X-ray crystallographic study

70 (with Ca +) 2AEJ, 2.10 (without Ca +) 2AEM. 2.80 (RCK domain) lORQ, 3.20 (contains Y46C mutation) 

Burkholderia Inward- K channel with 1P7B, 3.65 2003 Closed 2 

Aeropyrum pernix Voltage- K channel 2A0L 20 2005 Open r w 

Streptomyces lividans Voltage- K -channei 2IH1, 2.40 21 2006 Open  

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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... 

Chemical conversion of compounds to intermediates of known absolute configuration is a method routinely used to determine absolute configuration (86). This is necessary because x-ray analysis is not always possible suitable crystals are required and deterrnination of the absolute configuration of many crystalline molecules caimot be done because of poor resolution. Such poor resolution is usually a function of either molecular instability or the complex nature of the molecule. For example, the relative configuration of the macroHde immunosuppressant FK-506 (105) (Fig. 8), which contains 14 stereocenters, was determined by x-ray crystallographic studies. However, the absolute configuration could only be elucidated by chemical degradation and isolation of L-pipecoUc acid (110) (80). 

Kanglemycin. Kanglemycia (74) (Fig. 8) is isolated from the fermentation broth filtrate of Nocardia mediterranei var kanglensis (1747-64) and its stmcture determiaed by x-ray crystallographic studies. The antibiotic is active against gram-positive bacteria (28). 

X-ray crystallographic studies (59) have defined the conformations and hydrogen bonding of the tetracyclines under nonpolar and polar conditions. These are shown ia Figure 3. It is beheved that the equiUbrium between the 2witterionic and nonioni2ed forms is of importance for the broad-spectmm antibacterial activity, membrane permeation, and pharmacokinetic properties. 

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). 

The basic pteridine nucleus (1) consists, according to X-ray crystallographic studies (75JCS(P2)40), of a more or less planar molecule with no unexpected features in so far as... 

High accuracy molecular dimensions for the planar parent heterocycles in the gas phase have been obtained by microwave spectroscopy and are recorded in Table 2. These values have been corroborated for furan by a low-temperature X-ray crystallographic study... 

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. 

Schlichting, L, et al. Time-resolved x-ray crystallographic study of the conformational change in Ha-ras p21 protein on GTP hydrolysis. Nature 345 309-315,... 

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. 

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. ... 

According to an X-ray crystallographic study, 2-(A-phenylamino)thia-zole forms in the crystal dimers of the imino tautomers, which are linked together by the intramolecular NH—N bonds [77AX(B)106]. 

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. 

Tlie IR and and NMR spectra for thiazolo[5,4-h]quinolines 86 demonstrate their existence in the hydroxy form (96T11929). Tlie preference for IH,6H tautomer 87a in water and in the solid state is supported by and NMR spectroscopy, ab initio (STO-3G ) and X-ray crystallographic studies [75AX(B)1427 92H1399]. 

Tile data on H- and C-cliemical shifts, H-relaxation rates, and protonation for 2-arylimidazo[l,2- ]imidazoles (107) and their methyl derivatives 108 and 109 indicate that 107 exists predominantly as the 1/7 tautomer 107a in CDCI3 and as the IH tautomer 107b in DMSO-dg or CD3OD-D2O (91MRC1147). An X-ray crystallographic study has demonstrated the IH tautomer 107a in the lattice. 

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