Estudo teórico da fotofísica da 2-amino-6-(2-tiazolil) purina

Abstract

The nitrogenous bases that compose the DNA and RNA are known by their photochemical stability. This stability is related with the presence of conical intersections, connecting excited electronic states to the ground state, allowing deactivation in ultrafast fashion. In parallel to the study of natural nitrogenous bases, many artificial ones have been suggested, with potential to distinct applications, depending on their properties. In this work, the exited states of the artificial bases 2-amino-6-(2-thiazolyl)purine are investigated. This molecule can be obtained by the substitution of the oxygen atom in guanine by a thiazolyl ring. Both the isolated molecule and inserted in the DNA chain present fluorescence, property normally absent in the natural bases. Due to Its fluorescence, this molecule can be utilized as a site specific labeling in experimental applications. In this work, the potential energy curves of 2-amino-6-(2-thiazolyl)purine are constructed, the crossing of potential energy surfaces are identified, in geometries similar to the ones found in guanine. These crossings present an energetical barrier between 10 and 37 kcal/mol, depending on the geometry. Consequently, the excited state has an elevated lifetime and fluorescence becomes a competitive process. A crossing geometry preserving planarity, distinct from the intersections in all natural nitrogenous bases, was also found. The possibility for this type of special geometry is due to the presence of the thiazolyl group, not found in the natural bases. Exploratory calculations were conducted using TD-DFT and multireference methods in geometries close to the region of crossing between potential energy curves to characterize the intersections. The excitations are characterized with the analysis of the orbitals and by transition density matrix analysis methods.