Growing derivative crystals

Once the structure of pumiliotoxin 25ID was known from crystal structure analysis, the structures of a number of other alkaloids of the pumiliotoxin A class were derived from mass spectra and NMR spectra (see e.g., Fig. 15). A problem still remains in assigning the stereoconfigurations of the hydroxyl groups in the long side chains. This kind of problem could be easily resolved by crystallography, if it were possible to grow a crystal. 

Although a powder neutron diffraction analysis of PtF carried out by Ibers and Hamilton soon confirmed the formulation tentatively provided by the X-ray data (Ref. 13), they were obliged to tilt the O2 and three-fold disorder it in order to arrive at a suitable 0-0 distance. Recently (see Ref. 121), using anhydrous liquid HF as a solvent, we succeeded in growing single crystals of O RuF. With the more favorable heavy atom scattering factors of this salt we were able to prove the Ibers and Hamilton conjecture and also derive the 0-0 distance directly from the refinement. 

Note, however, that these agglomerates show evidence of growing out from a core of an aqueous derived crystal or from drops of oil that are known to form readily before the onset of crystallization. 

Ammonium sulphate is commonly used as a precipitant because of its great solubility and ready availability. It can liberate ammonia, which reacts with heavy atom derivatives. This problem can in some circumstances be alleviated by changing the mother liquor of the crystals to phosphate. Ammonium sulphate caused further problems in the analysis of the crystal structure of oxyhaemoglobin. In the presence of this salt and under irradiation, crystals of oxyhaemoglobin were oxidised rapidly to aqua-met-haemoglobin. The problem was solved by growing the crystals under usual conditions [34] and then transferring them to 3 M phosphate [16]. 

Alternatively, one can use sitting drop vapor diffusion where the drop is placed in a raised depression within the well. The sitting drop vapor diffusion experiment requires special plates or devices but set-up and retrieval of crystals is much simpler. Once the plate is prepared, it is then incubated at either 4 or 18°C for a period of time ranging from a few days to weeks. During incubation, each well of the plate is checked periodically under a microscope for the presence of crystals or precipitate. Conditions derived from the initial screen are usually further refined by adjusting pH, additives, and concentrations to grow suitable crystals for data collection and structnre analysis. Crystal seeding (Stura et al., 1992) can also be explored to improve the crystal size and quality. 

The double-hump shape of the free energy curve in Figure 15.3a has one other implication of interest to Earth scientists, metallurgists, materials scientists, and other people who work with solid solutions. This is a more subtle point, important in growing synthetic crystals or in analyzing exsolution properties in minerals. The two points marked Si and S2 are flex points of the free energy curve. Flex points (like the center point in the letter S) can be found from the second derivative of the curve, i.e., where... 

One of the more important uses of OM is the study of crystallization growth rates. K. Cermak constructed an interference microscope with which measurements can be taken to 50° (Ref 31). This app allows for study of the decompn of the solution concentrated in close proximity to the growing crystal of material such as Amm nitrate or K chlorate. In connection with this technique, Stein and Powers (Ref 30) derived equations for growth rate data which allow for correct prediction of the effects of surface nucleation, surface truncation in thin films, and truncation by neighboring spherulites... 

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