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Crystallization processes under different conditions are of fundamental interest in chemistry, pharmacy, and medicine. Therefore, we have studied the formation of micro- and nanosized crystals using water-caffeine (1,3,7-trimethyl-1H-purine-2,6(3H,7H)-dione) solutions under ambient conditions as a relevant model system. When droplets of an aqueous caffeine solution evaporate and eventually dry on surfaces (glass, polystyrene, and polyester), stable coffee tabletop rings with a perimeter of typically 3 mm are formed after 20 to 50 min. Using a micro focus X-ray beam available at the BESSY µSpot-beamline, the fine structure of different caffeine needles can be distinguished. Unexpectedly, both crystal modifications (α- and β-caffeine) are present, but locally separated in these rings. Furthermore, AFM studies reveal the presence of even smaller particles on a nanometer length scale. To eliminate influences of surface irregularities from the crystallization process, acoustic levitation of liquid samples was employed. Such levitated droplets are trapped in a stable position and only surrounded by air. The solvent in an ultrasonically levitated drop evaporates completely, and the resulting crystallization of caffeine was followed in situ by synchrotron X-ray diffraction. In this case, the diffraction pattern is in accordance with pure α-caffeine and does not indicate the formation of the room temperature polymorph β-caffeine. Hence, our investigations open new vistas that may lead to a controlled formation of cocrystals and novel polymorphs of micro- and nanocrystalline materials, which are of relevance for fundamental studies as well as for pharmaceutical and medical applications.
For the investigation of small sample volumes, the use of an acoustic levitator was tested as a `sample holder' for hovering droplets in a synchrotron beam. It might be advantageous to use levitated droplets instead of samples confined in solid holders, especially for the study of crystallization processes where the influence of containing walls has to be minimized. In a first experiment, the crystallization of sodium chloride in a small droplet of aqueous solution has been followed with a time resolution of 30 s. The collected diffraction peaks are compared with data in the ICSD database.
Many of todays analytical problems are characterized through small sample volumes and can only be solved through a corresponding miniaturisation of the analytical instrumentation. Handling of small sample volumes is inherently difficult due analytical blanks, contamination, and sorption processes on the walls of the containers which are employed during analytical procedures. Acoustic levitation is a powerful tool for contact-less sample handling of solid, liquid, and certain gaseous samples. In addition, levitation permits a chemical pre-treatment such as enrichment, extraction, and derivatisation as well as combination with other analytical techniques such as optical spectroscopy or mass spectrometry. Acoustically levitated liquid and solid samples are typically in a range between 0.005 - 5 µl (diameter 0.2 - 2 mm respectively) and are suspended in a gaseous environment by a stationary ultrasonic field (nodes of a standing wave). Acoustic levitation avoids sample contamination and sorption processes by container walls, but suffers from evaporation and loss of solvents. To balance evaporation and condensation on levitated drops during the experiments, techniques of contact-less droplet size monitoring and solvent and reagent supply have been developed. The advantages of acoustic levitation are demonstrated for on line analysis of crystallisation in a levitated droplet. The evaporation and formation of crystals is observed via time-resolved X-ray diffraction at BESSY. In this way, a high-throughput analysis for polymorphs seems feasible. Future work will be devoted to adaptation of a binding assay with receptors, antibodies or enzymes. Special emphasis will be on homogeneous immunoassays such as fluorescence polarisation immunoassays (FPIA) or apo-enzyme reactivation immunoassay systems (ARIS) combined with laser-induced fluorescence (LIF). This offers the opportunity to study the kinetics of the bio-macromolecule interactions without any wall effects which can have a significant influence on the apparent performance of bioanalytical assays.
Amyloid fibrils are polymers formed by proteins under specific conditions and in many cases they are related to pathogenesis, such as Parkinson’s and Alzheimer’s diseases. Their hallmark is the presence of a β-sheet structure. High resolution structural data on these systems as well as information gathered from multiple complementary analytical techniques is needed, from both a fundamental and a pharmaceutical perspective. Here, a previously reported de novo designed, pH-switchable coiled coil-based peptide that undergoes structural transitions resulting in fibril formation under physiological conditions has been exhaustively characterized by transmission electron microscopy (TEM), cryo-TEM, atomic force microscopy (AFM), wide-angle X-ray scattering (WAXS) and solid-state NMR (ssNMR). Overall, a unique 2-dimensional carpet-like assembly composed of large coexisiting ribbon-like, tubular and funnel-like structures with a clearly resolved protofilament substructure is observed. Whereas electron microscopy and scattering data point somewhat more to a hairpin model of β-fibrils, ssNMR data obtained from samples with selectively labelled peptides are in agreement with both, hairpin structures and linear arrangements.
The impact of the ovo proteins ovalbumin and lysozyme—present in the first stage of egg shell formation—on the homogeneous formation of the liquid amorphous calcium carbonate (LACC) precursor, was studied by a combination of complementing methods: in situ WAXS, SANS, XANES, TEM, and immunogold labeling. Lysozyme (pI = 9.3) destabilizes the LACC emulsion whereas the glycoprotein ovalbumin (pI = 4.7) extends the lifespan of the emulsified state remarkably. In the light of the presented data: (a) Ovalbumin is shown to behave commensurable to the 'polymer-induced liquid precursor' (PILP) process proposed by Gower et al. Ovalbumin can be assumed to take a key role during eggshell formation where it serves as an effective stabilization agent for transient precursors and prevents undirected mineralization of the eggshell. (b) It is further shown that the emulsified LACC carries a negative surface charge and is electrostatically stabilized. (c) We propose that the liquid amorphous calcium carbonate is affected by polymers by depletion stabilization and de-emulsification rather than 'induced' by acidic proteins and polymers during a so-called polymer-induced liquid-precursor process. The original PILP coating effect, first reported by Gower et al., appears to be a result of a de-emulsification process of a stabilized LACC phase. The behavior of the liquid amorphous carbonate phase and the polymer-induced liquid-precursor phase itself can be well described by colloid chemical terms: electrostatic and depletion stabilization and de-emulsification by depletion destabilization.
New experimental setup for on-line monitoring of the synthesis of oxides at the µ-spot Beamline
(2007)
A new setup is presented which enables simultaneous wide- and small-angle X-ray scattering (WAXS/SAXS) and Raman spectroscopic experiments during the synthesis of Mo-based mixed oxide catalyst precursors at the µ-spot beamline at the Berliner Elektronenspeicherring-Gesellschaft für Synchrotronstrahlung (BESSY). Furthermore, we report about the separate monitoring of the same reactions under comparable conditions by simultaneous combined ATR/UVvis/Raman spectroscopic measurements. For testing the performance of both experimental setups two syntheses were described comprising the precipitation of metal molybdates by mixing solutions of metal nitrates and ammonium heptamolybdate. Additionally, the effect of H3PO4 admixture on precipitation was investigated. The combined evaluation of spectroscopic and WAXS/SAXS data allows the discrimination between different molybdate species appearing in solution and precipitate. Furthermore, these molybdate species could be assigned to separate phases of different crystallinity.