Neutron tetrahedral intermediate

A. A. Kossiakoff, S. A. Spencer, Direct Determination of the Protonation States of Aspartic Acid-102 and Histidine-57 in the Tetrahedral Intermediate of the Serine Proteases Neutron Structure of Trypsin , Biochemistry 1981, 20, 6462-6474. 

A.A. Kossiakoff and S.A. Spencer. 1981. Direct determination of the protonation states of aspartic acid-102 and histidine-57 in the tetrahedral intermediate of the serine proteases Neutron structure of trypsin Biochemistry 20 6462-6474. (PubMed)... 

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 smdies, 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 from the imidazole.) Nucleophilic attack by the serine leads to an acylated serine through a tetrahedral intermediate. The newN-terminal end of the cleaved polypeptide chain diffuses away and is replaced by a water molecule. 

Molecular structures of the starting complex 14 and intermediates 15 (Fig. 22) and 17 were determined by X-ray analysis, and that of 18 by neutron diffraction. Intermediate 16 has not been isolated, but its methyl derivative ( i-H)Fe4(CO)i2(>7 -COCH3) was prepared separately and its structure determined by X-ray analysis.Except 14 which has a tetrahedral Fe4 core, clusters 15-18 have the butterfly structure. Details of steps between 16 and 17 are lacking and reactions beyond 18 are speculative, but the possible role of the carbide species in the reduction of CO is manifested by these studies. 

The nature of a spinel is described by a parameter X, the fraction of B atoms in tetrahedral holes some authors refer to the degree of inversion y (= 2X). For a normal spinel X = 0, and for an inverse spinel X =. Intermediate values are found (e.g. in a random spinel), and X is not necessarily constant for a given spinel but can in some cases be altered by appropriate heat treatment. For NiMn204 X varies from 0 37 (quenched) to 0-47 (slow-cooled). Values of X have been determined by X-ray and neutron diffraction, by measurements of saturation magnetization, and also by i.r. measurements. In favourable cases l.r. bands due to tetrahedral AO4 groups can be identified showing, for example, that in Li(CrGe)04 Li occupies tetrahedral positions. ... 

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