4-Aminobenzoic acid 4-methylpyridine/4-methylpyridinium 4-aminobenzoate 0.58/0.42: a redetermination from the original data

The title structure, 4-aminobenzoic acid 4-methylpyridine/4-methylpyridinium 4-aminobenzoate 0.58/0.42, 0.58(C6H7N·C7H7NO2)·0.42(C6H8N+·C7H6NO2−), has been redetermined from the data published by Kumaret al.(2015).Acta Cryst.E71, o125-o126. The improvement of the present redetermination consists in the introduction of disorder of the methyl group over two positions as well as in the correction of the positional parameters of the hydrogen atoms that are involved in the O—H...N or N—H...O hydrogen bonds. After the correction, the hydroxyl hydrogen atom turned out to be disordered over two positions about the centre of the O...N bond, which is relatively long [2.642 (2) Å], while the H atoms of the primary amine group account more realistically for the hydrogen-bond pattern after the removal of the positional constraints. All the O—H...N or N—H...O hydrogen bonds which are present in the title structure are of moderate strength.

Conformational phase transition of deuterated p-bromobenzyl alcohol as studied by neutron powder diffraction

A neutron powder diffraction study on the crystal structure of the title compound (p-Br–C6D4–CD2–OD) confirmed that a first-order phase transition at Tt1=229 K accompanied a drastic change in the molecular conformation caused by a discontinuous rotational shift of the hydroxyl hydrogen atom around the C(D2)–O(D) bond. At T<Tt1, a contraction of the unit cell volume of ∼1% was found when compared to that of the normal compound (p-Br–C6H4–CH2–OH).

Microwave Spectrum, Dipole Moment, and Intramolecular Hydrogen Bond of 2-Methoxyethanol

The minimum energy conformation of 2-methoxyethanol (CH3OCH2CH2OH) has been determined from an analysis of its microwave spectrum. The rotational constants of the normal species are: A = 12982.35, B = 2742.48, and C = 2468.10 MHz; the dipole moment components are μa = 2.03, μb = 1.15, [Formula: see text] and μ = 2.36 ± 0.03 D. For the CH3OCH2CH2OD species: A = 12385.71, B = 2724.74, and C = 2431.42 MHz. The conformation consistent with this data is gauche about each of the C—C, C—O(H) and C—O(ether) bonds, having dihedral angles of 57 ± 3°, 45 ± 5°, and 8 ± 3°, respectively. This distorted conformation is one in which the hydroxyl hydrogen atom is approximately aligned with the nearest sp3 lone pair electrons of the ether oxygen atom. Transitions in three excited torsional states have also been observed but no other rotational isomer was detected.

A MOLECULAR MECHANISM FOR ENZYMATIC DEHYDROGENATIONS INVOLVING PYRIDINE NUCLEOTIDES

The main kinetic results obtained with the dehydrogenases are briefly summarized. It is shown that the mechanism must involve a ternary enzyme–substrate–coenzyme complex, that acidic and basic groups on the enzyme surface must be involved in the reaction, and that there appears to be a transfer of the substrate from one site to another in the rate-determining step. It is suggested that this step is the actual hydrogen-transfer process. When the substrate is undergoing oxidation, this transfer is brought about by a nucleophilic attack by a basic group B−at the enzyme's active center. This attack may be on a hydroxyl hydrogen atom, and is considered to lead to the transfer of H−from the substrate to the coenzyme. The product is held to the enzyme by hydrogen bonding between the group BH (formed by the addition of a proton to the basic group B−) and the carbonyl group on the substrate.

A MOLECULAR MECHANISM FOR ENZYMATIC DEHYDROGENATIONS INVOLVING PYRIDINE NUCLEOTIDES

The main kinetic results obtained with the dehydrogenases are briefly summarized. It is shown that the mechanism must involve a ternary enzyme–substrate–coenzyme complex, that acidic and basic groups on the enzyme surface must be involved in the reaction, and that there appears to be a transfer of the substrate from one site to another in the rate-determining step. It is suggested that this step is the actual hydrogen-transfer process. When the substrate is undergoing oxidation, this transfer is brought about by a nucleophilic attack by a basic group B−at the enzyme's active center. This attack may be on a hydroxyl hydrogen atom, and is considered to lead to the transfer of H−from the substrate to the coenzyme. The product is held to the enzyme by hydrogen bonding between the group BH (formed by the addition of a proton to the basic group B−) and the carbonyl group on the substrate.