Showing papers in "Crystallography Reviews in 2006"
TL;DR: An overview of the early literature on C-H-O hydrogen bonding in the solid state is given in this article, where the structural characteristics and physical consequences of C-O-O bonds are described.
Abstract: An overview is given on C-H-O hydrogen bonding in the solid state Following a short survey of the early literature, the general properties of C-H-O bonding are discussed The structural characteristics and some physical consequences of C-H-O bonds are described A number of special systems are discussed in greater detail; these include water acceptors, inclusion complexes, and some biological structures
224 citations
TL;DR: Mechanical questions can now be addressed with the structural data available for atomic resolution enzyme complexes as well as from neutron diffraction data that have recently become available, providing new insight into the design of novel inhibitors that can target specific species with high selectivity of binding.
Abstract: The ubiquitous enzyme dihydrofolate reductase (DHFR) is responsible for the reduction of 5,6-dihydrofolate to 5,6,7,8-tetrahydrofolate in an NADPH-dependent manner. It is also a key pharmacological target for the treatment of cancer, as well as bacterial and opportunistic pathogenic infections. Interest in the design of potent and selective antifolate inhibitors has made DHFR one of the most studied enzymes, in particular its structural and biochemical properties. This review surveys more than 129 DHFR solution and crystal structures currently (02/07) reported in the Protein Data Bank representing 15 species of enzyme. Comparison of these DHFR sequences shows that while there is a high sequence homology among vertebrate species (75–95%), there is only about 30% homology between vertebrate and bacterial species. Despite the highly conserved nature of the ligand and cofactor binding sites, DHFR can bind a wide range of compounds that can have a high degree of flexibility. The enzyme itself can also undergo ...
49 citations
TL;DR: In this article, a system of structure symbols to classify and group uranyl-organic complexes is devised, which is applied to describe 604 published structures of uranylorganic complexes, then to consider the complex chemistry of URanyl carboxylates.
Abstract: A system of structure symbols to classify and group uranyl-organic complexes is devised. The method is applied to describe 604 published structures of uranyl-organic complexes, then to consider the complex chemistry of uranyl carboxylates. A review of the uranyl-organic structural units in monocarboxylates, polycarboxylates and mixed carboxylates is provided. The uranyl (U–O) and equatorial bond lengths are considered for well-refined structures and their trends are briefly discussed.
30 citations
TL;DR: In this article, a review traces the evolution of our understanding of the periodic patterns observed in crystals, as described by geometric crystallography and in related group theoretical methods, including aperiodic fractal, geometries and quasicrystals.
Abstract: This review traces the evolution of our understanding of the periodic patterns observed in crystals, as described by geometric crystallography and in related group theoretical methods. Succinct introductions are also presented on aperiodic fractal, geometries and quasicrystals, as well as the development of the field of crystal structure analysis, which is concerned with the methods and procedures for establishing crystal structure. No assumption is made as to the level of crystallographic training of the reader. The examination here is divided into four time periods: (1) The Hellenic period to the seventeenth century, which was concerned with the study of polyhedra; (2) The eighteenth century, which gave rise to the birth of modern crystallography from mineralogy; (3) The nineteenth century, wherein the theories of external and internal symmetry arose, along with their relation to physical properties; and (4) The twentieth century, including the development of X-ray crystallography through to the discove...
22 citations
TL;DR: In this paper, the behavior of two zeolite systems, those with the LTA and FAU topology, under hydrostatic pressure conditions is discussed, and the results clearly demonstrate the rich behaviour that is observed when the solids are pressurized in the presence of a liquid with molecules that are small enough to enter the pores.
Abstract: Zeolites are crystalline porous aluminosilicate framework solids. Although there are many reports of how the structures change as a function of temperature, similar studies as a function of high pressure are relatively scarce. With the increasing availability of synchrotron X-ray sources and greater knowledge in the chemistry community of high pressure techniques this is gradually changing. Still, only a handful of the many zeolite topologies have been studied. In this short review the behaviour of two zeolite systems, those with the LTA and FAU topology, under hydrostatic pressure conditions will be discussed. The results clearly demonstrate the rich behaviour that is observed when the solids are pressurized in the presence of a liquid with molecules that are small enough to enter the pores.
17 citations
TL;DR: In this article, small angle X-ray scattering (SAXS) is employed to characterize block copolymer (BCP) phase structure and orientation, and is used to observe the progress of poly(lactide) (PLA) hydrolysis through a section 5'mm long.
Abstract: The production of monoliths containing aligned nano-pores by selective etching of a poly(styrene)/poly(lactide) block copolymer (P(S-b-LA) is examined. Small angle X-ray scattering (SAXS) is employed to characterize block copolymer (BCP) phase structure and orientation, and is used to observe the progress of poly(lactide) (PLA) hydrolysis through a section 5 mm long. Orientation is found to be good, and complete etching of the PLA phase is observed and confirmed by weight loss. Diffusion of hydroxide ions through etched pores is tested using a diffusion cell, and found to obey the qualitative behaviour expected in a porous membrane. Breakthrough times are shown to be extended by the requirement of surface wetting. It is confirmed that nanoporous monoliths can be made from block copolymers, suggesting potential applications in membrane and chromatography applications. Contents 1. Introduction 14 1.1. Overview 14 1.2. Block copolymers structure 14 1.3. Phase etching and orientation 14 2. Exper...
15 citations
TL;DR: In this article, the authors apply the neutron diffraction analysis for the crystalline-state photoisomerization, in which a deuterium atom is replaced with the other hydrogen atom without degradation of the single crystal form.
Abstract: The crystal structure analysis with neutron diffraction can clearly make a distinction between the deuterium and hydrogen atoms although they cannot be distinguished by X-ray diffraction analysis. Applying the neutron diffraction analysis for the crystalline-state photoisomerization, in which a deuterium atom is replaced with the other hydrogen atom without degradation of the single crystal form, the mechanisms of the hydrogen atom transfer in the photoisomerization of several organic and organometallic compounds have been made clear. In the first example, the photoinversion of the chiral 1-cyanoethyl group bonded to the cobalt atom in a cobaloxime complex proceeded keeping the deuterium atom in this original position. For the larger alkyl group than the 1-cyanoethyl group in the second example, the chiral 1,2-bis(ethoxycarbonyl)ethyl group was also inverted to the opposite configuration in the same mechanism as that of the smaller 1-cyanoethyl group. The deuterium atom bonded to the chiral carbon atom wa...
10 citations
TL;DR: The following review will show that the molecular conformations and supramolecular interactions of inhibitors bound to DHFR match reasonably well-populated subsets found in the small-molecule context here.
Abstract: Because of the importance of dihydrofolate reductase (DHFR) inhibitors in the treatment of infections and cancer, a wealth of structural information has been collected. Small-molecule crystallography has been carried out on the drugs alone or in salts, and macromolecular crystallography on the drugs bound to DHFR from a number of different species. This review provides an analysis of the small-molecule data. The natural substrates of DHFR contain the pterin (2-amino-4-oxopteridine) ring system, and the ring geometry is compared as this system is changed to the 2,4-diamino analogue found in inhibitors and/or simplified to a single ring and/or subjected to steric hindrance. Clinically useful antifolates have separate ring systems linked by three rotatable bonds as in methotrexate, or two as in trimethoprim, or one as in pyrimethamine. In the latter two cases enough structures are available to provide a clear picture of the preferred disposition(s) of rings around the linkages. Because a very important inter...
9 citations
TL;DR: In this article, the structural response function of a system upon photoexcitation and/or the population changes of short-living intermediates during the time course of a reaction are investigated as a function of time.
Abstract: In time-resolved X-ray diffraction (TR-XRD), two different entities can be investigated as a function of time: the structural response function of a system upon photoexcitation and/or the population changes of short-living intermediates during the time course of a reaction. The time scales of these response functions might vary between hours and picoseconds, depending on the effects measured. By correlating the time evolution of structural changes with varying optical properties, it is possible to derive structure–function relationships in a time-dependent manner. The time-dependence of these relationships and the development of relevant methodologies is a major current interest in our group. Systems under investigation are organic materials with optical functionalities (molecular crystals, electron donor–acceptor systems) and solid state reactions (such as topochemical reactions). Contents 1. Introduction 26 2. Experimental technique: optical light pump/X-ray probe scheme 26 3. Correlation functions an...
6 citations
TL;DR: In this paper, a simple and novel method of directly depositing thin In x Ga1− x As or InP buffer layers on GaAs substrates is presented, instead of the strained-layer superlattice, two-step, graded or CS (compliant substrates) methods, while maintaining low dislocation density, high crystal quality, uniform and mirror-like surfaces.
Abstract: This is a review of direct growing high-quality In x Ga1− x As or InP buffer layers on GaAs substrates by metal-organic chemical vapour deposition (MOCVD). This low-temperature growth method benefits the improvement of metamorphic device performance. A simple and novel method of directly depositing thin In x Ga1− x As or InP buffer layers (<1 µm) on GaAs substrates is presented, instead of the strained-layer superlattice, two-step, graded or CS (compliant substrates) methods, while maintaining low dislocation density, high crystal quality, uniform and mirror-like surfaces. For the direct growth technique of In x Ga1− x As on GaAs, we found that an excellent quality In0.54Ga0.46As buffer of r.m.s surface roughness of only 0.686 nm by AFM and FWHM of 925 arcsec by XRD can be obtained at a low growth temperature of 440°C with a constant Ga/Ingas partial pressure of 5. An MIS-like Schottky diode with Au/i-InGaAs/n+-GaAs structure is investigated to check the film quality. A distinguished I–V characteristic wi...
3 citations
TL;DR: Winter et al. as discussed by the authors, D Le Messurier, CM Martin, D.R Winter, D LeMessurier and CM Martin present their work in Cryst Rev 12 (2006) 3.
Abstract: R Winter, D Le Messurier, CM Martin; Cryst Rev 12 (2006) 3 Sponsorship: EPSRC, CCLRC, Pilkington