X-ray crystallographic methods

Microchemical or ultramicrochemical techniques are used extensively ia chemical studies of actinide elements (16). If extremely small volumes are used, microgram or lesser quantities of material can give relatively high concentrations in solution. Balances of sufficient sensitivity have been developed for quantitative measurements with these minute quantities of material. Since the amounts of material involved are too small to be seen with the unaided eye, the actual chemical work is usually done on the mechanical stage of a microscope, where all of the essential apparatus is in view. Compounds prepared on such a small scale are often identified by x-ray crystallographic methods. 

Studies of cyclodecane derivatives by X-ray crystallographic methods have demonstrated that the boat-chair-boat conformation is adopted in the solid state. (Notice that boat is used here in a different sense than for cyclohexane.) As was indicated in Table 3.7 (p. 146), cyclodecane is significantly more strained than cyclohexane. Examination of the boat-chair-boat conformation reveals that the source of most of this strain is the close van der Waals contacts between two sets of three hydrogens on either side of the molecule. 

Peptoids based on a-chiral aliphatic side chains can form stable helices as well [43]. A crystal of a pentameric peptoid homooligomer composed of homochiral N-(1-cyclohexylethyl)glycine residues was grown by slow evaporation from methanol solution, and its structure determined by X-ray crystallographic methods. In the crystalline state, this pentamer adopts a helical conformation with repeating cis-... 

In contrast to the allyl system, where the reduction of an isolated double bond is investigated, the reduction of extensively delocalized aromatic systems has been in the focus of interest for some time. Reduction of the systems with alkali metals in aprotic solvents under addition of effective cation-solvation agents affords initially radical anions that have found extensive use as reducing agents in synthetic chemistry. Further reduction is possible under formation of dianions, etc. Like many of the compounds mentioned in this article, the anions are extremely reactive, and their intensive studies were made possible by the advancement of low temperature X-ray crystallographic methods (including crystal mounting techniques) and advanced synthetic capabilities. 

This type of side-on bending, which has been established by X-ray crystallographic methods for the related acyl complexes (r 5-C5H5)2Zr(COMe)Me (38) and (T>5-C5H5)2Ti(COMe)Cl (39), could overcome the thermodynamic objection, previously discussed, against the formation of a normal, linearly bonded formyl by CO insertion into a metal-hydride bond. Thermochemical data obtained from alcoholysis of zirconium tetralkyl species show that the mean bond energy of Zr—O is 50 kcal/mole greater than that of Zr—C (40). 

Extensive biochemical and spectroscopic studies have been undertaken on hCP in order to investigate the nature of the copper centers and their role in structure-function relationships. However, the protein is very susceptible to aggregation, proteolysis, loss of copper, and other chemical degradations and requires careful preparation and handling in these circumstances it is difficult to review all the literature objectively and comprehensively. A three-dimensional crystal structure of hCP has been reported at a nominal resolution of 3.1A [7], but this resolution has been extended to just beyond 3.0 A. This chapter will summarize some of the more important biochemical and spectroscopic studies of the protein. It will then focus on the structural results recently obtained by X-ray crystallographic methods and attempt to explain putative functions of the protein in terms of its molecular structure. 

Subsequently, 30 was oxidized by MCPBA at —10 °C to produce the corresponding oxirane, 31. This compound proved to be unstable to either mild acid or mild base. Thus, work-up of the oxidation reaction product with either dilute aqueous NaHCOs or via column chromatography on silica gel converted 31 into the corresponding cis diol, 32 (Scheme 10). The structure of 32 was established unequivocally via application of X-ray crystallographic methods [22]. 

This minor product has been now isolated and converted into the corresponding pentacyclo[5.3.0.0 .0 .0 ]decanedicarboxylic acid via intramolecular [2 - - 2] photocyclization. The material thereby obtained was converted into the corresponding cage di(p-nitrobenzyl ester) derivative via the method shown in Scheme 12. The structure of the resulting diester was established unequivocally as 40 via application of X-ray crystallographic methods (see Fig. 3.2) [28]. 

All these fuels belong to a group of high-energy-density fuels with compact molecular structure rendered by the presence of pentacyclic cages. They are stable and nonvolatile at room temperature and pressure. Three formulations are solid and the fourth is a viscous liquid. Their S3mthesis and molecular structure analysis that uses X-ray crystallographic methods have been described by Marchand [5, 6]. Their molecular structure and physical properties are presented briefly below. Measured thermophysical and thermochemical properties follow. 

A X-ray crystallographic method for detecting the transient accumulation of intermediates in enzyme catalysis, protein folding, ligand-binding interactions, and other processes involving macromolecules. The approach is premised on the well documented retention of substantial reactivity of biological macromolecules, even in the crystalline state. 

Sharma and Reed, 1976)]. In proteins the coordination number 4 is most common, where the zinc ion is typically coordinated in tetrahedral or distorted tetrahedral fashion. The coordination polyhedron of structural zinc is dominated by cysteine thiolates, and the metal ion is typically sequestered from solvent by its molecular environment the coordination polyhedron of catalytic zinc is dominated by histidine ligands, and the metal ion is exposed to bulk solvent and typically binds a solvent molecule (Vallee and Auld, 1990). The inner-sphere coordination number of catalytic zinc may increase to 5 during the course of enzymatic turnover, and several five-coordinate zinc enzyme—substrate, enzyme product, and enzyme-inhibitor complexes have been studied by high-resolution X-ray crystallographic methods (reviewed by Matthews, 1988 Christianson and Lipscomb, 1989). The coordination polyhedron of zinc in five coordinate examples may tend toward either trigonal bipyramid or octahedral-minus-one geometry. 

Carboxypeptidase A was the first zinc enzyme to yield a three-dimensional structure to the X-ray crystallographic method, and the structure of an enzyme-pseudosubstrate complex provided a model for a precatalytic zinc-carbonyl interaction (Lipscomb etai, 1968). Comparative studies have been performed between carboxypeptidase A and thermolysin based on the results of X-ray crystallographic experiments (Argosetai, 1978 Kesterand Matthews, 1977 Monzingoand Matthews, 1984 Matthews, 1988 Christianson and Lipscomb, 1988b). Models of peptide-metal interaction have recently been utilized in studies of metal ion participation in hydrolysis (see e.g., Schepartz and Breslow, 1987). In these examples a dipole-ion interaction is achieved by virtue of a chelate interaction involving the labile carbonyl and some other Lewis base (e.g.. 

Fig. 36. The three-dimensional structure of the binuclear zinc cluster of leucine aminopeptidase, as determined by X-ray crystallographic methods (Burley etai, 1990 S. K. Burley and W. N. Lipscomb, personal communication).

Salvinorin, isolated from Salvia divinorum has been shown by spectroscopic and X-ray-crystallographic methods to be a tran -neoclerodane diterpene of structure (1). Crystals of compound (1) are orthorhombic, space group P2iP2iP27 with a = 6.368 (2), h = 11.338(3), c = 30.7100 (6) A, and Z = 4. The structure was refined by least-squares to R 0.052 and R 0.056. 

Important structural information is provided by the 27A1 MAS NMR spectrum of silicalite, a porous solid isostructural with zeolite ZSM-S (see Section III,F). It has been argued (81) that silicalite has no aluminum and no cations in this structure and therefore no ion-exchange properties. In other words, it was claimed not to be a zeolite, and the original patent (80) describes it as a porous form of silica, and asserts that such aluminum as may be present is in the form of A1203 impurity. Because it is not possible to monitor the coordination of very small amounts of A1 (of the order of hundreds of ppm) by X-ray crystallographic methods, these claims could not be tested directly. Fyfe et al. (82) have demonstrated that ... 

Very few of these compounds have been characterized by X-ray crystallographic methods. As with alkylperoxo complexes, they can have a linear or a bridged structure, but no triangular form... 

Chemists have BEEN concerned predominantly with the chemistry of species that exist in their molecular ground state, often stable for an indefinite period. Structures can, in principle, be obtained by X-ray crystallographic methods, and physical data such as NMR, IR, and UV/VIS spectra can be obtained with conventional spectrometers. 

A review of the chemistry and biological actions of colchicine and its derivatives has been published.536 The 13C n.m.r. spectra of colchicine and its derivatives have been studied537-540 and the structure (119) has been determined for epoxycolchicine (prepared from colchicine by the action of water and sodium peroxide) by X-ray crystallographic methods 541 other n.m.r. studies have shown that the double-bond in dehydro-7-deacetamidocolchicine is placed as shown in (120), and not in the 6,7-position, as previously supposed.542 The treatment of deacetyl-colchicine with isocyanates affords the related substituted ureas, which can be converted into A-nitroso-derivatives by treatment with nitrous acid these have been evaluated as inhibitors of neoplasms and as virucides.543 /V-Demethylation and O-demethylation of A-methylcolchiceinamide by micro-organisms has been achieved.544... 

The basic relationship of x-ray crystallographic methods is Bragg s law, which will be derived here by considering a beam of x-rays hitting two... 

In a continuation of the series of bibliographic articles on the structures of polysaccharides as established by X-ray crystallographic methods, Sundararajan (Mississauga, Ontario) and Marchessault... 

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