Ab initio study of the electron transfer in an ionized stacked complex of guanines.

The charge transfer process in an ionized stacking of two consecutive guanines (G(5')G(3'))(+) has been studied by means of state-averaged CASSCF/MRCI and RASSCF/RASPT2 calculations. The ground and two first excited states of the radical cation have been characterized, and the topology of the corresponding potential energy surfaces (PESs) has been studied as a function of all intermolecular geometrical parameters. The results demonstrate that the charge transfer process in (G(5')G(3'))(+) is governed by the avoiding crossing between the ground and first excited states of the complex. Relative translation motions of both guanines in their molecular planes are shown to lead to the charge migration between G(5') and G(3'). Five stationary points (three minima and two saddle points) have been characterized along the reaction path describing the passage of the positive charge from G(5') to G(3'). The global minimum on the PES is found to correspond to the charge configuration G(5')(+)G(3'). The existence of an intermediate minimum along the reaction path has been established, characterizing a structure where the positive charge is equally distributed between the two guanines. The calculated energy profile allowed us to determine the height of the potential energy barrier (7.33 kcal/mol) and to evaluate the electronic coupling at a geometry close to the avoiding crossing (3.6 kcal/mol). Test calculations showed that the topology of the ground state PES of the complex GG(+) is qualitatively conserved upon optimization of the intramolecular geometrical parameters of the stationary points.