Agreement with Crystal Structure

The slow rate is usually small enough that the contribution from structured amides is negligible, thus a peptide that is 50% a-helix and 50% random coil in the native protein should become approximately 50% exchanged and no more over the first few seconds. This makes it possible to test the extent to which a crystal structure reflects native structural ensemble of a protein in solution [6, 21] if the observed level of exchange in a particular peptide is substantially above that expected from the crystal structure, then the protein is less structured than reported. Alternatively, if the observed level of exchange is substantially lower than that expected from the crystal structure, then the protein is more structured than reported. 

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The sodium tetraborates, borax (the decahydrate) and tincalconite (the pentahydrate), have been studied by proton (33, 281), boron-11 (101, 111), and sodium-23 (102, 111) magnetic resonance, and the data is compatible with X-ray studies. Similar agreement with crystal structures has been found with the "B NMR of the hexaborate ion in tunellite (100) and pentaborates (26, 254). Wegener et al. (192, 430)... 

The effect of Lewis acids has also been examined computationally. In agreement with crystal structure determinations, Lewis acids such as BF3 normally adopt an anti structure for aldehydes. Despite the unfavorable steric effect in acetone, the calculated (MP2/6-31G) energy of complexation with BF3 is nearly as high as for acetaldehyde, presumably owing to the additional electron donation by the methyl groups. ... 

The structure and energy of D-sorbitol has been calculated using an empirical force-field method bond lengths and angles are in reasonable agreement with crystal structure X-ray and neutron... 

The Niccolite Structure. The substances which crystallize with the niccolite structure (B8) are compounds of transition elements with S, Se, Te, As, Sb, Bi, or Sn. The physical properties of the substances indicate that the crystals are not ionic, and this is substantiated by the lack of agreement with the structural rules for ionic crystals. Thus each metal atom is surrounded by an octahedron of non-metal atoms but these octahedra share faces, and the edges of the shared faces are longer than other edges (rather than shorter, as in ionic crystals). Hence we conclude that the bonds are covalent, with probably some metallic character also. 

Figure 6(b). This structure is in a fairly good agreement with the structure determined by x-ray analysis [Figure 6(c)]. This result suggests that the deformation in the crystal of the poly( ethylene oxide) chain from the uniform helix is due principally to intermolecular interaction. 

Moreover, in the case of the low level of hydration h = 0.25 the evolution of the density of states as a function of the temperature is less pronounced than in the case of h = 0.5. This is in agreement with the structural study [43] at the lower hydration (h = 0A 75), which only detected small changes when the temperature is lowered from room temperature to 77 K, and with further structural studies of low-hydrated Vycor samples. Low temperatures do not significantly affect the overall structure of the protein and the bound water molecule, and no crystallization of water has been observed. This could reflect the fact that at room temperature the interfacial water behaves like a dense supercooled liquid. 

Up to date, two AIMD studies have been performed in this field. The first dealt with self-assembled polypeptides nanotubes [28]. These systems have a variety of potential application in biochemistry and material science, from optoelectronics to the construction of drug delivery vehicles. Calculations carried out on Cyclo[D-Ala-Glu-D-Ala-Gln]2 in the crystal phase showed also in this case excellent agreement with available structural data and provided novel information on intra- and intermolecular H-bond patterns. 

The obtained polyhydroxyoligosilanes are relatively thermally stable, colorless solids that can be purified by crystallization or precipitation from pentane or acetone, except for the liquid silanol 3a, which was purified by distillation under vacuum. The NMR spectra of the compounds are rather straightforward and are in full agreement with the structures proposed. 

Treatment of a suspension of an excess of the disodium salt 1 with 3% sulfuric acid afforded a compound in 41 % yield whose elemental analysis is in agreement with the structure of the mesoionic pentazine 2.14 15N-Labeling experiments are also in agreement with the proposed structure. In the IR spectra of 2, the following bands can be observed 995, 1050, 1100, 1165, 1200 and 1410 cm - l tetrazole anion 1636 cm-1 C=N 1938 cm -1 N-N = N group of pentazine 3410-3470 cm-1 water of crystallization 3570 cm-1 N —H. 

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