Structural isomers and tautomerism of chlorophyll a in the ground state studied by semiempirical MNDO-d method

Abstract

The structural isomers and proton transfer phenomena of the chlorophyll a system in the ground state has been studied using semiempirical MNDO-d molecular orbital calculations. The preferred conformations, total energies, charge densities, electronic dipole moments and bond orders have been determined for the keto, enol tautomers and the ‘a prime’ (27-S-epimeric isomer) of chlorophyll a. Models constructed allow examination of distortions in the pyrroles and porphyrin macrocycles of these magnesium complexes. They permit estimation of changes in bond lengths and angles produced by changes in hybridization, those produced by an inversion of the asymmetry of the carbometoxy group, changes in the π-electronic system of the chlorin systems and the distortions produced by closure of the five-membered isocyclic ring when either epimerization or enolization takes place. The results obtained are in reasonable agreement with the experimental findings and reproduce many of the observed tends. The Mg complex was more stable in the keto form where the distortions are lesser in magnitude and the conjugative effects are stronger. The structure of the transition state for the interconversion process between the enol and the keto forms was found highly strained and lead to high activation energy (90 kcal mol −1). This result indicates no evidence for fast proton transfer from the enol to the keto form, which is in quite agreement with the experimental results. A survey of several neutron diffraction crystal structure, NMR and infrared analysis provides experimental evidence in qualitative support of these theoretical predictions.