Theoretical Study of (3, 6) Cyclohemiketal Form of L-Ascorbic Acid

Sažetak The question of the stability of the bicyclic form of AA is addressed by several theoretical methods varying in the level of sophistication. Both semi- empirical and ab initio approaches indicate that the bicyclic form is somewhat more stable than the 2,3-enediol tautomer (7). However, the tautomer I has a significantly higher dipole moment indicating that it gains additionally in stability if dissolved in polar solvents due to strong electrostatic solvent-solute interactions

Lowdin Atornic Charges for Modeling Long Range Deuteriurn Isotope Shifts in C-13 NMR Spectra of Binuclear Arornatic Compounđs

An approach for modeling long range deuterium isotope effects on C-13 chemical shifts, based on ab initio calculations of the differences of atomic charges between parent and deuterated molecules, is proposed. The total atomic charges were calculated for a series of molecules consisting of two benzene rings linked direct1yor via different groups, the so called binudear aromatic molecules: biphenyl, trans-stilbene, cis-stilbene, diphenylacetylene, diphenylbutadiene and their 4-deuterated derivatives. The basis sets used were: STO-3G, 6-31G, 6-31G** and MIDI. The deuteration site, i.e. C-D bond was simulated by the shortening of the corresponding C-H bond length by 0.012 A or by the reduction of CCH in-plane or out-of-plane bending angle by 1.3° and 2°, respectively. Calculations were performed both for the X-ray geometries and 6-31G fully optimized geometries. The charge differences from Lčwdin but not from Mulliken population analysis, calculated with 6-31G, 6-31G**and MIDI basis sets, showed good correlaation with the measured longrange deuterium isotope effects on C-13 chemical shifts of C-atoms, five, six, seven, eight, ten and twelve bonds away from the site of deuteration. This correlation holds only for bond shortening but not for bending angle changes, which corresponds to the predominance of stretching over bending mode contributions to isotope effects. The vibrational analysis (6-31G) revealed no coupling of C-D stretching with the vibrations of remote C-atoms. Therefore, we assume that the symmetry breaking due to unsymmetrical deuterium substitution produces a small electric dipole moment by an unbalance in the normal vibrations which in turn polarizes n-electrons, giving rise to charge changes throughout the molecule. Calculated charge redistribution due to deuterium, experimentally also observed by microwave spectroscopy, might induce long-range deuterium isotope effects on C-13 chemical shifts in agreement with the postulated C-C n-bond polarization contribution to C-13 chemical shif

ENZO: A Web Tool for Derivation and Evaluation of Kinetic Models of Enzyme Catalyzed Reactions

We describe a web tool ENZO (Enzyme Kinetics), a graphical interface for building kinetic models of enzyme catalyzed reactions. ENZO automatically generates the corresponding differential equations from a stipulated enzyme reaction scheme. These differential equations are processed by a numerical solver and a regression algorithm which fits the coefficients of differential equations to experimentally observed time course curves. ENZO allows rapid evaluation of rival reaction schemes and can be used for routine tests in enzyme kinetics. It is freely available as a web tool, at http://enzo.cmm.ki.si