Structure crystallographic studies

The primary structure of a peptide is its ammo acid sequence We also speak of the secondary structure of a peptide that is the conformational relationship of nearest neighbor ammo acids with respect to each other On the basis of X ray crystallographic studies and careful examination of molecular models Linus Pauling and Robert B Corey of the California Institute of Technology showed that certain peptide conformations were more stable than others Two arrangements the a helix and the (5 sheet, stand out as... 

Baeyer assigned a cis-form to indigotin, but x-ray crystallographic studies indicated that the dye molecule has a center of symmetry that is only possible if the molecule has a trans-configutation (119). Many derivatives of indigotin have been prepared that would not have been possible if indigotin had a cis-structure, eg, (36) (120). 

This electron microscopy reconstruction has since been extended to high resolution (3 A) where the connections between the helices and the bound retinal molecule are visible together with the seven helices (Figure 12.3c). The helices are tilted by about 20° with respect to the plane of the membrane. This is the first example of a high-resolution three-dimensional protein structure determination using electron microscopy. The structure has subsequently been confirmed by x-ray crystallographic studies to 2 A resolution. 

One of the most striking results that has emerged from the high-resolution crystallographic studies of these icosahedral viruses is that their coat proteins have the same basic core structure, that of a jelly roll barrel, which was discussed in Chapter 5. This is true of plant, insect, and mammalian viruses. In the case of the picornaviruses, VPl, VP2, and VP3 all have the same jelly roll structure as the subunits of satellite tobacco necrosis virus, tomato bushy stunt virus, and the other T = 3 plant viruses. Not every spherical virus has subunit structures of the jelly roll type. As we will see, the subunits of the RNA bacteriophage, MS2, and those of alphavirus cores have quite different structures, although they do form regular icosahedral shells. 

X-ray crystallographic studies of serine protease complexes with transition-state analogs have shown how chymotrypsin stabilizes the tetrahedral oxyanion transition states (structures (c) and (g) in Figure 16.24) of the protease reaction. The amide nitrogens of Ser and Gly form an oxyanion hole in which the substrate carbonyl oxygen is hydrogen-bonded to the amide N-H groups. 

The influence of electron-count on cluster geometry has been very elegantly shown by a crystallographic study of the deep-red compound [K(ctypt)]g [Ge9]- [Ge9] .2.5en, prepared by the reaction of KGe with cryptand in ethylenediamine. [Ge9] has the C4, unicapped square-antiprismatic structure (10.10c) whereas [Ge9]- , with 2 less electrons, adopts a distorted Dit, structure which clearly derives from the tricapped trigonal prism (p. 153).The field is one of... 

X-Ray crystallographic studies of solid compounds have been used to infer the position in the structure of potentially tautomeric hydrogen atoms. These positions are either found directly by high pre-... 

An X-ray crystallographic study of 2-hydroxy-4,6-dimethylpyrimi-dine led to no conclusions regarding its structure. Because of the rapid exchange of the NH protons of pyrimidin-2-one both in dimethyl sulfoxide and in water, nuclear magnetic resonance spectroscopy does not afford positive evidence for either the oxo or the hydroxy formulation. The statement that 4,6-dimethylpyrimidin-2-one had been isolated in two modifications, 94 and 95, was soon disproved. ... 

Similarly to aminoazoles, diaminoazoles prefer their amino forms 210 for 3,5-diaminopyrazole [76AHC(S1), p. 460 84CHEC-I(5)167 96CHEC-11(3)1] 211 for 3,5-diamino-l,2,4-triazole and 212 for 3,5-diaminoisothiazole [76AHC(S1), p. 460]. Ilie triamino structure 213 was proven by an X-ray crystallographic study of 4-aminoquinazine hydrobromide [73JCS(P2)lj. 

The resulting derivatives, unlike most organotin carboxylates, are hydrolyzed relatively readily in air. An X-ray crystallographic study 399) showed that trimethyltin glycinate has an intermolecularly associated, polymeric structure, with bridging amino groups. 

Two versions of the Ba-Cd phase diagram are very similar and, but for some minor differences in the critical point parameters, differ only in the stoichiometry of the compound at 17 at% Ba-Ba2Cd9 or BaCd . Since ref. 5 includes a structural analysis of the intermediate compounds, this phase diagram is recommended. There are five intermediate compounds in the system. Four of these melt congruently, BaCd (SSS C), BaCdj (642°C), Ba2Cd, (620°C) and BaCd (672°C) the other, BajCd (409°C), is formed in a peritectic reaction. The existence of Ba2Cd BaCd, BaCdj, and BaCd], has been verified in separate crystallographic studies. 

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