Enhanced Structure Determination

Most experimental structures are solved by X-ray crystallographic methods. The standard procedure for the determination of the 3-D arrangement of atoms involves the raw diffraction data (the intensities of the reflected X-ray beams or structure factors) and the refinement by a theoretical model based on an atomic model of the molecule in the case of proteins, this is the primary structure. The introduction of constraints is necessary for most diffraction experiments, in particular in the area of protein crystallography. Only with these can a chemically reasonable structure with the positions of all atoms in the molecule be obtained [288]. 

An energy minimization is performed using the following function for the energy  

The R-factor is used as one of the main criteria for the quality of a crystal structure 

In the case of mixed methods for the refinement, the Emodei term of Eq. (7.1) is composed of QM and MM terms  

It has been shown that this procedure may lead to impressive local resolution enhancements ]288, 289]. In addition, protonation states of metal-bound solvent molecules, or problems with the oxidation state of metals and structural disorder have been detected ]288]. 

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Glycine receptor function is modulated by alcohols and anesthetics [4]. Amino acid residue al(S267) is critical for alcohol potentiation, as mutation to small residues (Gly, Ala) enhance, and mutation to large residues (His, Cys, Tyr) diminish the ethanol effect. Glycine recqrtor modulation by Zn2+ involves structural determinants located within the large N-terminal domain. Additional glycinergic modulators include neuroactive steroids and the anthelmintic, ivermectin, which activates glycine receptors by a novel, strychnine-insensitive mechanism. 

The band shape variability of VCD coupled with its frequency leads to an enhanced sensitivity to secondary structure in folded proteins as compared to IR and ECD structure determinations alone (Pancoska etal., 1989). However, by combining IR and ECD or VCD and ECD techniques, the best structural determinations are obtained, as the strengths of each compensate for the other (Pancoska et al., 1995 Baumruk et al., 1996 Keiderling, 2000). Differentiation of different unfolded structures is also possible, but is most useful when data from several techniques are combined. The distinction of IR and VCD from ECD for unfolded proteins lies in the short-range response that dominates... 

CRYSTAL STRUCTURE DETERMINATION BY IMAGE DECONVOLUTION AND RESOLUTION ENHANCEMENT... 

The technique of X-ray crystallography has been, and will remain, indispensable for the determination of the unusual structures of S—N compounds. A more recent development is the application of N NMR spectroscopy in S—N chemistry. Despite the necessity to employ N-enriched materials for these studies, the judicious application of this technique in both structural determinations and in monitoring the progress of reactions will undoubtedly accelerate the progress of the subject. The advent of MCD spectroscopy and the use of the perimeter model have also enhanced our understanding of the electronic structures of cyclic S—N molecules. Rapid advances in this area are to be expected. 

The quantitative measurement of homonuclear (interproton) Overhauser enhancements [1, 2] is usually a very important tool for the structure determination of organic molecules, in particular when dealing with stereochemical aspects such as configuration and conformation, due to the dependence of the enhancements. 

The mechanism can only be retained if a base very much better than water is present at the enzyme active site to deprotonate the Zn(OH2)2+ species. In fact, the structure determination reveals a histidine residue with its side chain positioned approximately halfway down the -15 A-deep cleft in the protein structure within which the Zn site is located. This arrangement could act as a proton shuttle between the Zn(OH2)2+ and external solvent water, possibly via another two water molecules also found within the cleft. As a consequence, the enhancement of ligand acidity by Zn11 is more important in the kinetic than the equilibrium sense (taken from http //www.chem.uwa.edu.aU/enrolled students/BIC sect4/sect4.2.htmll. 

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