Comparison of the deactivation mechanism of 5-fluorouracil with that of its parent system, Uracil: The need of the use of the MS-CASPT2 method

摘要

Because of their biological significance in radiation damage and repair of the genetical material, a detailed understanding of the photophysical properties of DNA bases is very important. The key property in DNA is the ultrafast radiationless decay to the ground state that allows dissipating the energy before harmful photoproducts may be formed. Experimental results in the investigation of uracil suggest that there exist a pair of competitive mechanism for the radiationless deactivation of this basis. It is well established that the key point of its ultrafast radiationless decay is a Sπ/S0 conical intersection [where Sπ is the excited state populated by the photoexcitation, characterized by a ~1(π-π7z.ast;) excitation], but there are still some point of controversy, like the possible existence and involvement of a stable Sπ species in the deactivation mechanism. Nucleobases tautomers and derivatives usually have different photochemical properties, and in general, they show a slower deactivation mechanism. The study of some derivatives of uracil can be interesting for comparative purposes and to help in the reassurance of the deactivation mechanisms proposed for uracil. This is one of the reasons for developing our study on 5-fluorouracil (5-FU), which exhibits much longer lived excited states than the parent system uracil. On top of this, 5-FU is also of interest by itself because it is used in cancer treatment. We have studied the ground and first excited singlet states of 5-FU using the complete active space self-consistent field method (CASSCF)/MS-complete active space perturbation theory to second order level (CASPT2) protocol that is known to provide accurate results for this type of systems. Minima, potential energy surface (PES) crossings and paths have been characterized and compared with those of uracil. Our results show that for 5-FU, as opposite to uracil, a minimum exists in the PES of the Sπ state. The involvement of this minimum in the deactivation mechanism together with the presence of a small barrier in the relaxation path toward the Sπ/S0 conical intersection explain the longest decay time of the excited state of 5-FU.