Photochemical studies of DNA and RNA bases, nucleosides, nucleotides, and dinucleotides

The 254 nm photodestruction quantum yield of a wide range of DNA model compounds were determined in the presence or absence of additives that react specifically with previously identified reactive intermediates. Competition experiments suggested that the formation of the electron adducts of the purine derivatives was one of the principal reactive species. The major products of the adenine and guanine nucleosides were identified and quantified upon 254 nm irradiation or 266 nm laser excitation. It was shown that one of the principal effects of UV excitation is the breakage of the N-glycosidic bond and opening of the imidazole ring.; The mechanism through which the one-electron photo-oxidation (PO) of a wide range of purine and pyrimidine derivatives, and dinucleotides, upon 266 nm laser excitation, and the effect of sequence, conformation, base stacking, and ground state concentration of the dinucleotides on the one-electron PO yield were investigated. It was demonstrated that the photoionization (PI) occurs through a combination of one- and two-photon pathways. The net PI yields were used as a novel and more direct approach to get relative IPs of different dinucleotides and to predict the efficiency of hole-trapping and charge transport in DNA. To support these experimental results, the ionization thresholds of the DNA bases and of its nucleosides in vacuum and aqueous phase were determined by ab initio calculations using the 6-31++G(d,p) basis set at the Hartree-Fock and Möller-Plesset level of theory. For the first time, the vertical and adiabatic gas and condensed phase ionization potentials (IP) of various DNA nucleosides were calculated at the MP2/6-31++G(d,p) level of theory. It was demonstrated that long-range bulk polarization and short-range hydrogen bonding interactions dramatically stabilize the first vertical and adiabatic IP of the bases and nucleosides.; Lifetimes of the first singlet excited states of various dinucleotides, their time resolved anisotropy, the rotational diffusion coefficient, and correlation times were also measured. These physical parameters were used to probe the effect of sequence and base stacking in the dynamics of the single excited states of the dinucleotides.