Institut für Physikalische Chemie und Theoretische Chemie
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Kinetic analysis of the thermic effect of food and its relationship to body composition in humans
(2000)
The course of energy expenditure after a meal can vary widely with regard to the slope of onset, amplitude, and duration of the thermic effect. The aim of the present study was to explore the relationship between the thermic effect of food (TEF), as characterized by kinetic analysis of postprandial energy expenditure, body composition, and variables related to the metabolic syndrome including central obesity, hypertension, and glucose tolerance. A total of 181 men and women (body mass index [BMI] range, 19.4 to 52.2 kg/m2) were characterized for body composition, blood pressure, oral glucose tolerance, and energy expenditure after a test meal. Energy expenditure, as measured by indirect calorimetry, was analyzed over a 6-hour period by 3-parameter curve fitting using equations derived from kinetics describing a biphasic reaction involving 2 consecutive first-order reactions (A->B->C). Apart from total thermic effect of food (TEFk), the curve also provided an estimate of time of peak (Tp) and amplitude of peak (Ap) for each subject. Multiple stepwise regression analysis with TEFk, Ap, and Tp as dependent variables showed significant effects of sex, age, body weight, body fat, -blockade, and body composition on TEF curve parameters. Cluster analysis based on Tp shown 2 distinct clusters with significant differences in age and body fat mass. This study shows that kinetic analysis of postprandial energy expenditure can be used to examine the determinants of the time course of the thermic effect of food in man.
In situ Laser-induced fluorescence (LIF) analysis pf petroleum product-contaminared soil samples
(2000)
Diffuse reflectance measurements and photon migration studies with near infrared (NIR) diode lasers were employed to elucidate experimental methods for determining absorption and scattering coefficients and species concentrations in highly scattering solutions. Applicability of theoretical approaches were established by investigating model systems with absorbing (e.g. ink, malachite green) and scattering (e.g. milk powder, caolinit) species in aqueous solution. While diffuse reflectance measurements practically requires calibration procedures, photon migration studies allow quantitative determination of absorption and scattering coefficients of turbid solutions consistent with absorptions coefficients obtained from Lambert-Beer's law. Furthermore, NIR absorption spectra of water, chlorinated hydrocarbons (chloroform, 1,2-dichloroethane, trichloroethene) and of various sugars ($alpha$-D-glucose, sucrose, maltose) are discussed. Spectral variations of NIR water absorption with temperature and solvents are exammined. Exemplary, NIR diode laser detection of water in acetone/water mixtures is performed.
Synthese und Charakterisierung monomolekularer dicker Polyisobutenfilme mit ionischen Ankergruppen
(1999)
Polymerbedeckte Lipidvesikel
(1999)
Proteinsekundärstrukturen an der fluiden Grenzfläche - in situ - Untersuchungen mit der IRRAS
(1999)
Investigation on the photoreactions of Nitrate and Nitrite ions with selected Azaarenes in water
(1999)
Polyelektrolyt-Tensid-Komplexe mit fluor- und siliciumhaltigen Tensiden : Struktur und Eigenschaften
(1998)
In this work we present theoretical studies of the hydrolytic reaction of methyl formate, formamide and urea with one water molecule. The studied systems contain two additional water molecules which can act as bifunctional acid- base catalysts. These water molecules catalyze proton transfers between the primary reacting species. Our models describe the concerted transfer of two protons in every reaction step. The calculations have been carried out with the Becke3LYP/6-31G* method. Unspecific solvation effects have been included by means of a polarizable continuum model. Substrate reactivity differences as well as preferences for different reaction pathways can be discussed with the aid of these molecular systems. The studied alternative mechanisms include the common addition-elimination mechanism via a tetrahedral intermediate, and a concerted SN-like mechanism without a reaction intermediate. Our results suggest that the proved decreasing substrate reactivity in the order ester, amide, urea is caused by a rising resonance stabilization of the reaction centre, and not by a different positive partial charge of the carbonyl carbon. It is also concluded, that the probability of a concerted addition of a nucleophile and elimination of a leaving group without a tetrahedral intermediate rises in the order ester, amide, urea. The ordering of reactivity is not influenced by this behaviour.
Basicities of 16 acyl compounds including selected aldehydes, ketones, esters, amides and ureas in the gas phase and in water were calculated with the Becke3LYP/6-31G* method. Solvent effects were modeled using a polarizable electrostatic continuum representation of the solvent. The properties of the electron densities of protonated molecules were described by localized bond orbitals. Our results suggest that the carbonyl oxygen is the preferred site of protonation for all molecules studied. Calculated pKa values in water range from -12 for aldehydes to + 1.5 for ureas. They agree well with published experimental data. We found that a high basicity of acyl compounds at the carbonyl oxygen is coupled with a large amount of additional resonance stabilization at the carbonyl group. The protonation of the leaving group of eaters, amides and ureas is less preferred, but the basicity difference between C=O and -OR or -NR2 decreases from esters to ureas. Calculated pKa values for this site range from -18 for esters to + 3 for ureas. These values are often not accessible by experiment. The structure of carboxylic acid derivatives protonated at the leaving group is determined by prefragmentation of the molecules into an acylium ion or positively charged isocyanate and an alcohol or amine. (C) 1998 Elsevier Science B.V.
The influence of polyelectrolytes on structure formation in liquid crystalline Na-dodecylsulfate/decanol/water systems was investigated by means of small angle X-ray diffraction, rheology, NMR spectroscopy, and microscopy. By adding Na-polyacrylate (PAA) into the mesophase, the one-phase region is left and phenomena of phase separation into a solvent-rich and a polymer/surfactantrich phase occurs. By incoporating an anionic and cationic polyelectrolyte step by step the tendency of phase separation is increased drastically. The self-organization process can be regulated directly by varying the water content of the system. However, at a water content of 30% the properties of the resulting liquid crystal were changed drastically. X-ray diffraction shows a multitude of Bragg peaks, NMR shows a peak-splitting, and rheology shows a change from non-Newtonian to Newtonian-flow behavior. On the basis of the experimental results an ordered multilayer associate structure can be assumed.