Teaching conductometry : another perspective

A presentation of the concepts of electrolytic conduction in a way that avoids the drawbacks of the typical, historical presentation

Hydride-donation reaction of reduced nicotinamide adenine dinucleotide. 1. MINDO/3 and STO-3G calculations on analog reactions with cyclopropene, tropylidene, and 1,4-dihydropyridine as hydride donors and the cyclopropenium cation as acceptor

On the basis of MO arguments it is made plausible that the hydride-transfer reactions are valid and useful analogues of the NAD+/NADH hydride-transfer reaction. By means of MIND0/3-enthalpy contour maps a suitable reaction coordinate is found for each reaction. For the simplest system cyclopropene/cyclopropenium cation, the enthalpy profile along the reaction coordinate is compared with ab initio (STO-3G) results. It is demonstrated that both methods produce very similar results with regard to the overall course of the reaction (the reaction pathway), but that MIND0/3 yields a much lower activation enthalpy than STO-3G. The results indicate that the studied hydride-transfer reactions can all be conveniently described with reference to a supermolecule of C, symmetry, containing a linear C-.H.-C fragment. The reaction consists of a concerted and gradual breaking of one, and forming of the other one of the C-H bonds in this fragment. Simultaneously transfer of negative charge takes place, the overall result being the migration of a hydride ion

Quantumchemical (MNDO SCF/CI) calculations of the chiroptical properties of twisted conjugated diene and dione chromophores

In this study the utility of wavefunctions, generated with the MNDO method is investigated for the computation of the oscillator strength and the rotational strength of molecules containing twisted conjugated diene and dione chromophores. In order to improve the quality of the excited-state wavefunctions, limited configuration interaction has been applied. For a-dienes (1,3-butadiene, 1,3-cyclohexadiene, 1,4- dimethylcyclohexa-2,4-diene) the method produces results which are comparable with earlier ab initio SCF/CI calculations. The results demonstrate that for these compounds the twisting of the a-diene fragment is the main factor determining the optical activity. On the other hand, for a-diones (glyoxal, bornane-2,3-dione) it is found that significant differences exist between the chiroptical properties of unsubstituted and substituted glyoxal, even if the twist angle of the a-dione fragment is kept unaltered. It appears that for the calculation of the chiroptical properties of such compounds it is essential to be able to treat relatively large molecular systems. It is suggested that the MNDO-based approach used offers a convenient tool for such calculations

The hydride-donation reaction of reduced nicotinamide adenine dinucleotide. 2. MINDO/3 and STO-3G calculations on the role of the carbamoyl group in enzymic reactions

The enzyme-catalyzed stereospecific hydride-transfer equilibrium RH + NAD’ R+ + NADH has been studied with the help of semiempirical (MIND0/3) and ab initio (STO-3Gc)a lculations on corresponding model compounds. It seems possible to relate the stereospecificity simply to an out-of-plane orientation of the CONH2 group in the transition state of the reaction. One of the important functions of the enzyme would be to freeze the otherwise almost freely rotating CONHz group in a favorable orientation

A model study of the role of the CONH2-group in the redox coenzyme NAD(P)-NAD(P)H

A model is offered for the role of the CONH2-group in the stereospecific incorporation of hydride at C(4) in NAD(P)+-like compounds

Stereoselective hydride uptake in model systems related to the redox-couple NAD+/NADH

The present work deals with the mechanistic investigations of the hydride transfer reactions concerning the redox couple NAD+/NADH. Based on the theoretical and experimental investigatins of NAD(H) model compounds as 3-carbamoyl pyridinium cations (3-carbamoyl-1,4-dihydropyridine) it was found that the out-of-plane rotation of the carbonyl function controls the stereo-and regiospecificity of the introduced hydride anion. It was found that the hydride anion transfered in the reaction, is always syn-positioned with respect to the carbonyl group. The unique stereoselectivity exhibits a strong coherence with the recent crystallographic 3D-data for the ternary complex of NAD bonded horse liver alcohol dehydrogenase. The results show that the amide group is 30° out of the plane with the carbonyl directed toward the A side. There are observations that the absolute configuration of the introduced chirality in the 3-carbamoyl pyridinium cations selects between the hydride uptake coresponding with the enzymatic A or B specificity