Computational Chemistry for Photosensitizer Design and Investigation of DNA Damage

Computational chemistry can be used for the prediction of photochemical reactivity and the design of photosensitizers for cancer phototherapy. For example, the activity of a photosensitizer for DNA damage can be estimated from the calculation of the HOMO energy of the molecules. In general, DNA damage is mediated by the following two processes: 1) photo-induced electron transfer from the DNA base to the photoexcited photosensitizer and 2) base modification by singlet oxygen generation through photoenergy transfer from the photosensitizer to oxygen. The DNA-damaging activity of the photosensitizer through electron transfer is closely related to the HOMO energy level of the molecule. It has been demonstrated that the extent of DNA damage photosensitized by xanthone analogues is proportional to the energy gap between the HOMO level of the photosensitizer and that of guanine. In addition, computational chemistry can be used to investigate the mechanism of the chemopreventive effect on phototoxicity. Furthermore, the molecular orbital calculation is useful to design a photosensitizer in which the activity of singlet oxygen generation is controlled by DNA recognition. Singlet oxygen is an important reactive oxygen species to attack cancer. The control of singlet oxygen generation by DNA is necessary to achieve the tailor-made cancer photo-therapy. Several porphyrin photosensitizers have been designed on the basis of the molecular orbital calculation to control the activity of singlet oxygen generation.

Keywords: Ab initio molecular orbital calculation, density functional treatment (DFT), DNA damage, electron transfer, highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), molecular mechanics calculation, photosensitizer, porphyrin, redox potential, singlet oxygen (1O2), Zerner’s intermediate neglect of differential overlap (ZINDO) procedure.