Background: Recent studies of melanin biosynthesis have uncovered an unusual enzymatic activity which converts the non-naturally occurring D-isomer of 2-carboxy-2, 3-dihydroindole5,6-quinone (dopachrome) into 5,6-dihydroxyindole-2-carboxylic acid (DHICA). The aim of the present investigation was to isolate and characterize the enzyme catalyzing this tautomerization reaction. Materials and Metho...

The DFT calculations at the B3LYP level with 6-311G** basis set were carried out in order to reveal whether tautomerization or decarboxylation is responsible for the instability of 2,2-di(pyridin-2-yl)acetic (DPA) and 1,8-diazafluorene-9-carboxylic (DAF) acids. The carboxyl protons in both compounds are involved in the intramolecular hydrogen bonds (the pyridine nitrogen atoms are the hydrogen ...

2,3,7,8,12,13,17,18-Octaethyl-5,15-diphenylporphyrin (1) is characterized by an inner cavity with a rectangular shape and small NH⋅⋅⋅N distances. It resembles porphycene, which is a constitutional isomer of porphyrin known for its strong intramolecular hydrogen bonds and rapid tautomerization. Such distortion of the porphyrin cavity leads to tautomeric properties of 1 that are intermediate betw...

MP2, DFT and CCSD methods with 6-311++G** and aug-cc-pvdz basis sets have been used to probe the structural changes and relative energies of E-prop-2-ynylideneamine (I), Z-prop-2-ynylideneamine (II), prop-1,2-diene-1-imine (III) and vinyl cyanide (IV). The energy near-equivalence and provenance of preference of isomers and tautomers were investigated by NBO calculations using HF and B3LYP metho...

High-level quantum-chemical and quantum-dynamics calculations are reported on the tautomerization equilibria and rate constants of isolated and monohydrated cytosine and guanine molecules. The results are used to estimate the fraction of the bases present in the cell during DNA synthesis as the unwanted tautomers that forms irregular base pairs, thus giving rise to a spontaneous GC→AT point mut...

A concise synthesis of rubioncolin B is described, which features an unprecedented intramolecular Diels-Alder reaction involving an ortho-quinone methide and a naphthofuran moiety. The ortho-quinone methide is generated through a surprisingly facile tautomerization of a para-quinone.

Relative tautomerization energies, dipole moments and polarizabilities for the tautomers of (R)-4-amino-1,2oxazolidin-3-one (Cycloserine) was studied by quantum-chemical calculations, using the HF and DFT(B3LYP) level of theory with the 6-311++G(d,p) basis set in the gas phase and different solvents using SCRF model, with full geometry optimization. Entropies, enthalpies, Gibbs free energies an...

The binding of an excess electron to HCN and HNC was studied at the coupled cluster level of theory with single, double, and noniterative triple excitations and with extended basis sets to accommodate the loosely bound excess electron. The HCN molecule, with a dipole moment of 3.05 Debye, binds an electron by 10 cm, whereas the HNC tautomer possesses a similar dipole moment ~3.08 Debye! and bin...