Remarkable supramolecular catalysis of glycoside hydrolysis by a cyclodextrin cyanohydrin

(6AR,6DR)-6A,6D-di-C-cyano-beta-cyclodextrin (3) was synthesized and shown to catalyze hydrolysis of nitrophenyl glycosides with the reaction following Michaelis-Menten kinetics. At pH 7.4 and 25 degrees C, hydrolysis of 4-nitrophenyl-beta-glucopyranoside (2) was catalyzed with KM = 15 mM, kcat = 8.2 x 10-6 s-1, and kcat/kuncat = 1217. Catalysis was observed with concentration of 3 as low as 10 muM. Hydrolysis of the corresponding alpha-glucoside, alpha-galactoside, alpha-mannoside, and 2-nitrophenyl-beta-galactoside was also catalyzed by 3, with kcat/kuncat ranging from 283 to 2147. A series of analogues of 3 was prepared and investigated for catalysis of the hydrolysis of 2: (6AR,6DR)-6A,6D-di-C-propyl-beta-cyclodextrin (9) was not catalytic, while 6A,6D-di-C-cyano-6A,6D-dideoxy-beta-cyclodextrin (12) had a low catalytic activity (kcat/kuncat = 4). A kcat/kuncat = 48 was found for 6A,6D-dialdehydo-beta-cyclodextrin dihydrate (11). It was proposed that 3 acts by general acid catalysis on the bound substrate.     

An evaluation of solid-phase microextraction for analysis of volatile organic compounds in drinking water

Solid-phase microextraction (SPME) has been applied to the quantitative analysis of 60 volatile organic compounds (VOCs) in drinking water. Equilibration curves for the partitioning of the VOCs between the fiber coating and fortified water obtained at 20, 50, and 80 °C are found between the theoretical curves for completely agitated and non-agitated samples. Two important factors for the amount adsorbed by the SPME fiber coating are the extraction time and the fiber coating/water distribution coefficient, K_FW . Both depend on the sample temperature, but in a counteracting manner: Increasing the temperature shortened the equilibration times, especially for the heavier VOCs, but also lead to lower K_FW values, and consequently a lower sensitivity of the method. K_FW values are determined for 33 of the VOCs at 40, 60, and 80°C and the heats of adsorption,–ΔH, are calculated. The nature of the adsorption is found to be exothermic which explains the decreasing sensitivity of the method with increasing temperature. Detection limits were typically from 20 ng/l to 200 ng/l, except for the very light VOCs with which detection difficulties were encountered. For all of the VOCs the linear range extended from the lowest concentration at which they were actually detected to at least 5 mg/l. The precision, 3% average standard deviation when an internal standard was used, was satisfactory for most quantitative routine analysis. SPME was also applied to head-space (HS) analysis of drinking water through the coupled equilibrium between water/head-space/fiber coating. HS-SPME is demonstrated to have shorter equilibration times than SPME directly from the water and equal sensitivities, except for the very light VOCs. Water samples from a drinking water plant contaminated in the low μg/l range with 1,1,1-trichloroethane, trichloroethene and tetrachloroethene were analyzed. There seems to be a reasonable agreement between results obtained by SPME and purge & trap. It is concluded that SPME has a great potential for drinking water analysis.     

Controlling Through‐Space and Through‐Bond Exchange Pathways in Bis‐Cobaltocenes for Molecular Spintronics

Pinching molecules via chemical strain suggests intuitive consequences, such as compression at the pinched site and clothespin‐like opening of other parts of the structure. If this opening affects two spin centers, it should result in reduced communication between them. We show that for naphthalene‐bridged biscobaltocenes with competing through‐space and through‐bond pathways, the consequences of pinching are far less intuitive: despite the known dominance of through‐space interactions, the bridge plays a much larger role for exchange spin coupling than previously assumed. Based on a combination of chemical synthesis, structural, magnetic, and redox characterization, and a newly developed theoretical pathway analysis, we can suggest a comprehensive explanation for this non‐intuitive behavior. These results are of interest for molecular spintronics, as naphthalene‐linked cobaltocenes can form wires on surfaces for potential spin‐only information transfer.     

Dependence relation in pregeometries

The aim of this paper is to set a foundation to separate geometric model theory from model theory. Our goal is to explore the possibility to extend results from geometric model theory to non first order logic (e.g. ). We introduce a dependence relation between subsets of a pregeometry and show that it satisfies all the formal properties that forking satisfies in simple first order theories. This happens when one is trying to lift forking to nonelementary classes, in contexts where there exist pregeometries but not necessarily a well-behaved dependence relation (see for example [HySh]). We use these to reproduce S. Buechler's characterization of local modularity in general. These results are used by Lessmann to prove an abstract group configuration theorem in [Le2]. Received February 2, 1999; accepted in final form April 30, 2000.     

Comprehensive analysis of chromatographic data by using PARAFAC2 and principal components analysis

The most straightforward method to analyze an obtained GC–MS dataset is to integrate those peaks that can be identified by their MS profile and to perform a Principal Component Analysis (PCA). This procedure has some important drawbacks, like baseline drifts being scarcely considered or the fact that integration boundaries are not always well defined (long tails, co-eluted peaks, etc.). To improve the methodology, and therefore, the chromatographic data analysis, this work proposes the modeling of the raw dataset by using PARAFAC2 algorithm in selected areas of the GC profile and using the obtained well-resolved chromatographic profiles to develop a further PCA model. With this working method, not only the problems arising from instrumental artifacts are overcome, but also the detection of new analytes is achieved as well as better understanding of the studied dataset is obtained. As a positive consequence of using the proposed working method human time and work are saved. To exemplify this methodology the aroma profile of 36 apples being ripened were studied. The benefits of the proposed methodology (PARAFAC2 + PCA) are shown in a practitioner perspective, being able to extrapolate the conclusions obtained here to other hyphenated chromatographic datasets.     

Spectroscopic properties and electronic structure of pentammineruthenium(II) dinitrogen oxide and corresponding nitrosyl complexes: binding mode of N(2)O and reactivity

The spectroscopic properties and the electronic structure of the only nitrous oxide complex existing in isolated form, [Ru(NH(3))(5)(N(2)O)]X(2) (1, X = Br(-), BF(4)(-)), are investigated in detail in comparison to the nitric oxide precursor, [Ru(NH(3))(5)(NO)]X(3) (2). IR and Raman spectra of 1 and of the corresponding (15)NNO labeled complex are presented and assigned with the help of normal coordinate analysis (NCA) and density functional (DFT) calculations. This allows for the identification of the Ru-N(2)O stretch at approximately 300 cm(-)(1) and for the unambiguous definition of the binding mode of the N(2)O ligand as N-terminal. Obtained force constants are 17.3, 9.6, and 1.4 mdyn/A for N-N, N-O, and Ru-N(2)O, respectively. The Ru(II)-N(2)O bond is dominated by pi back-donation, which, however, is weak compared to the NO complex. This bond is further weakened by Coulomb repulsion between the fully occupied t(2g) shell of Ru(II) and the HOMO of N(2)O. Hence, nitrous oxide is an extremely weak ligand to Ru(II). Calculated free energies and formation constants for [Ru(NH(3))(5)(L)](2+) (L = NNO, N(2), OH(2)) are in good agreement with experiment. The observed intense absorption at 238 nm of 1 is assigned to the t(2g) --> pi(*) charge transfer transition. These data are compared in detail to the spectroscopic and electronic structural properties of NO complex 2. Finally, the transition metal centered reaction of nitrous oxide to N(2) and H(2)O is investigated. Nitrous oxide is activated by back-donation. Initial protonation leads to a weakening of the N-O bond and triggers electron transfer from the metal to the NN-OH ligand through the pi system. The implications of this mechanism for biological nitrous oxide reduction are discussed.