Methylated β-cyclodextrins: influence of degree and pattern of substitution on the thermodynamics of complexation with tauro- and glyco-conjugated bile salts

The complexation of 6 bile salts with various methylated β-cyclodextrins was studied to elucidate how the degree and pattern of substitution affects the binding. The structures of the CDs were determined by mass spectrometry and NMR techniques, and the structures of the inclusion complexes were characterized from the complexation-induced shifts of (13)C nuclei as well as by 2D ROESY NMR. Thermodynamic data were generated using isothermal titration calorimetry. The structure-properties analysis showed that methylation at O3 hinders complexation by partially blocking the cavity entrance, while methyl groups at O2 promote complexation by extending the hydrophobic cavity. Like in the case of 2-hydroxypropylated cyclodextrins, the methyl substituents cause an increased release of ordered water from the hydration shell of the bile salts, resulting in a strong increase in both the enthalpy and the entropy of complexation with increased number of methyl substituents. Due to enthalpy-entropy compensation the effect on the stability constant is relatively limited. However, when all hydroxyl groups are methylated, the rigid structure of the free cyclodextrin is lost and the complexes are severely destabilized due to very unfavorable entropies.     

OMP decarboxylase: phosphodianion binding energy is used to stabilize a vinyl carbanion intermediate

Orotidine 5'-monophosphate decarboxylase (OMPDC) catalyzes the exchange for deuterium from solvent D(2)O of the C-6 proton of 1-(β-d-erythrofuranosyl)-5-fluorouracil (FEU), a phosphodianion truncated product analog. The deuterium exchange reaction of FEU is accelerated 1.8 × 10(4)-fold by 1 M phosphite dianion (HPO(3)(2-)). This corresponds to a 5.8 kcal/mol stabilization of the vinyl carbanion-like transition state, which is similar to the 7.8 kcal/mol stabilization of the transition state for OMPDC-catalyzed decarboxylation of a truncated substrate analog by bound HPO(3)(2-). These results show that the intrinsic binding energy of phosphite dianion is used in the stabilization of the vinyl carbanion-like transition state common to the decarboxylation and deuterium exchange reactions.     

Structural chemistry, monoclinic-to-orthorhombic phase transition, and CO2 adsorption behavior of the small pore scandium terephthalate, Sc2(O2CC6H4)CO2)3, and its nitro- and amino-functionalized derivatives

The crystal structure of the small pore scandium terephthalate Sc(2)(O(2)CC(6)H(4)CO(2))(3) (hereafter Sc(2)BDC(3), BDC = 1,4-benzenedicarboxylate) has been investigated as a function of temperature and of functionalization, and its performance as an adsorbent for CO(2) has been examined. The structure of Sc(2)BDC(3) has been followed in vacuo over the temperature range 140 to 523 K by high resolution synchrotron X-ray powder diffraction, revealing a phase change at 225 K from monoclinic C2/c (low temperature) to Fddd (high temperature). The orthorhombic form shows negative thermal expansivity of 2.4 × 10(-5) K(-1): Rietveld analysis shows that this results largely from a decrease in the c axis, which is caused by carboxylate group rotation. (2)H wide-line and MAS NMR of deuterated Sc(2)BDC(3) indicates reorientation of phenyl groups via π flips at temperatures above 298 K. The same framework solid has also been prepared using monofunctionalized terephthalate linkers containing -NH(2) and -NO(2) groups. The structure of Sc(2)(NH(2)-BDC)(3) has been determined by Rietveld analysis of synchrotron powder diffraction at 100 and 298 K and found to be orthorhombic at both temperatures, whereas the structure of Sc(2)(NO(2)-BDC)(3) has been determined by single crystal diffraction at 298 K and Rietveld analysis of synchrotron powder diffraction at 100, 298, 373, and 473 K and is found to be monoclinic at all temperatures. Partial ordering of functional groups is observed in each structure. CO(2) adsorption at 196 and 273 K indicates that whereas Sc(2)BDC(3) has the largest capacity, Sc(2)(NH(2)-BDC)(3) shows the highest uptake at low partial pressure because of strong -NH(2)···CO(2) interactions. Remarkably, Sc(2)(NO(2)-BDC)(3) adsorbs 2.6 mmol CO(2) g(-1) at 196 K (P/P(0) = 0.5), suggesting that the -NO(2) groups are able to rotate to allow CO(2) molecules to diffuse along the narrow channels.     

Increased cubic–tetragonal phase transition temperature and resistivity hysteresis of surface vacuum annealed SrTiO3

Electrical properties of SrTiO₃ single crystal samples treated by an anisotropic surface annealing technique under reducing conditions have been investigated in the temperature range of 35 K–300 K. Optical and atomic force microscopy show that annealing gives rise to polycrystallization and the formation of colored dendritic structures. Carrier concentrations and mobilities determined by Hall measurements as well as resistivities detected by van der Pauw measurements show the expected metallic behavior due to oxygen vacancy doping. Moreover, the temperature dependent resistivities indicate a cubic-to-tetragonal phase transition, which to our knowledge has not been reported before. Additionally, the transition occurred up to 53 K above the known bulk transition temperature T _C at 105 K with a hysteresis up to a temperature of 220 K. Both phenomena possibly arise from dislocations and associated strain fields introduced by surface annealing that are assumed to lower the free energy of the tetragonal phase and simultaneously pin tetragonal domains. Thus, microregions of the tetragonal phase persist above T _C causing the hysteresis in resistivity up to ∼12%. This effect possibly provides new chances for future oxide based non-volatile data-storage devices.     

Electrically enhanced liquid‐phase microextraction of three textile azo dyes from wastewater and plant samples

An electromembrane extraction procedure coupled with HPLC and visible detection was applied for the extraction of three textile azo dyes as organic salts. The extraction parameters such as extraction time, applied voltage, pH range, and concentration of salt added were optimized. A driving force of 60 V was applied to extract the analytes through 2‐nitrophenyl octyl ether, used as the supported liquid membrane, into a neutral aqueous solution. This method required 20 min extraction time from a neutral sample solution. The proposed microextraction technique provided good linearity with correlation coefficients from 0.996 to 0.998 over a concentration range of 1.0–1000.0 ng/mL. The LODs of dyes were 0.30–0.75 ng/mL, while the reproducibility ranged from 6.7 to 12.9% (n = 6). Also, enrichment factors of 96–162 that corresponded to the recoveries ranging from 48 to 81% were achieved. Finally, the application of this new method was demonstrated on wastewater samples and some plants grown in contaminated environments. Excellent selectivity was obtained as no interfering peaks were detected.     

Generalized frieze pattern determinants and higher angulations of polygons

Frieze patterns (in the sense of Conway and Coxeter) are in close connection to triangulations of polygons. Broline, Crowe and Isaacs have assigned a symmetric matrix to each polygon triangulation and computed the determinant. In this paper we consider d-angulations of polygons and generalize the combinatorial algorithm for computing the entries in the associated symmetric matrices; we compute their determinants and the Smith normal forms. It turns out that both are independent of the particular d  -angulation, the determinant is a power of d−1$d-1$, and the elementary divisors only take values d−1$d-1$ and 1. We also show that in the generalized frieze patterns obtained in our setting every adjacent 2×2$2\times 2$-determinant is 0 or 1, and we give a combinatorial criterion for when they are 1, which in the case d=3$d=3$ gives back the Conway–Coxeter condition on frieze patterns.     

A potent antimalarial trichothecene from hyphomycete species

Antimalarial screening of diverse fungal samples resulted in the isolation and identification of a new sesquiterpene, trichothecinol D (1), which exhibited potent in vitro activity against Plasmodium falciparum with a 200-fold selectivity for parasite versus mammalian cytotoxicity. Trichothecinol D was obtained by bioassay-guided fractionation of two hyphomycete species. Its chemical characterization was performed by detailed NMR spectroscopy and high resolution mass spectrometry. The absolute configuration of trichothecinol D was determined by X-ray crystallography.     

A study of salt effects on the complexation between β-cyclodextrins and bile salts based on the Hofmeister series

The Hofmeister series is the ranking of ions according to their ability to strengthen (kosmotropic ions) or weaken (chaotropic ions) hydrophobic interactions. Such ions are therefore expected to affect the strength of cyclodextrin (CD) inclusion complexes and may thereby affect the release of CD bound drug molecules. The influence of Hofmeister ions on the binding constants of complexes between CDs (β-CD and hydroxypropylated β-CD) and bile salts (glycocholate and glycochenodeoxycholate) were examined by isothermal titration calorimetry. The chaotropic anions tended to weaken these inclusion complexes. Conversely, kosmotropic ions increased the binding strength and this effect scaled with the buried hydrophobic surface area. Both effects are relatively weak at physiological ion concentrations and may be neglected for most pharmaceutical purposes.