Spectroscopic characterization of cyclic GMP in dimethylsulfoxide and water solutions

We report the spectroscopic characterization of cGMP, which has an important biological role as a second messenger. NMR and Raman spectroscopies were used to characterize the cGMPH and cGMPNa structure in solution (water and dimethylsulfoxide). The spectroscopic results show that for cGMPH in dimethylsulfoxide the N7 position is free and the proton is localized on the phosphate group. By addition of 1 and 2 HCl equivalents to cGMPNa in the same solvent the first equivalent protonates the phosphate group while the second is localized on the N7 position of the guanine ring. On the contrary, for cGMPH in water the proton is mainly localized on the N7 guanine ring with an equilibrium depending on pH. Therefore, the solvent has a major effect in determining the protonation site in cGMPH. Solid state CP-MAS 13 C NMR spectrum of cGMPH suggests N7 protonation, phosphate group ionization and a sugar conformation different from that of the salt.