X-ray crystal studies

A relationship between the redox state of an iron—sulfur center and the conformation of the host protein was furthermore established in an X-ray crystal study on center P in Azotobacter vinelandii nitroge-nase (270). In this enzyme, the two-electron oxidation of center P was found to be accompanied by a significant displacement of about 1 A of two iron atoms. In both cases, this displacement was associated with an additional ligation provided by a serine residue and the amide nitrogen of a cysteine residue, respectively. Since these two residues are protonable, it has been suggested that this structural change might help to synchronize the transfer of electrons and protons to the Fe-Mo cofactor of the enzyme (270). 

A ruthenium(VI) nitrido complex containing a tetrapyrrolic macrocycle ligand, [Ru (N)(L)] (82) (H4L = mcio-octamethylporphyrinogen), has been synthesized by the reaction of diphewldiazomethane with [Ru"(L)] , which is prepared from [Ru(cod)(Cl)2] and Na4(L) 4THF. X-ray crystal studies reveal that [Ru (N)(L)] has a Cjv symmetry, and it has the usual saddle conformation, with the metal atom displaced 0.482 A out of the N4 mean plane. The reactivities and redox chemistry of this complex are summarized in Scheme 12 (see also Section 5.6.5.2). 

One of the simplest molecules in which it is customary to invoke outer d-orbital participation in a bonding is the triiodide ion. This ion has been observed with a large number of different cations and X-ray crystal studies have revealed both symmetrical and unsymmetrical species, although in both forms it is essentially linear. If the bonding involves only the valence p-orbitals then the Hiickel orbitals for the symmetric species are those shown in Fig. 12. This description is exactly equivalent to the covalent-ionic resonance formulation VII. 

Controlled O2 oxidation of the MoIII,ni IV,lv tetramer yields a product which titrates for MoIV and from an incomplete X-ray crystal study appears to be basically the same tetrameric structure, except that all the bridging ligands are now fi-oxo. This Mo tetramer is also an intense red colour, Figure 10. The reduction potential for the couple involving the two tetrainers is +0.07 V. Rapid reactions of both tetramers with 02 are of interest in that there is no site for binding unless the coordination number increases. Mild oxidants such as [Co(NH3)5Cl]2+ react rapidly (fi/2 < 1 min) with both tetramers to give the Mo)1 dimer. 

However, an X-ray crystal study of one of the derivatives of 21, 3-phenyl-5-(4-methoxyphenyl-1,2-oxatellurolyl-l-ium chloride 21 (R = Ph, Ar = C6H4OMe-4) justifies its T-shaped tricoordinate tellurium structure (83JA875), which is portrayed in Fig. 4. 

Another remarkable influenza virus sialidase inhibitor, derivative 25 [119], was developed based on a pyrrolidine core by Abbott Laboratories [119, 123], X-ray crystal studies surprisingly observed hydrophobic interactions of the d.s-propenyl group within S2, which is normally involved in charge-charge interaction as with the guanidino moiety in zanamivir [69], The side-chain methyl and methoxy groups of... 

The dissymmetic (C2 symmetry) dihydrodinaphthothiepin 43 and related bridged diaryl derivatives are of interest because the stereoisomers can be separated (atropisomers) and have potential applications as chiral reagents. An X-ray crystal study of compound 43 shows that it has a cisoid arrangement around the biaryl bond with an interplane angle of 66.1 <1995T787, CHEC-III(13.03.3.1)102>. 

In principle, hydrolitic exchange (35, 36) also might influence the adsorption capacity however, such an assumption is difficult to bring into agreement with the data of x-ray crystal studies of calcium and... 

Few X-ray crystal studies on 1,3-dithiolium system have been performed. The structures of 2,4-diphenyl- and 2,4,5-triphenyl-l,3-dithiolium perchlorates have been reported. 

From a variety of X-ray crystal studies (e.g., didehydro-QCI/QCD and didehydro-quinidine), we have established that alkyne quinuclidines are twisted consistently less than the vinyl and ethyl analogues. Overall torsion angles, that is, the sum of the three torsion angles, vary from 26 to 49° compared with up to 65° for ethyl and 60° for ethenyl derivatives, respectively (Figure 12.3). Fine-tuning of nitrogen polarity (and basicity), which may be important for catalysis and perhaps pharmacological activity, is thus possible ([28] R. Wartchow, personal communication). 

Structural characterization of f3-2 -pyridylaminocrotonoyl-2-pyridylamide. Gawi-necki and co-workers reported the results of a solution and X-ray crystal study of p-2 -pyridylaminocrotonoyl-2-pyridylamide (180) that have yielded interesting insights into the solution behavior of the molecule. The amide studied has a dimeric form whereas the monomer molecules are held together by two intermolecular hydrogen bonds. The NMR data also show that there is an intramolecular hydrogen bond in each monomer subunit. 

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