Rate of photochemical protonation and electronic relaxation of excited 1, N6-ethenoadenosine in its aqueous solution

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Abstract
Fluorescence lifetimes and intensities have been examined for 1, N6‐ethenoadenosine (εAdo), 1‐methylethenoadenosine (m1εAdo+), 9‐methylethenoadenosine (m9εAdo+), N1‐deazaethenoadenosine (N1 deaza‐εAdo), and 9‐methyl‐N1‐deazaethenoadenosine (m 9N1deaza‐εAdo+) at various pH′s in the 7.5–0.5 region and at 25 °C. From an analysis, the following conclusions are reached: (1) The strong fluorescence of εAdo is caused by the neutral form (but not protonated form) of its excited state (εAdo*). (2) The protonation of εAdo* takes place at position 9 (but not position 1) with the rate constant of 1.45×1010 s−1 M−1, and the deprotonation of H9 εAdo+* with the rate constant of 1.15×109 s−1, so that the excited state pK*a is 1.10. (3) In the lower pH solution, the fluorescence of εAdo is lowered partly by a proton‐induced radiationless transition of the neutral εAdo* itself, and partly through the protonation of εAdo into (εAdoH+)*, which is also subjected to a proton‐induced radiationless transition. All of the ten rate constants involved in the relaxation process of εAdo* have been unequivocally determined.