Acceptable crystal structures

The canons outlined by the correlations presented in the previous sections of this chapter lack any detailed predictive power, but can be used at least to outline the boundaries within which a crystal structure may be considered acceptable. The two necessary, if not sufficient, requisites for an acceptable organic crystal structure are  

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The 3D crystal structure of Dsm. baculatum [NiSeFe] hydrogenase has been solved 185), and it was indicated that the enzyme contains three [4Fe-4S] centers. A cysteine (replacing a proline usually found near the [3Fe-4S] core) provides an extra ligand, enabling the acceptance of a fourth iron site at this cluster. 

While wild-type PAMO was unable to convert 2-phenylcyclohexanone efficiently, all deletion mutants readily accepted this ketone as substrate. All mutants also displayed a similar thermostability when compared with the parent enzyme. The most active mutant (deletion of S441 and A442) was used for examining its enantioselective properties. It was found that the mutant preferably formed the (/ )-enantiomer of the corresponding lactone E = 100). While CHMO also shows a similar enantioselective behavior, this PAMO deletion mutant is a better candidate for future applications due to its superior stability. This clearly demonstrates that PAMO can be used as parent enzyme to design thermostable BVMO variants. It also illustrates that the available crystal structure of PAMO will be of great help for BVMO redesign efforts. ... 

Aleshin and coworkers (49) have reported the X-ray crystal structure at 2.2-A resolution of a G2-type variant produced by Aspergillus awamori. Meanwhile, an attempt was made to determine the amino acid residues that participate in the substrate binding and catalysis provided by G2 of A. niger (52). The results of the chemical approach indicated that the Asp-176, Glu-179, and Glu-180 form an acidic cluster crucial to the functioning of the enzyme. This conclusion was then tested by site-specific mutagenesis of these amino acid residues, which were replaced, one at a time, with Asn, Gin, and Gin, respectively (53). The substitution at Glu-179 provided an inactive protein. The other two substitutions affected the kinetic parameters but were not of crucial importance to the maintenance of activity. The crystal structure (49) supports the conclusion that Glu-179 functions as the catalytic acid but Asp-17 6 does not appear to be a good candidate for provision of catalytic base. Thus, there still exists considerable uncertainty as to how the disaccharide is accepted into the combining site for hydrolysis. Nevertheless, the kind of scheme presented by Svensson and coworkers (52) almost surely prevails. 

When Herman Mark first evaluated the crystal structure of rubber (with E. A. Hauser) and cellulose (with J. R. Katz) in 1924 and 1925, it was generally accepted that these materials were low molecular weight or monomeric. The unusual properties of these substances, now known to be related to high molecular weight, were then attributed to aggolomeration or "association" of the low molecular weight precursors. A common explanation for the associations were secondary forces such as Johannes Thiele s partial valences. 

Most hydrogen-bond-containing crystal structures are homomolecular, i.e. both the hydrogen-bond accepting and donating functionality exist in the same molecule. Examples of such co-crystals are urea and thiourea derivatives as well as non-cyclic imides156,179,180. 

Ti ossbauer spectroscopy is the term now used to describe a new ana-lytical technique which has developed using y-ray nuclear resonance fluorescence or the Mossbauer effect. For most of the time since Rudolf Mossbauer s discovery in 1958 it was the physicist who utilized this new tool. Starting approximately in 1962 some chemists realized the potential of this new technique. Since then they have applied Mossbauer spectroscopy to the study of chemical bonding, crystal structure, electron density, ionic states, and magnetic properties as well as other properties. It is now considered a complimentary tool to other accepted spectroscopic techniques such as NMR, NQR, and ESR. 

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