Photon absorption produces highly reactive excited molecules which can undergo chemical changes. On the one hand, these changes result in the activation of photoreceptor proteins. On the other hand, light-induced chemical modifications of biomolecules can interfere with biological functions. Mechanisms of light-induced reactions are determined by the electronic structure of absorbing molecules which can be studied using computational quantum-chemistry. Nowadays, state-of-the-art quantum-chemistry program suites allow computing electronic transitions involved in biological photoreactions and vibrations of excited molecules. Even complete pathways which excited molecules follow when returning to the electronic ground state can be determined.
We use quantum-chemical methods to investigate excited molecules implicated in biological photoactivation and photodamage. Our research is focused on: (1) light activation and signaling in blue-light photoreceptors and (2) DNA photodamage and photorepair. In both fairly complex processes, light-induced electron transfer and short-lived radical species play a role. Our effort is directed to identify the underlying photoreaction mechanisms from the first principles.