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Tautomeric Equilibria and pi-Electron Delocalization for the Substrate (Pyruvate) and Inhibitor (Oxamate) of Lactate Dehydrogenase - DFT Studies

Raczyńska E. D., Duczmal K., Hallmann M.

Polish Journal of Chemistry

2007

Vol. 81, nr 9

1655-1666

Keto-enol tautomerism for the substrate (pyruvate) of lactate dehydrogenase (LDH) and amideiminol tautomerism for its inhibitor (oxamate) were studied at the DFT(B3LYP)/6-31++G(d,p) level. Both anions (and also both radicals) prefer the C=O forms, i.e. the keto and amide form, respectively. The OH forms (enolpyruvate and iminoloxamate) have higher Gibbs free energies. Their amounts in the tautomeric mixtures are larger than 0.01% for anions and lower than 0.001% for radicals. pi-Electron delocalization for OCO fragments is greater than that for XCO fragments for both anions and radicals.

Prediction of tautomeric equilibrium constant in the gas phase for simple compounds containing the amidine group

Raczyński E.D., Taft R.W.

Polish Journal of Chemistry

1998

Vol. 72, nr 6

1054-1067

Prototropic tautomerism together with conformational and configurational isomerism with respect to the C-N single and C=N double bond have been considered for simple compounds containing the amidine group, -NH-C(R)=N-. General methods, based on the theoretical (AM1) calculations for prediction of the tautomeric equilibrium constant (as pK-T) in the gas phase have been discussed and illustrated by their application to cyclic and acyclic N,N'-disubstituted amidines containing substituent(s) without heteroatoms. The results obtained for simple cyclic amides are similar to those reported in solution. The pK-T values calculated for simple acyclic amidines are in good agreement with those found on the basis of the experimental substituent effects observed for non tautomerizing amidines in the gas phase.