Mixed Crystals

Kitaigorodsky, Alexander I.

doi:10.1007/978-3-642-81672-7

Cited by 382 publications

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“…Whereas twinning in crystals of low molecular weight substances, both organic and inorganic, is comparatively frequent (see [1][2][3][4][5][6][7][8] and references therein), twinning in polymers is rather an exceptional phenomenon: the first widely accepted observation of twinning in polymer crystals [7] was reported as late as 1963 (in the following, the term "polymer" refers primarily to synthetic macromolecules made up of one or a few repeat units, lacking the very specific monomeric succession and interactions found in biological macromolecules such as proteins; coordination and purely inorganic polymers are also excluded from the present work).…”

Section: Introductionmentioning

confidence: 99%

Twinning of Polymer Crystals Suppressed by Entropy

Karayiannis¹,

Foteinopoulou²,

Laso³

2014

Symmetry

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Abstract:We propose an entropic argument as partial explanation of the observed scarcity of twinned structures in crystalline samples of synthetic organic polymeric materials. Polymeric molecules possess a much larger number of conformational degrees of freedom than low molecular weight substances. The preferred conformations of polymer chains in the bulk of a single crystal are often incompatible with the conformations imposed by the symmetry of a growth twin, both at the composition surfaces and in the twin axis. We calculate the differences in conformational entropy between chains in single crystals and chains in twinned crystals, and find that the reduction in chain conformational entropy in the twin is sufficient to make the single crystal the stable thermodynamic phase. The formation of cyclic twins in molecular dynamics simulations of chains of hard spheres must thus be attributed to kinetic factors. In more realistic polymers this entropic contribution to the free energy can be canceled or dominated by nonbonded and torsional energetics.

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Section: Introductionmentioning

confidence: 99%

Twinning of Polymer Crystals Suppressed by Entropy

Karayiannis¹,

Foteinopoulou²,

Laso³

2014

Symmetry

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“…From the phase diagrams, Fig. 1, it follows that there is complete subsolidus miscibility: there is a single solid±liquid loop joining the (stable) melting points of the pure substances [for a part in contrast to results published earlier by Kitaigorodsky (1984)]. The existence of uninterrupted series of mixed crystals is supported by the plots of cell dimensions as a function of composition (see Fig.…”

Section: Mixed Crystals Of 2-¯uoronaphthalene With Other Members Of Tmentioning

confidence: 61%

“…The situation was circumscribed by Kitaigorodsky (1984) as follows:`All the naphthalene -derivatives are supposed to possess similar energy surfaces which have two minima approximately of the same depth. One of them leads to the¯uoronaphthalene packing, the second to the naphthalene type packing'.…”

Section: Introductionmentioning

confidence: 99%

Isomorphism and mixed crystals in 2-R-naphthalenes: evidence of structural subfamilies and prediction of metastable forms

Haget¹,

Chanh²,

Meresse³

et al. 1999

J Appl Cryst

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Solid±liquid binary phase diagrams and isothermal unitcell parameters as a function of composition are given for the high-temperature form (P2 1 /a, Z = 2) mixed crystals generated by 2-¯uoronaphthalene, naphthalene and 2-naphthol with four other derivatives of naphthalene. The study leads to the distinction between two high-temperature forms (or types of packing). The ®rst one (type 1) is taken by ®ve 2-R-naphthalenes (R = F, Cl, Br, SH, CH 3 ; the ®rst subfamily), the second one (type 2) by naphthalene itself and 2-naphthol (R = H, OH; the second subfamily). The crystallographic data also allow an estimation of unit-cell parameters for the metastable forms of naphthalene (type 1, a = 8.0, b = 5.95, c = 8.6 A Ê , = 116) and 2-¯uoronaphthalene (type 2, a = 8.336, b = 5.915, c = 9.000 A Ê , = 122.23 ), supporting an interpretation, in terms of crossed isodimorphism, of the observed two-phase regions in systems in which the components do not belong to the same subfamily.

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“…The proportionality factor is determined by the similarity of the UA structure to the coprecipitating compound. Processes of coprecipitation are related to the ability of crystallizing compounds to form a mixed crystal (13 ), which is a solid solution (14 ) containing ions or molecules of one compound in the crystal lattice of another. Crystals of purines are formed by molecules and classified as molecular crystals (15 ).…”

Section: Discussionmentioning

confidence: 99%

“…Solid solutions of organic compounds are always substitutional, formed by replacing some solvent molecules with the admixture (16 ). The necessary condition for 2 or more organic compounds to form a solid solution is the similarity of shape and size of their molecules (13 ). This condition is fulfilled by purine derivatives identified in urinary stones (Fig.…”

Section: Discussionmentioning

confidence: 99%

Methylated Purines in Urinary Stones

Safranow¹,

Machoy²

2005

Clinical Chemistry

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Background: The aim of the study was to measure the content of methylated purines that appear as admixtures in uric acid stones. Methods: We analyzed urinary calculi from 48 residents of Western Pomerania who underwent surgery at the urology ward in Szczecin. Stone samples were dissolved in 0.1 mol/L NaOH. Extracts were diluted in 50 mmol/L KH 2 PO 4 and analyzed by reversed-phase HPLC with ultraviolet detection and use of a gradient of methanol concentration and pH. Results: Uric acid was the main component of 9 stones. All 9 showed admixtures of 9 other purine derivatives: endogenous purine breakdown products (xanthine, hypoxanthine, and 2,8-dihydroxyadenine) and exogenous methyl derivatives of uric acid and xanthine (1-, 3-, and 7-methyluric acid; 1,3-dimethyluric acid; and 3-and 7-methylxanthine). Amounts of these purine derivatives ranged from the limit of detection to 12 mg/g of stone weight and showed a strong positive correlation (Spearman rank correlation coefficients, 0.63-0.94) with the uric acid content of the samples. The main methylated purine in the stones was 1-methyluric acid. Conclusions: Urinary purines at concentrations below their saturation limits may coprecipitate in samples supersaturated with uric acid and appear as admixtures in urinary stones. The amount of each purine depends on its average urinary excretion, similarity to the chemical structure of uric acid, and concentration of the latter in the stone. These findings suggest that purines in stones represent a substitutional solid solution with uric acid as solvent. Methylxanthines, which are ubiquitous components of the diet, drugs, and uric acid calculi, may be involved in the pathogenesis of urolithiasis. © 2005 American Association for Clinical ChemistryMany purines are poorly soluble in aqueous solutions and may precipitate from urine, forming urinary calculi, if their concentrations exceed the limit of saturation. Uric acid (UA) 1 is the end product of human purine metabolism and is excreted mainly in the urine. Acidic urine is often a supersaturated solution of uric acid, even when the daily output remains in the normal range, and uric acid stones may form. Crystallization is usually prevented by inhibitors (citrates, glycosaminoglycans, and some proteins) (1 ), although not always effectively enough, particularly at low urine pH. Calculi composed of UA also form in males with hypoxanthine-guanine phosphoribosyltransferase (EC 2.4.2.8) deficiency (complete or partial). Xanthine (Xan) may appear in xanthinuria as well as during treatment with allopurinol (2 ). 2,8-Dihydroxyadenine (2,8-DHA) is excreted in urine and can precipitate in renal tissue and the urinary tract in patients with dihydroxyadeninuria attributable to adenine phosphoribosyltransferase (EC 2.4.2.7) deficiency (3 ).Limited attention has been paid to the possible role of metabolites of methylxanthines (caffeine, theophylline, and theobromine) in the pathogenesis of urolithiasis, although their average daily output in urine, depending on dietary intake, is ϳ500...

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Mixed Crystals

Cited by 382 publications

References 0 publications

Twinning of Polymer Crystals Suppressed by Entropy

Twinning of Polymer Crystals Suppressed by Entropy

Isomorphism and mixed crystals in 2-R-naphthalenes: evidence of structural subfamilies and prediction of metastable forms

Methylated Purines in Urinary Stones

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