Charge distribution in 3'-deoxythymidine-fullerene: Mass spectrometry, laser excitation, and computational studies

Electrospray ionization of the donor-spacer-acceptor model system 3'-imino[60]fulleryl-3'-deoxythymidine molecule (FdT) produces deprotonated negatively charged species (dFdT). In this paper, we investigate where the negative charge is localized and whether its location can be manipulated. The fragmentation of dFdT is studied experimentally by mass spectrometry using both collisional and photoactivation. Besides fragmentation, photoexcitation of anions stored in an ion trap leads to electron photodetachment. The competition between the two channels is studied as a function of the excitation wavelength. Starting from the neutral parents, two families of dFdT molecules are computationally identified. Deprotonation takes place on the 3'-deoxythymidine (dT) subunit, either on the thymine at N3 or on the deoxyribose residue at O5'. Deprotonation in N3 leads to negatively charged molecules with an extended geometry and the excess charge largely localized on the dT. The O5'-deprotonation leads to lower-energy folded conformers stabilized by an additional bond (C-O or C-H) with the nearby C-60-N acceptor part, and the negative charge is mostly localized on the fullerene. The calculated electron detachment energies are higher for the extended N3dFdT conformers than for the O5'dFdT ones. Multiphoton photodetachment experiments at 1064 nm indicate the negative charge to be on the C-60 unit. No indication for a photoinduced charge transfer was found. In MS beside the C-60 anion a C60NH, - fragment is observed, which implies a double intramolecular H transfer. The computed energy of the corresponding dFdT, stabilized by two H-C-60 bonds, is intermediate between N3 and O5' deprotonated molecules.