Automated crystallization

Solution-phase synthetic methods, as they were described for synthetic organic libraries, can also be applied to materials science and are devoid of the diffusion problems encountered in thin-film deposition. The reagent solutions are mixed and incubated following an appropriate procedure, and the final products are usually isolated by precipitation or crystallization. Automated liquid dispensing units with extreme precision and high rehabiUty can be used in synthetic protocols. No major differences are presented in respect to solution-phase organic library synthesis (see Section 8.2.4). Several examples are briefly illustrated below to provide a quick overview of the currently reported synthetic methods in solution for materials libraries. 

Motherwell, W. D. S., Shields, G. P. and Allen, F. H. (1999). Visualization andcharac-terization of non-covalent networks in molecular crystals automated assignment of graph-set descriptors for asymmetric molecules. Acffl Crystallogr. B, 55, 1044-56. [57]... 

The syntheses were performed on 0.1 mmol/g Pepsyn KA resins (Nova-biochem) under standard conditions (7) using a Crystal automated synthesizer (Novabiochem). The 18-mer target (H) which constituted the C-terminal section of a larger 44-residue target was originally prepared without side-chain protection for Asn or Gln and differential protection of the cysteine residues as Cys (Trt) and Cys (Acm). 

The experimental apparatus commonly involve a sealed-tube or rotating-anode X-ray source, a 2-circles (for powders) or a 4-circles (for single crystals) automated diffractometer for sample orientation with respect to the incident beam, and one or more detector banks for detection of the diffracted signal. The angular position, the integrated intensity, and the peak profile shape of the diffracted signal are important for the subsequent analysis. 

It may be desirable to predict which crystal structure is most stable in order to predict the products formed under thermodynamic conditions. This is a very difficult task. As of yet, no completely automated way to try all possible crystal structures formed from a particular collection of elements (analogous to a molecular conformation search) has been devised. Even if such an effort were attempted, the amount of computer power necessary would be enormous. Such studies usually test a collection of likely structures, which is by no means infal-... 

The crystallization kinetics defines the open time of the bond. For automated industrial processes, a fast crystallizing backbone, such as hexamethylene adipate, is often highly desirable. Once the bond line cools, crystallization can occur in less than 2 min. Thus, minimal time is needed to hold or clamp the substrates until fixturing strength is achieved. For specialty or non-automated processes, the PUD backbone might be based on a polyester polyol with slow crystallization kinetics. This gives the adhesive end user additional open time, after the adhesive has been activated, in which to make the bond. The crystallization kinetics for various waterborne dispersions were determined by Dormish and Witowski by following the Shore hardness. Open times of up to 40 min were measured [60]. 

A regularly formed crystal of reasonable size (typically >500 pm in each dimension) is required for X-ray diffraction. Samples of pure protein are screened against a matrix of buffers, additives, or precipitants for conditions under which they form crystals. This can require many thousands of trials and has benefited from increased automation over the past five years. Most large crystallographic laboratories now have robotics systems, and the most sophisticated also automate the visualization of the crystallization experiments, to monitor the appearance of crystalline material. Such developments [e.g., Ref. 1] are adding computer visualization and pattern recognition to the informatics requirements. 

The data measurement in an x-ray analysis is typically performed using a computer controlled diffractometer. Except for the crystal mounting and some preliminary procedures, the measurement of intensities is fully automated. It is the next... 

There are many variants of this system which can be envisaged as means by which the current possibilities for automation in data collection can be applied for specific purposes. There are considerable dangers in this approach in that it may be all too easy to build in restrictions which predetermine the results. These dangers, however, are not likely to be worse than those normally encountered in electron microscopy or single crystal diffraction where the one particularly "good-looking picture is taken as being "typical" of a sample. 

In the current era many medicinal chemists are unaware of the very important role of compound soUd state properties on aqueous solubility and therefore to oral absorption. In many organizations compound purification by crystallization has disappeared being replaced by automated reverse-phase HPLC purification. If medicinal chemists isolate a compound as a white powder from evaporation of... 

Visual inspechon frequently cannot differenhate between an amorphous or crystalline material, e.g. at Pfizer medicinal chemists were required to submit only crystalline and not amorphous compounds to an automated thermodynamic solubility assay. In prachce half the white powders that they produced for the assay and that they thought were crystalline were actually amorphous. Prior to 2000 the vast majority of these medicinal chemistry labs had no melting point equipment and it was only in 2000 that the pharmaceuhcal sciences department started a workshop to teach medicinal chemists the importance of solid state properhes, how to crystallize compounds and the importance of salt forms. 

Once a structure of the desired protein has been solved, it is a very rapid process to produce subsequent high-quality structures and, in fact, some groups have even linked various scripts together, or modified software tools to provide much more automated software aids to repeated crystal structure solution, such as when solving multiple ligand complexes of the same protein [7]. 

Figure A2.1 Waters ProMonix On-Line HPLC analyzer. The upper compartment door contains a keypad for programming and operation of the analyzer. The upper window allows viewing of indicator lights and a liquid crystal display that provides the operator with analyzer interface, programmed parameters, and instrument status results. The lower chamber contains the pumps, valves, injector, and detector(s) required for the chromatographic separation. The sample conditioning plate for online process monitoring is to the right of the analyzer. This is a typical process HPLC. (From Cotter, R.L. and Li, J.B., Lab Rob Autom., 1, 251,1989. With permission of VCH Publishers.)...

Now that automated diffractometers and sophisticated computer data-reduction systems have become routinely used, the task involved in conducting an X-ray crystal-structure examination has been enormously simplified. As a result, the number of structures determined each year by crystallography has escalated dramatically. The logistics of maintaining this series in the original form would now entail such stupendous demands on our space that a volume of Advances could be completely occupied by this one article alone. As the original purpose of this series... 

EXPERIMENTAL PROCEDURES AUTOMATED TURBIDIMETRIC TITRATION. A method for the automated aqueous turbidimetric titration of surfactants has been published (10) in which anionic surfactants are titrated against N-cetylpyridinium chloride to form a colloidal precipitate near the equivalence point. N-cetylpyridinium halides have a disadvantage in that they have the tendency to crystallise out of solution (15), consequently the strength of the solution may alter slightly without the knowledge of the operator, also the crystals suspended in solution may cause damage to the autotitrator. In view of these drawbacks hyamine was preferred as the titrant. 

In video microscopy, for instance, background is normally subtracted using differential interference contrast (DIC) [18]. This technique, which requires a number of manipulations from the user, may now be automated using a new method called polarization-modulated (PMDIC) [19,20], It requires the introduction of a liquid crystal electro-optic modulator and of a software module to handle difference images. PMDIC has been shown to bring improvements in imaging moving cells, which show a low contrast, as well as thick tissue samples. 

The three dimensional structure was obtained by means of single crystal X-ray diffraction. CuKa radiation, a graphite monochromator, and a photomultiplier tube were used to collect 1825 total reflections on an automated diffractometer. Of these, 1162 were used for the analysis. Figure 2 shows a computer generated drawing of halcinonide. The position of the chlorine atom was not clear from the Patterson map, but the direct method program "MULTAN" gave its position. 

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