Polymorphism crystallographic studie

Comparison with the Crystalline Structure of A- and B-Type Starch. In the most recent crystallographic studies on the crystalline part of starch (14.15) the structure of both polymorphs are based on a parallel arrangement of left-handed double-helices. In the two observed structures the double-helices are slightly different since small variations away from the perfect six-fold symmetry are found. Nevertheless, they correspond closely to the model studied here. The essential result is that in these two structures the closest interactions between two neighboring double-helices correspond closely to the duplex described as PARA 1. 

Foster, R. (1969). Organic charge transfer molecules. Academic Press, London. [153] Foxman, B. M., Goldberg, P. L. and Mazurek, H. (1981). Confrontational polymorphism of Ni(NCS)2[P(CH)2CH2CN)3]2. Crystallographic study of three polymorphs. J. Inorg. Chem., 20,4368-75. [163]... 

Theocharis, C. R., Jones, W. and Rao, C. N. R. (1984). An unusual photo-induced conformational polymorphism A crystallographic study of bis(j9-methoxy)-rranx-stilbene./. Chem. Soc., Chem. Commun., 1291-3. [234]... 

Structural and Physical Aspects. - The stability of the various conformers of the phosphines oxides (269)- 211) has received theoretical consideration. A new triclinic polymorph of triphenylphosphine sulfide has been structurally characterised, together with a related triclinic polymorph of triphenylphos-phine. Two reports of the solid state crystal structure of the phenolic phosphine oxide (272) have appeared. A crystallographic study has confirmed that the product of electrochemical oxidation of o-diphenylphosphinoben-zenethiol is the disulfide-bridged bis(phosphine oxide) (273). Solid-state structural studies of the dioxides (274), the (i )-(-I-)-isomer of (275), 1-hexynyl(diphenyl)phosphine oxide,tribenzylphosphine oxide, and tris(t-butyl)phosphine selenide," have also been reported. 

Apostol MI, Sawaya MR, Cascio D et al (2010) Crystallographic studies of PrP segments suggest how structural changes encoded by polymorphism at residue 129 modulate susceptibility to human prion disease. J Biol Chem 285 29671-29675... 

Crystallographic studies of [PtCu2(CCPh)4] show this compound can adopt at least three polymorphs. The most singular of these comprises a near linear trimer of trans-Pt2Cu4 octahedra with inter-octahedral Pt Pt distances of 300 pm. The emission spectra are different for each polymorph and are concentration dependent in solution.281 EXAFS shows that [Pt30903oL36] (L = sulfonated phen) has cubic close packed Pt with a 1% contraction in the cluster compared to the pure metal.282... 

Conformational differences can result in variations in local electron density and NMR can be an ideal probe for this type of behavior, via the chemical shielding tensor [31, 32]. For example detailed inter- and intramolecular conformations can be described at high resolution. Proton sites and hydrogen bonds, which are usually averaged in solution state NMR and not easily assessed in crystallographic studies, can be directly determined by solid state NMR. In Fig. 3.2.14 is presented a typical example in which the different polymorphs of sulfanilamide can be distinguished by their 13C CPMAS spectra [33],... 

An extremely important tool for the characterization of polymorphs and solvates is that of microscopy, since the observable habits of differing crystal structures must necessarily be different and therefore useful for the characterization of such systems [27]. Common sense would dictate that the visual observation of such materials would immediately follow an x-ray crystallographic study, which would in principle make the science of optical crystallography [28-30] an essential aspect of any program of study. A review of crystallography from the pharmaceutical viewpoint is available [31]. 

For routine crystallographic studies of polymorphs or pseudopolymorphs containing small and medium-sized molecules, the methodology of crystal structure determination is well estabHshed and the reader is referred to standard monographs on the subject [122,123]. However, a few points on ciurent routine methodology are worth emphasising in order to place some of the newer developments (to be discussed later) in perspective. 

An X-T2iy crystallographic study of the a-form of undecasulphur dinitride (S Na) has shown the molecule to consist of two rings, each shaped like the cyclo-octasulphur ring, fused at the 1,3-position and with the bonds from nitrogen coplanar (36). Several properties of the molecule have been studied, namely solubility, polymorphism, enthalpy of formation, and... 

In these relatively simple phenmy-substituted cyclotriphosphazenes, conformation plays a role in their structure. An x-ray crystallographic study of hexa-(4-fluoropheno>y)cyclotriphosphazene (17) yielded four polymorphs. The polymorphs, three of which have not been previously reported, were identified through variable temperature x-ray crystallographic analysis and differential scanning calorimetry (DSC). Crystals of the a form were found to melt at 128.4 °C and then re-crystallize on cooling to 53.4 °C. A p polymorph was found to have an endotherm at 98.4 °C and then a melt at 128.4 °C,which is the same as the a polymorph. This suggested a transition from the p to the a form at 98.4 °C. Further studied revealed that the various forms all undergo transitions from one form to another. 

As might be expected, the literature associated with studies of polymorphism and solvatomorphism has grown in proportion to the interest in the field. As a result, an annual review of the area has been initiated, and will be continued in succeeding volumes in the Profiles series. The citations in this article are drawn primarily from the major pharmaceutical and crystallographic journals, and therefore are not represented to be comprehensive. However, they should represent the bulk of work that was conducted with pharmaceutical interest in mind. 

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