Analysis crystallographic aids

When a diffraction grating, such as a crystal, interacts with X rays, the electron density that causes this diffraction can be described by a Fourier series, as discussed in Chapter 6. The diffraction experiment effects a Fourier analysis, breaking down the Fourier series (of the electron density) into its components, that is, the diffracted beams with amplitudes, F[hkl). The relative phases a(hkl) are, however, lost in the process in all usual diffraction experiments. This loss of the phase information needed for the computation of an electron-density map is referred to as the phase problem. The aim of X-ray diffraction studies is to reverse this process, that is, to find the true relative phase and hence the true three-dimensional electron density. This is done by a Fourier synthesis of the components, but it is now necessary to know both the actual amplitude F[hkl) and the relative phase, a[hkl), in order to calculate a correct electron-density map (see Figure 8.1). We must be able to reconstruct the electron-density distribution in a systematic way by approximating, as far as possible, a correct [but so far unknown) set of phases In this way the crystallographer, aided by a computer, acts as a lens for X rays. 

In view of the efficient inhibition of a particular crystalline glycan hydrolase by the epoxylbutyl -cellobioside 7 (n = 2), we decided to prepare the deoxy iodo derivative 21 to aid in the X-ray crystallographic analysis. We soon found that, although the alkene 22 was easily available as a direct precursor to our target, the epoxide functionality had to be introduced indirectly using bromo-hydrin 23 technology any direct oxidation of 22 invariably led to some loss of the iodine atom [23]. 

The sialyltransferases investigated were found to accept many variations at C5 and C9 of the sialic acid moiety. Halcomb and Chappell confirmed this fact with studies of sialyltransferases from different sources [45]. The C9- and C5-modified substrates in Scheme 37 were good substrates. However no activity was detected when compounds 111 and 117 were assayed. In addition to these examples, other CMP-NeuAc analogs that have been prepared and successfully transferred include C9 fluorescent compounds, C9 thioacetyl, C5 thioacetyl, and C4 dexoy [37]. A particularly useful modification introduced by the Wong lab incorporates a mercury atom at C9 (Scheme 38) [48]. Transfer of this sialic acid to glycoproteins could gready aid in X-ray crystallographic analysis of these biomolecules. 

Owing to the availability of only small amounts of material, the structure of another novel alkaloid, lycopecurine (19 R1 = OH, R2 = H) from Lycopodium alopecuroides has been elucidated with the aid of X-ray crystallographic analysis. A future report will deal with the isolation of dehydrolycopecurine (19 R1 + R2 = O) from L. inundatum, and its correlation with lycopecurine.6... 

Several acetylated glycals have been subjected to this reaction, and the main products obtained after deacetylation are (despite an initial report to the contrary ) the 2,6-anhydro-3-deoxyalditols. Thus, for example, 1,5-anhydro-4-deoxy-D-lj/xo-hexitol (42) (a 2,6-anhydro-3-deoxyhexitol), and the it-ribo isomer, were isolated in high combined yield, in the ratio 1 0.7, from di-O-acetyl-D-arabinal, after hydroformylation followed by deacetylation and reduction of the formyl compounds (which are produced together with the anhydrodeoxyalditols). Structural analyses of the products were carried out with the aid of periodic acid degradations, nuclear magnetic resonance spectroscopy, and x-ray crystallographic analysis. ... 

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