Structure elucidation, by X-ray diffraction

Ribonucleases are a widely distributed family of en-zymes that hydrolyze RNA by cutting the P—O ester bond attached to a ribose 5 carbon (fig. 8.12). A good representative of the family is the pancreatic enzyme ribonuclease A (RNase A), which is specific for a pyrimidine base (uracil or cytosine) on the 3 side of the phosphate bond that is cleaved. When the amino acid sequence of bovine RNase A was determined in 1960 by Stanford Moore and William Stein, it was the first enzyme and only the second protein to be sequenced. RNase A thus played an important role in the development of ideas about enzymatic catalysis. It was one of the first enzymes to have its three-dimensional structure elucidated by x-ray diffraction and was also the first to be synthesized completely from its amino acids. The synthetic protein proved to be enzymatically indistinguishable from the native enzyme. 

M(I) to M(II), the M olefin n back-bonding becomes weaker, resulting in longer M—C and shorter C=C bonds. The M-N interactions become stronger. The structures elucidated by X-ray diffraction are consistent with DFT calculations and EPR data. 

Fig. 13.21 The structures, elucidated by X-ray diffraction, of (a) [Si802o], determined for the salt [Me4N]g[Si802o]-65H20 [M. Wiebcke et al. (1993) Microporous Materials, vol. 2, p. 55], and (b) [Sii203o], determined for the salt Ki2[a-cyclodextrin]2[Sii203o]-36H20 [K. Benner et al. (1997) Angew. Chem., Int. Ed. Engl., vol. 36, p. 743]. The silicon atoms in (a) and (b) define a cube and hexagonal prism respectively. Colour code Si, purple O, red.

A compound with the stoicheiometry Ph4P4, originally obtained by the reaction of sulphur with PhjPg, can also be prepared from PhPCl2 and potassium sulphide. The structure, elucidated by X-ray diffraction, contains a... 

The fluorination also produces Sip4 in the course of the reaction, which is very difficult to separate from the desired product, as it forms a 1 2 adduct, which could be obtained in crystalline form and therefore structurally elucidated by X-ray diffraction. The result is shown in Pig. 3. 

The successful use of these X-ray crysallographic techniques in studying the enzyme-substrate interactions of lysozyme (21) and chymotrypsin (22) has recently been reviewed by Blow and Steitz (16) and Blow (23). To date, however, these methods have had only limited application, since the detailed structures of only about ten enzymes have been elucidated by X-ray diffraction... 

During the addition of Ph2CN2 to a solutions of 44a in toluene at -78°C, the intense purple color of the diazomethane disappeared, yielding the yellow, crystalline [2+ l]-cycloadduct 52 (Eq. 15).14 Its molecular structure was elucidated by X-ray diffraction analysis (Fig. 15). 

With the rapid development of modem analysis techniques, especially the popularization of single-crystal X-ray diffraction equipment, the origin of various physical and chemical properties, and the clear elucidation of correlations between the structure and properties of Pcs have become possibilities. Together with the maturity and diversification of their modifications, many Pcs have been synthesized and their molecular structures characterized by X-ray diffraction analysis in the past 5 years [15-77]. 

The synthesis of this salt started with the enantiomerically pure 1,2-diamine 106, that was converted into the corresponding thiourea derivative 107 (Scheme 12). Exposure of the thiourea 107 to oxalyl chloride in toluene at 60 °C cleanly afforded the desired imidazolinium chloride 108. These two salts were used to produce new palladium and nickel carbene complexes. The structure of both palladium carbene complexes 96a and 96b has been elucidated by X-ray diffraction <2005CEJ1833>. 

Over 25 million compounds are known, but only about one percent of them have their crystal structures elucidated by X-ray and neutron diffraction methods. Many inorganic structures are closely related to each other. Sometimes one basic structural type can incorporate several hundred compounds, and new crystalline compounds may arise from the replacement of atoms, deformation of the unit cell, variation of the stacking order, and the presence of additional atoms in interstitial sites. 

The molecular structures of several polysulfane tetroxides of this type have been elucidated by X-ray diffraction on single crystals. The S-S bonds between the sulfone groups and the neighboring sulfane sulfur are weaker (210-214 pm) than ordinary S-S single bonds (205 pm). ... 

The first secondary stibine, the structure of which was elucidated by X-ray diffraction, was MeSjSbH . This hydride is exceptionally stable and possesses (as expected) a trigonal pyramidal structure (CSbC, 101.7°). Reaction according to equation 26 formed the planar (X-ray) Cu2Sb2 heterocycle containing essentially identical Cu-Sb distances of 270 pm in agreement with a real Cu-Sb single bond (sum of covalent radii = 268 pm) . 

---