X-ray crystallographic techniques

Infrared and nmr spectroscopy have been used to help distinguish between vitamins D2 and D (87—89). X-ray crystallographic techniques are used ... 

X-Ray diffraction studies on the 3-imino-l-azetine (205 Ar = p-FC6H4), show that the four-membered ring is planar with an unusually long endocyclic C=N bond (74ZN(B)399). The structure of the 1-azetine A7-oxide (275) has also been determined by X-ray crystallographic techniques (79CC993). 

These compounds have been extensively investigated because they are important nucleic acid constituents. Uracil has been shown conclusively to exist predominantly as pyrimidine-2,4-dione (99) both by a refined X-ray crystallographic technique in which the positions... 

In 1882 Baeyer and Oekonomides advanced formula 72 (R = H) for isatin on chemical grounds, but shortly thereafter the dioxo structure 73 (R H) was proposed since the ultraviolet spectrum of isatin resembled that of the N—Me derivative (73, R Me) and not that of the O—Me derivative (72, R = Me). " It was later shown, despite a conflicting report, that the ultraviolet spectrum of isatin is very similar to the spectra of both the O— and N—Me deriva-tives - the early investigators had failed to take into consideration the facile decomposition of the O—Me derivative. Although isolation of the separate tautomers of isatin has been reported, - these claims were disproved. A first attempt to determine the position of the mobile hydrogen atom using X-ray crystallographic techniques was inconclusive, but later X-ray work," dipole moment data, and especially the infrared spectrum demonstrated the correctness of the... 

Several complexes that involve intercalation of an acridine in a portion of a nucleic acid have been studied by X-ray crystallographic techniques. These include complexes of dinucleoside phosphates with ethidium bromide, 9-aminoacridine, acridine orange, proflavine and ellipticine (65-69). A representation of the geometry of an intercalated proflavine molecule is illustrated in Figure 6 (b) this is a view of the crystal structure of proflavine intercalated in a dinucleoside phosphate, cytidylyl- -S ) guano-sine (CpG) (70, TV). For comparison an example of the situation before such intercalation is also illustrated in Figure 6 (a) by three adjacent base pairs found in the crystal structure of a polynucleotide (72, 73). In this latter structure the vertical distance (parallel to the helix axis) between the bases is approximately... 

X-ray crystallographic techniques when extended to polymeric solids some interesting features of the internal structure of these substances. It was found that good majority of polymers diffract X-rays like any crystalline substance but many behave like amorphous materials giving very broad and diffuse X-ray diffraction patterns. This is seen in following figure. 

In reference 190, the authors describe the spectroscopic and X-ray crystallographic techniques they used to determine the pMMO structure. First, EPR and EX AFS experiments indicated a mononuclear, type 2 Cu(II) center hgated by histidine residues and a copper-containing cluster characterized by a 2.57 A Cu-Cu interaction. A functional iron center was also indicated by Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES). ICP-AES uses inductively coupled plasma to produce excited atoms that emit electromagnetic radiation at a wavelength characteristic of a particular element. The intensity of this emission is indicative of the concentration of the element (iron in this case) within the sample. 

The X-ray crystallographic techniques for characterizing crystals, data collection, and solving structures of proteins and protein-DNA complexes have been discussed elsewhere in this book. 

X-ray crystallographic techniques have greatly aided progress in deducing the chemistry of 1,2,4-thiadiazoles. The structures of a number of 1,2,4-thiadiazoles and 1,2,4-thiadiazolidines have been determined by x-ray techniques, and are listed in Table 1. 

Crystallographic analysis was based primarily on the results of difference Fourier maps in which the interactions between residues in the active site and the inhibitor could be characterized. During these studies, about 35 inhibitor complexes were evaluated by x-ray crystallographic techniques. It is noteworthy that the resolution of the PNP model extends to only 2.8 A and that all of the difference Fourier maps were calculated at 3.2 A resolution, much lower than often considered essential for drug design. Crystallographic analysis was facilitated by the large solvent content that allowed for free diffusion of inhibitors into enzymatically active crystals. 

The structures of five different homotropenylium or closely related systems determined using X-ray crystallographic techniques have been reported. These ions are 1891,1992,2019, 2193 and 2294. 

Using X-ray crystallographic techniques, McPhail and Miller determined that the normal conformation of the A/B ring juncture in 8 is significantly altered by the presence of the silicon atom (51). Presumably, this alteration in stereochemistry, along with the bulk of the gem-dimethyl grouping (not found in active estrogens), is sufficient to cause loss of hormonal activity. 

The isolation of single substances has to be combined with a detailed characterization and structure elucidation. For this purpose, the following three techniques were mostly applied in the past Mass spectrometry, nuclear magnetic resonance spectroscopy, and X-ray crystallographic techniques. They will be discussed in more details in the following subsections. 

In coordinately saturated donor-acceptor complexes, the formation of new bonds as described with the thallium(I) tetracarbonylcobaltate (vide supra) is not readily accomplished. However, strong charge-transfer interactions between donor and acceptor moiety may lead to partial or complete electron transfer, which can be revealed by (UV-Vis or IR) spectroscopic and X-ray crystallographic techniques. For example, CT crystals of nitrosonium (NO+) with aromatic donors show various degrees of charge transfer as detected by gradual shifts in the N-O stretch (IR)... 

Dioxaphosphorinane derivatives containing a substituent with different steric arrangement at the C5 position (529)-(532) have been prepared. Their conformations and configurations were determined by H, NMR and X-ray crystallographic techniques. Both chair-twisted-chair and chair-boat equilibra were observed in solution. X-Ray analysis revealed in one case two independent molecules per asymmetric unit, one with chair and the other one with a boat conformation (Figure 111). ... 

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