SOLVENT AND ELECTRONIC EFFECTS ON THE REACTION OF GUANOSINE DERIVATIVES WITH SINGLET OXYGEN

The total rate constants (sum of chemical reaction and physical quenching, k(r) + k(q)) for singlet oxygen removal by a number of 8-substituted guanosine derivatives in different solvents were measured by time-resolved infrared singlet oxygen luminescence decay. The rate constants for chemical quenching were determined separately by competition experiments. For all of the derivatives except the 8-oxo, quenching is 2 orders of magnitude faster than reaction. For the 8-ore derivative, the ratio is only around 3. There is a slight increase in both (k(r) + k(q)) and k(r) in polar solvents, and the increase is larger for the more reactive substrates. The reactivities of guanosine derivatives in photooxygenation correlate with their ionization potentials and with the Brown sigma(+) parameter. The initial interaction with singlet oxygen is discussed in terms of these parameters.