Dinâmica de íons temporários de biomoléculas halogenadas

Abstract

We present results on the anion spectrum and on the vibrational dynamics induced by electron capture, for a series of halogenated molecules. The anion states were characterized by means of elastic scattering calculations, in the fixed nuclei approximation, performed with the Schwinger multichannel method with pseudopotentials. Quantum dynamics calculations of the nuclear wavepacket were performed by the propagation on potential energy surfaces described in the local approximation. The target molecules comprise chloromethane, chloroethene, uracil, 5-fluorouracil, 5-chlorouracil, 5-bromouracil, 5-iodouracil, 6-chlorouracil, 2-thiouracil, adenine, 2-chloroadenine and 8-chloroadenine. For chloromethane we computed vibrational excitation cross sections for the C−Cl stretching, and showed this mode is promptly activated by the presence of the extra charge. For chloroethene, we demonstrated that the direct mechanism of dissociation is very inefficient, and also revealed the interesting topology of its complex potential energy surfaces. The derivatives of uracil presented rich anionic spectra, as each one has three π * delocalized states, a σ * state located at the bond between the substituted atom and the ring, and a dipole bound state. Overall, the obtained energetics are in very good agreement with experimental data. Analysis of the anionic spectra and the potential energy surfaces indicate mechanisms in which the anion is formed in a long-lived π * resonance and changes its character to the dissociative σ * state. For 5-chlorouracil in particular, this coupling is mediated by an out-of-plane movement of the chlorine atom. As the anion states progressively stabilize as the halogen atomic number increases, the couplings become more favorable, and account for the increasing dissociation cross sections. Our results explain many of the observed features of dissociative electron attachment to halouracils, providing a theoretical basis for its radiosensitizing ability. In chloroadenines, we found a σ * resonance and four π * resonances. We support they could also act as potential radiosensitizers.