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Andreas F Voegele

Publications and source records attributed to Andreas F Voegele.

8 recordsLinked to original sources
PubMed2004

Double hydrogen tunneling revisited: the breakdown of experimental tunneling criteria.

Formic acid dimer was chosen as a model system to investigate synchronous double proton transfer by means of variational transition state theory (VTST) for various isotopically modified hydrogen species. The electronic barrier for the double proton transfer was evaluated to be 7.9 kcal/mol, thus being significantly lower than it was determined in previous studies. The tunneling probabilities were evaluated at temperatures from 100 up to 400 K and typical Arrhenius behavior with enhancement by tunneling is observed. When comparing the transmission factors kappa in dependence of the mass of the tunneling hydrogen, it was found that there are two maxima, one at very low masses (e.g., 0.114 amu, corresponding to the muonium entity) and one maximum at around 2 amu (corresponding to deuterium). With the knowledge of the VTST-hydrogen transfer rates and the corresponding tunneling corrections, various tunneling criteria were tested (e.g., Swain-Schaad exponents) and were shown to fail in this reaction in predicting the extent of tunneling. This finding adds another aspect in the ongoing "Tunneling-Enhancement by Enzymes" discussion, as the used tunneling criteria based on experimental reaction rates may fail to predict tunneling behavior correctly.

Journal Article↗
PubMed2004

The ground-state tunneling splitting of various carboxylic acid dimers.

Carboxylic acid dimers in gas phase reveal ground-state tunneling splittings due to a double proton transfer between the two subunits. In this study we apply a recently developed accurate semiclassical method to determine the ground-state tunneling splittings of eight different carboxylic acid derivative dimers (formic acid, benzoic acid, carbamic acid, fluoro formic acid, carbonic acid, glyoxylic acid, acrylic acid, and N,N-dimethyl carbamic acid) and their fully deuterated analogs. The calculated splittings range from 5.3e-4 to 0.13 cm(-1) (for the deuterated species from 2.8e-7 to 3.3e-4 cm(-1)), thus indicating a strong substituent dependence of the splitting, which varies by more than two orders of magnitude. One reason for differences in the splittings could be addressed to different barriers heights, which vary from 6.3 to 8.8 kcal/mol, due to different mesomeric stabilization of the various transition states. The calculated splittings were compared to available experimental data and good agreement was found. A correlation could be found between the tunneling splitting and the energy barrier of the double proton transfer, as the splitting increases with increased strength of the hydrogen bonds. From this correlation an empirical formula was derived, which allows the prediction of the ground-state tunneling splitting of carboxylic acid dimers at a very low cost and the tunneling splittings for parahalogen substituted benzoic acid dimers is predicted.

Journal Article↗
PubMed2003

Extended method for adiabatic mode reordering.

The task of vibrational mode reordering is very important for reaction valley studies and for the determination of small curvature tunneling effects. An extended algorithm for adiabatic mode reordering is presented. It is based on the method introduced by Konkoli et al. [J Comput Chem 1997, 18, 1282], which is shown to suffer from numerical problems in the region of frequency-crossings and avoided crossings. One improvement is the use of cubic splines for the interpolation of the projected matrix of force constants, which allows larger step sizes between the discrete points along the reaction path, where vibrational analysis is performed. The main improvement is the use of perturbation theory to resolve crossings and avoided crossings. Within this theoretical framework it becomes clear why the method of the maximal overlap between the normal modes cannot work properly, as eigenvectors associated with nearby eigenvalues tend to become "wobbly". Thus a perturbative procedure is designed that is used for all cases where two harmonic frequencies approach each other and the overlap of the associated normal modes is of no practical use. Advantages of the new procedure are the use of larger step sizes along the minimum energy path and the much more reliable resolution of mode-crossings and avoided crossings independent of the systems symmetry. In addition to that it is shown that one should be very cautious in all computational situations when working with eigenvectors associated with nearby eigenvalues.

Journal Article↗
PubMed2002

About the stability of sulfurous acid (H2SO3) and its dimer.

The characterization and isolation of sulfurous acid (H2SO3) have never been accomplished and thus still remain one of the greatest open challenges of inorganic chemistry. It is known that H2SO3 is thermodynamically unstable. In this study, however, we show that a Ci-symmetric dimer of sulfurous acid (H2SO3)2 is 3.5 kcal mol-1 more stable than its dissociation products SO2 and H2O at 77 K. Additionally, we have investigated the kinetic stability of the sulfurous acid monomer with respect to dissociation into SO2 and H2O and the kinetic isotope effect (KIE) on this reaction by transition-state theory. At 77 K, the half-life of H2SO3 is 15 x 10(9) years, but for the deuterated molecule (D2SO3) it increases to 7.9 x 10(26) years. At room temperature, the half-life of sulfurous acid is only 24 hours; however, a KIE of 3.2 x 10(4) increases it to a remarkable 90 years. Water is an efficient catalyst for the dissociation reaction since it reduces the reaction barrier tremendously. With the aid of two water molecules, one can observe a change in the reaction mechanism for sulfurous acid decomposition with increasing temperature. The most likely mechanism below 170 K is via an eight-membered transition-state ring; yet, above 170 K, a mechanism with a six-membered transition state ring becomes the predominant one. For deuterated sulfurous acid, this change in reaction mechanism can be observed at 120 K. Consequently, between 120 and 170 K, different predominant reaction mechanisms occur for the decomposition of normal and deuterated sulfurous acid when assisted by two water molecules. However, the much longer half-life of deuterated sulfurous acid and the stability of the sulfurous acid dimer at 77 K are encouraging for future synthesis and characterization under laboratory conditions.

Journal Article↗
PubMed2002

Characterization of the vitamin E-binding properties of human plasma afamin.

Human plasma afamin, the fourth member of the albumin gene family, is shown to be a specific binding protein for vitamin E. A radio ligand-binding assay followed by Scatchard and Hill analysis are used to show that afamin has a binding affinity for both alpha-tocopherol and gamma-tocopherol, two of the most important forms of vitamin E, in vitro. The binding-dissociation constant was determined to be 18 microM, indicating that afamin plays a role as vitamin E carrier in body fluids such as human plasma and follicular fluid under physiological conditions. Additionally, we demonstrate that afamin has multiple binding sites for both alpha- and gamma-tocopherol. Due to the large binding capacity of afamin for vitamin E, it might take over the role of vitamin E transport in body fluids under conditions where the lipoprotein system is not sufficient for vitamin E transport. To confirm the experimental results, we performed homology modeling and docking calculations on the predicted tertiary structure, which showed coincidence between calculated and in vitro results.

Binding Sites↗
PubMed2002

Towards the experimental decomposition rate of carbonic acid (H2CO3) in aqueous solution.

Dry carbonic acid has recently been shown to be kinetically stable even at room temperature. Addition of water molecules reduces this stability significantly, and the decomposition (H2CO3 + nH2O --> (n+1)H2O + CO2) is extremely accelerated for n = 1, 2, 3. By including two water molecules, a reaction rate that is a factor of 3000 below the experimental one (10 s(-1)) at room temperature was found. In order to further remove the gap between experiment and theory, we increased the number of water molecules involved to 3 and took into consideration different mechanisms for thorough elucidation of the reaction. A mechanism whereby the reaction proceedes via a six-membered transition state turns out to be the most efficient one over the whole examined temperature range. The determined reaction rates approach experimental values in aqueous solution reasonably well; most especially, a significant increase in the rates in comparison to the decomposition reaction with fewer water molecules is found. Further agreement with experiment is found in the kinetic isotope effects (KIE) for the deuterated species. For water-free carbonic acid, the KIE (i.e., kH2CO3/kD2CO3) for the decomposition reaction is predicted to be 220 at 300 K, whereas it amounts to 2.2-3.0 for the investigated mechanisms including three water molecules. This result is therefore reasonably close to the experimental value of 2 (at 300 K). These KIEs are in much better accordance with the experiment than the KIE for decomposition with fewer water entities.

Carbonic Acid↗
PubMedDate not supplied

Afamin is a novel human vitamin E-binding glycoprotein characterization and in vitro expression.

Hydrophobic vitamins are transported in human plasma and extravascular fluids by carrier proteins. No specific protein has been described so far for vitamin E, which plays a crucial role in protecting against oxidative damage and disease. We report here the purification of a 75-kDa glycoprotein with vitamin E-binding properties by stepwise chromatography of lipoprotein-depleted human plasma and monitoring of vitamin E (alpha-tocopherol)-binding activity. Partial sequencing identified this protein as afamin, a previously described member of the albumin gene family with four or five potential N-glycosylation sites. Glycosylation analysis indicated that >90% of the glycans were sialylated biantennary complex structures. The vitamin E-binding properties were confirmed using recombinantly expressed afamin. Qualitative and quantitative analysis of plasma and extravascular fluids revealed an abundant presence of this protein not only in plasma (59.8+/-13.3 microg/mL) but also in extravascular fluids such as follicular (34.4+/-12.7 microg/mL) and cerebrospinal (0.28+/-0.16 microg/mL) fluids, suggesting potential roles for afamin in fertility and neuroprotection. Afamin is partly (13%) bound to plasma lipoproteins. Afamin and vitamin E concentrations significantly correlate in follicular and cerebrospinal fluids but not in plasma. The vitamin E association of afamin in follicular fluid was directly demonstrated by gel filtration chromatography and immunoprecipitation which complements the in vitro findings for purified native and recombinant afamin.

Biological Transport↗