Molecular mechanics study on conformation of perylene-quinonoid photosensitizers

Abstract

Using molecular mechanics method, values of the heat of formation (HF) of different conformations of perylenequinonoid photosensitizer, hypocrellin A (HA) and hypocrellin B (HB) were calculated and the variances of HF after phenolic protons’ dissociation were calculated as well. The following was found: (i) The HF values of four conformational isomers of HA and HB are similar to each other, so the four isomers can transform to each other at room temperature. (ii) There exists the difference between the ability of dissociation of phenolic protons of HA and that of HB, the former is higher than the latter. (iii) There exist two intramolecular hydrogen bonds in HA and HB. The bond energy is approximately 8 kJ/mol and the energy of conformation I is lower than that of conformation II The bond energy of HA is lower than that of HB. (iv) There exists a low energy spot when phenolic hydroxyl bond twists 180 from the psition where hydrogen bond is formed, which suggests that this kind of conformation probably exists. (v) The phenolic hydrogen of HBMC forms hydrogen bond with the neighbouring nitrogen, which is its structure basis of possessing photosensitizing activity.