Protonation and quaternization of 1, N6-ethenoadenosine: What are the species responsible for the fluorescence of 1, N6-ethenoadenosine?

Methylation of 1,N⁶-ethenoadenosine (εAdo) gives a mixture of N1- and N9-quaternized methyl-3-β-D -ribofuranosylimidazo[2,1-i] purinium salts (m¹εAdo⁺ and m⁹εAdo⁺, respectively). The ratio of the two forms of the protonated εAdo [H¹εAdo⁺]/[H⁹εAdo⁺] has been estimated to be approximately 0.10 by comparing the uv absorption spectra of the protonated species of εAdo and the two nontautomerizable model compounds. In relation to a study on the protonation effect on the fluorescence of εAdo, we have now determined the effect of quaternization on the fluorescence spectra at 293 and 77 K. We have found that m¹εAdo⁺ and m⁹εAdo⁺ are both fluorescent, and the high degree of coincidence between the fluorescence spectra of εAdo and m¹εAdo⁺ at pH 7 is noted. The m¹εAdo⁺ singlet form is a more efficient fluorescer than the m⁹εAdo⁺ ion at room temperature (quantum yields of 0.43 and 0.11, respectively). All the results which are presented in this paper are consistent with the picture that there exist more than one species responsible for the fluorescence of εAdo, depending on the environment of the molecule in aqueous solution (temperature and pH of solvent).