Polymorphism crystal structure

The recently developed high-resolution solid-state NMR technique proton CRAMPS NMR has become a very useful research tool, corresponding to X-ray crystallography, and enabling the study of crystal structure polymorphs of amino acids. In this chapter, we first discuss a recent research example application to crystal structure analysis of polymorphic forms of some typical a-amino acids in order to test the power of H CRAMPS NMR, compared with C and N NMR methods. 

Caspar DL, Cohen C, Longley W (1969) Tropomyosin crystal structure, polymorphism and molecular interactions. J Mol Biol 41 87107 Cavaille F, Janmot C, Ropert S, dAlbis A (1986) Isoforms of myosin and actin in human, monkey and rat myometrium. Comparison of pregnant and non-pregnant uterus proteins. Eur J Biochem 160 507513... 

T. Shoda, K. Yamahara, K. Okazaki, and D. E. Williams, ]. Mol. Struct. (THEOCHEM). 3J3,267 (1995). Molecular Packing Analysis of Benzene Crystals. Part 2. Prediction of Experimental Crystal Structure Polymorphs at Low and High Pressure. 

Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals. The complete morphology of a material is described by polymorphism and other variables such as crystal habit, amorphous fraction or crystallographic defects. 

The most common crystalline forms are polymorphs, hydrates, and solvates (pseudopolymorphs). Polymorphs are formed when a substance crystallizes in two or more crystal structures. Polymorphism significantly impacts on physicochemical properties of materials, such as stability, density, melting point, solubility, bioavailability, and so on. Hence the characterization of all possible polymorphs, identifying the stable (thermodynamic) polymorph, and design of reliable processes for consistent production are critical in modem day drug development. 

In this chapter, we discussed crystal structure, the body-centered cubic crystal structure, and the ability of a metal to experience a change in its crystal structure (polymorphism). A knowledge of these concepts helps us understand the transformation of BCC iron to martensite (which has another crystal structure) in Chapter 10. This relationship is represented by the following concept map ... 

Fig. 2.1. Some metals hove more than one crystal structure. The most important examples of this polymorphism ore in iron and titanium.

Pressure-induced phase transitions in the titanium dioxide system provide an understanding of crystal structure and mineral stability in planets interior and thus are of major geophysical interest. Moderate pressures transform either of the three stable polymorphs into the a-Pb02 (columbite)-type structure, while further pressure increase creates the monoclinic baddeleyite-type structure. Recent high-pressure studies indicate that columbite can be formed only within a limited range of pressures/temperatures, although it is a metastable phase that can be preserved unchanged for years after pressure release Combined Raman spectroscopy and X-ray diffraction studies 6-8,10 ave established that rutile transforms to columbite structure at 10 GPa, while anatase and brookite transform to columbite at approximately 4-5 GPa. 

Rawlings and Lingafelter [69] studied the hydrated phases of sodium alcohol sulfates ranging from C6 to C20 and their crystal structures by X rays. The a phase is almost identical to that of the alkylsulfonates but all other phases are different. The crystals of all phases are monoclinic. This work was completed by Prins and Prins [70] who gave more precise details of the polymorphism of sodium alcohol sulfates. 

In 1933, Bernal and Crowfoot [1] reported on the solid state polymorphism of p-azoxyanisole. They found two crystalline modifications of this compound, a stable yellow form and an unstable white polymorph. Krigbaum et al. [31 reexamined the crystal structure of the stable yellow form. The compound shows an imbricated structure which is the basic packing required for nematic behaviour according to Gray [132]. 

Many of the investigated mesogenic compounds show solid state polymorphism. In order to obtain useful information about the arrangement of the molecules in the mesophase from the X-ray data of the single crystals, it is important to investigate the crystal structure of those solid phase which transforms into the liquid crystalline phase. For instance, only the crystal structures of the low temperature solid phases of the compounds MBBA [138, 139], MHPOBC [159], and T15 [81] could be determined, but the... 

The reaction of the trimethyl tritiophosphite and triphenyl tritiophosphite with the gold derivative [Au(C6F5)(tht)] leads to the gold(I) complexes [57] shown in Figure 3.6. The crystal structure of the trimethyl tritiophosphite gold (I) complex was studied by X-ray diffraction and two different polymorphs were discovered. 

Identification of proteins that bind to Z-DNA added one further step to the establishment of the presence of Z-DNA in vivo and its possible biological role. Herbert and Rich [22] demonstrated an in vitro assay system where one type of double-stranded RNA adenosine deaminase, called DRAD-binding Z-DNA. There are evidences that topoisomerase II from Drosophila, hiunan and calf thymus recognizes a number of DNA shapes, including Z-DNA [34,35]. Bloomfield and coworkers [36] have found that the condensation of plasmids is enhanced by Z-DNA conformation in d(CG)n repeats. The information related to B-Z transition [31], the effect of ligands on it [28,29] and X-ray crystal structure data [37,38] appear to suggest that the possible biological role of this polymorphic form of DNA will be soon established. 

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