about the early stage of its formation. Therefore, contactless devices are designed that are capable of providing informative investigations on the early stages of the precipitation pathway of calcium carbonate in supersaturated solutions using classical scattering methods such as wide‐angle X‐ray scattering (WAXS) and small‐angle X‐ray scattering (SAXS) techniques. In particular, SAXS is exploited for investigating the size of entities formed from supersaturated solutions before the critical conditions for amorphous calcium carbonate (ACC) nucleation are attained. The saturation level is controlled and kept constant by mixing four diluted solutions (i.e., NaOH, CaCl2, NaHCO3, H2O) at constant T and pH. The scattering data are collected on a liquid jet generated about 75 s after the mixing point. The data are modeled using parametric statistical models providing insight about the size distribution of denser matter in the liquid jet. Theoretical implications on the early stage of solid formation pathway are inferred.

In this article, a practical procedure for absolute intensity calibration for smallangle scattering (SAXS) studies on liquid microjets is established. A gold nanoparticle suspension is used as standard so that the intercept at Q = 0 of the SAXS scattering curve provides a scaling reference. In order to obtain the most precise extrapolation at Q = 0, an extension of the Guinier approximation has been used, with a second-order term in the fit that adapts to a larger Q range.

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In article number https://doi.org/10.1002/ppsc.201800482, Andrea Testino and co‐workers investigate calcium carbonate clusters in‐situ by small angle X‐ray scattering using a liquid jet setup at constant pH, temperature, and saturation level. The saturation is kept constant and below the critical condition for amorphous calcium carbonate formation. Data analysis allows for extracting information about entities in the liquid jet characterized by a higher density: highly hydrated small clusters (<2 nm) and large aggregates (superclusters, broad size distribution) are detected.

Calcium carbonate, one of the most studied biominerals, has major applications across a broad spectrum of technologies. In order to investigate the crystallization kinetics, in-situ wide-angle (WAXS) and small-angle (SAXS) synchrotron X-ray scattering experiments are being performed at the MS-X04AS Beamline of the SLS synchrotron at the PSI, Villigen, Switzerland.[1] In particular, the SAXS signal is important for detecting aggregates and their size, independently of their atomic structure, while the WAXS will enable us to distinguish between amorphous clumps and crystalline NPs. The feasibility of the WAXS data collection has been established in recent tests, while a SAXS experiment has been performed in situ on a horizontal liquid microjet. This was generated using a nozzle connected to a mixer. Four HPLC pumps were delivering solutions in order to obtain the desirable pH and saturation level of the system. The liquid was collected in a catcher where T and pH of the solution, under stirring, were monitored on line. After micro-jet optimization (pulsation damping, liquid jet diameter, solution composition, time delay between mixing point and liquid, X-ray beam focusing), measurements were carried out with stainless steel nozzles of 125 μm and 250 μm and with delay times of 0, 1, 60 s. The SAXS data (figure 1 shows a representative example of the data), collected using a Mythen II detector [2], revealed very clearly pre-nucleation spherical amorphous clusters of 20-30 nm size. Correlations between supersaturations, delay time after mixing, particle size, and concentrations are analyzed and discussed [3]. The data are corrected from air and sodium carbonate solution background.

The type IIB receptor protein tyrosine phosphatases (R2B RPTPs) are cell surface transmembrane proteins that engage in cell adhesion via their extracellular domains (ECDs) and cell signaling via their cytoplasmic phosphatase domains. The ECDs of R2B RPTPs form stable, homophilic, trans interactions between adjacent cell membranes. Previous work has demonstrated how one family member, PTPRM, forms homodimers; however, the determinants of homophilic specificity remain unknown. We have solved the X-ray crystal structure of the membrane-distal, N-terminal domains of PTPRK that form a head-to-tail dimer consistent with intermembrane adhesion. Comparison with the PTPRM structure demonstrates inter-domain conformational differences that may define homophilic specificity. Using small-angle X-ray scattering we determined the solution structures of the full-length ECDs of PTPRM and PTPRK, identifying that both are rigid, extended molecules that differ in their overall long-range conformation. Furthermore, we identify one residue, W351, within the interaction interface that differs between PTPRM and PTPRK and show that mutation to glycine, the equivalent residue in PTPRM, abolishes PTPRK dimer formation in vitro. These data support that homophilic specificity within this receptor tyrosine phosphatase family is driven by a combination of shape complementarity and specific but limited sequence differences.

Chitin is a major source of energy and macroelements for many organisms. An important step in its degradation is the deacetylation of chitin or its fragments. Deacetylase from the extremophile Pyrococcus chitonophagus has been analyzed by X-ray crystallography, small-angle X-ray scattering, differential scanning calorimetry, isothermal titration calorimetry and NMR to determine its structure, thermodynamics and enzymatic properties. It is a hexameric, zinc-containing metalloenzyme that retains its structural integrity up to temperatures slightly exceeding 100 °C. It removes the acetyl group specifically from the non-reducing end of the sugar substrate. Its main substrate is N,N-diacetylchitobiose but it also active, at a reduced level, toward N-acetyl-d-glucosamine or a trimer of N-acetyl-d-glucosamine units. Crystallographic analysis includes the structure of the enzyme with its main substrate approaching the active site in a monodentate manner, replacing the single water molecule that is bound at the Zn2+ cation when the ligand is absent. The Zn2+ cation remains tetrahedrally coordinated, with three of its ligands provided by the protein’s conserved His-Asp-His triad. The crystal structures are consistent with the reaction mechanism proceeding via an anhydride intermediate. Hydrolysis as the first step cannot be ruled out in a hydrated environment but no defined ‘hydrolytic water’ site can be identified in the analyzed structures.

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