Hydrogen crystallographic studies

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. 

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-... 

The interest in the structures of simple R2Si(OH)2 compounds lies in the fact that one of them, Bu 2Si(OH)2, forms a discotic liquid crystalline phase (308,309). Despite many attempts, it has not proved possible to obtain crystals of Bu 2Si(OH)2 suitable for a crystallographic study, the material obtained from various solvents usually being of a fine fibrous nature. The discotic phase of Bu 2Si(OH)2 has been proposed (309) to be due to the formation of dimeric disks of molecules which remain on breaking the interdimer hydrogen bonds in a structure of type 65 at the transition between crystal and mesophase. As has been described, structure type 65 is found for several diols similar to Bu 2Si(OH)2, and it is thus quite likely that Bu 2Si(OH)2 does indeed have the proposed structure. 

The exact nature of the lesion in DNA is unknown, and so is the type of DNA that is attacked. Recent X-ray crystallographic studies, as well as other physicochemical studies, have made it clear that DNA is not simply a polynucleotide, folded as Watson and Crick (106) proposed. There are three main conformational types of DNA they each keep the hydrogen-bonded bases in the center of the helix, but may tilt them by a "propellor twist," may slide them from the center of the helix in the plane of the base pairs, and may vary the amount of rotation from one base pair to the next up the helical axes. 

The reaction mechanism in phosphine-Rh complex catalyzed hydrogenation was elucidated by Halpern16 and Brown and Maddox17 on the basis of NMR and X-ray crystallographic studies of the reaction intermediates, as well as detailed kinetic analyses. The well-recognized mechanism proposed by Halpern is presented in Figure 6-3. 

In the course of the synthetic efforts to isolate 59, a co-crystal containing the starting aminotriazole compound 60 could be obtained depending on the mode of purification (chromatography or crystallization) <2004AXC733>. Crystallographic study of these products showed that the molecules 59 are linked into simple chains, whereas in 60 the components are linked by combination of two-center N-H- N and N-H- O hydrogen bonds. 

When we study the case of cyclodecane, we find that x-ray crystallographic studies reveal that the most stable conformation has C—C bond angles of 117° which indicates some angle strain from the normal 109°28. So the wide bond angles allow the molecule to expand and minimise unfavourable repulsions between hydrogens across the ring. 

Much of the impetus for the study of hydrogen bonded network structures of 24 and bipy or other linear N-donor ligands has come from the desire to perform time-resolved crystallographic studies of photoactive guest species embedded within the network as a guest [58,62,63]. Embedding a photoactive species within a network structure effectively dilutes it in the solid state. The advantages of this include improved uniformity of illumination of the crystal, less photons are... 

In all these reactions, crystallographic studies demonstrated the crystal control responsible for formation of the observed products. Thus, the prereaction ketone oxygen-steroidal hydrogen distance is in all cases 3.4 0.5 A, the ketonic carbon-steroidal (to-be-attacked) carbon distance is of the order of 4 A, and the... 

Recently-reported pyrimido[4,5-, pyrimidine 168, prepared as in Equation (90), has been reported as a Janus-AT heterocycle, where the molecule is designed to have the hydrogen-bonding characteristics of both a diaminopurine and uracil/thymine <2007JOC466>. Crystallographic studies show that 168 forms a linear formate-bridged supra-molecular array. 

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