Quantum Mechanics in Biology: Photoexcitations in DNA Eric R. Bittner and Arkadiusz Czader 1 Department of Chemistry and the Texas Center for Superconductivity, University of Houston, Houston, TX 77204 bittner@uh.edu 2 Department of Chemistry and the Texas Center for Superconductivity, University of Houston, Houston, TX 77204 Abstract. We consider here the theoretical and quantum chemical description of the photoexcitated states in DNA duplexes. We discuss the motivation and limi- tations of an exciton model and use this as the starting point for more detailed excited state quantum chemical evaluations. In particular, we focus upon the role of interbase proton transfer between Watson/Crick pairs in localizing an excitation and then quenching it through intersystem crossing and charge transfer. 1 Quantum Biology But when a biochemist begins to use quantum-mechanical language . . . we may justifiably suspect he is talking nonsense. H. C. Longuet- Higgins, “Quantum mechanics and biology”, Biophys. J. 2, 207-213 (1962). At the risk of being too broad and perhaps too conservative, very few processes that occur in a biological system require a deep understanding of quantum theory. While all intermolecular forces and chemical structures are ultimately of quantum mechanical origin and their proper description does require the use of quantum theory, very few reactions are truly quantum me- chanical. The reason is that in order for something to exhibit quantum like behavior, quanta of energy being exchanged must be discrete and large com- pared to the thermal energy. Thus, few ordinary chemical processes meet this criteria. Those that do, however, typically involve excitations of the electronic states, excitations of highly-local high-frequency vibrational modes, such as a CO group bound to the metal center on a heme, or tunneling of either an electron or proton between a donor and acceptor. The quotation at the beginning of this section comes from an address given by H. C. Longuet-Higgins in the early 1960 at a workshop on “Emerging techniques in Biophysics”. In it, he concludes that at the moment (in 1960) there was very little point in trying to conjure up a quantum theory to explain © Springer-Verlag Berlin Heidelberg 2009 , I. Burghardt et al. (eds.), Energy Transfer Dynamics in Biomaterial Systems Springer Series in Chemical Physics 93, DOI 10.1007/978-3-642-02306-4_4, 103