Neutron diffraction studies residues

Conformation. Neutron diffraction studies of sucrose revealed the presence of two strong intramolecular hydrogen bonds 0-2—HO-1 and 0-5—HO-6 in the crystal form (7,8). These interactions hold the molecule in a weU-ordered and rigid conformation. The two rings are disposed at an angle close to 90°, with the glucopyranosyl and fmctofuranosyl residues adapting chair and T" twist conformations, respectively. 

FIGURE 9.5. Nuclear density on a computer graphics screen. Shown is a histidine residue in a neutron-diffraction study. (Courtesy Benno P. Schoenborn.)... 

In many database systems it is common practice to record the presence/absence of certain specified features in terms of the on/off (1/0) setting of bits in a bit-map. Items in the CSD that are suitable for such space-saving treatment are e.g. yes/no flags indicating a neutron diffraction study, absolute configuration determination, residual errors in entry, etc. A total of 124 bits record information in this way and... 

After chymotrypsin has bound its protein substrate, the serine residue at position 195 is ideally situated to attack the acyl carbon of the peptide bond (Fig. 24.19). This serine residue is made more nucleophilic by transferring its proton to the imidazole nitrogen of the histidine residue at position 57. The imidazolium ion that is formed is stabilized by the polarizing effect of the carboxylate ion of the aspartic acid residue at position 102. (Neutron diffraction studies, which show the positions of hydrogen atoms, confirm that the carboxylate ion remains as a carboxylate ion throughout and does not actually accept a proton... 

The mechanism by which serine peptidases, particularly serine endopep-tidases (EC 3.4.21), hydrolyze peptide bonds in peptides and proteins has been extensively investigated by X-ray crystallography, site-directed mutagenesis, detection of intermediates, chemical modification, H-NMR spectroscopy, and neutron diffraction [2-14], These studies revealed that all serine peptidases possess a catalytic triad, composed of a serine, a histidine, and an aspartate residue, and a so-called oxyanion hole formed by backbone NH groups. 

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