A theoretical study on isomerization process of hypocrellin A in ground state

Abstract

The isomerization of hypocrellin A (HA), a naturally occurring perylenequinonoid photosensitizer, has aroused much interest in recent years, which comprises intramolecular H-atom transfer (IHT) and helix interconversion processes. Although much effort has been devoted to investigating the processes by means of various spectroscopy methodologies, there is no theoretical study on the isomerization of HA, because the molecule is too large to do high-level quantum chemical calculations. In this paper, a combined ab initio method RHF/6-31G(d,p)//AM1 was employed to calculate the ground-state-energies associated with the different isomers and the transition states for the IHT of HA. It was found that: (i) the most stable isomers for HA were the left-handed-4,9- or 3,10-quinones, in good agreement with the experimental findings; (ii) the barriers for four IHT processes are around 40 kJ/mol, very similar to the barriers for active center of HA, suggesting that the side chains of HA have little influence on the IHT barriers; and (iii) the dipole moments for the IHT transition states vary a little compared with those of the initial states, suggesting that the isomerization of HA in ground state will be slightly influenced by solvent polarity.