A TD-DFT study on photo-physicochemical properties of hypocrellin A and its implications for elucidating the photosensitizing mechanisms of the pigment

As a naturally occurring perylenequinonoid pigment, hypocrellin A (HA) has gained much attention in recent years owing to its excellent photosensitive properties. However, there is little theoretical study on the photo-physicochemical behaviors of HA, which stimulated our interest to perform the study by means of time-dependent density functional theory (TD-DFT) calculations. In this paper, the TD-DFT calculations provide various excited state properties of HA in polar and non-polar solvents, including absorption spectrum, lowest triplet excited state energy, vertical electron affinity and vertical ionization potential, by which we can get some deeper insights into the photosensitive mechanisms of hypocrellin. First, although HA can photo-generate 1O2 through energy transfer in both non-polar and polar solvents, it gives birth to O2− only in polar solvents, and it is the HA anion (generated from autoionization) that are responsible for the O2−-generation. Second, HA cation (also generated from autoionization) is more favored in thermodynamics than triplet excited HA to accept electron from DNA bases. As a result, HA cation likely takes more responsibility than excited HA for the pigment's photo-damage to DNA in oxygen-free media.