Infrared spectroscopy of nucleotides in the gas phase 2. The protonated cyclic 3',5'-adenosine monophosphate

The intracellular signal transductor cAMP, generated as a protonated species ([cAMP + H](+)) in an electrospray ionization source, has been assayed by infrared multiple photon dissociation (IRMPD) spectroscopy. IRMPD spectra have been recorded both in the 800-1800 cm(-1) mid-IR range and in the N-H and O-H stretching region, between 3200 and 3700 cm(-1). Extensive quantum chemical calculations have been performed at both B3LYP and MP2 levels of theory using the 6-311+G(2df,2p) basis set. At both levels of theory, the N7 protonated structures are predicted to be much higher in energy than the N1 and N3 protonated ones. Overall, the experimental spectrum is in very good agreement with the calculated IR spectra of the two lowest energy structures corresponding to protonation on either the N1 or the N3 adenine site. For both structures a syn conformation about the glycosidic bond is predicted at either level of theory. The NH stretching region appears to be the most structurally informative in terms of orientation about the glycosidic bond. It also suggests that the energetics derived at the MP2/6-311+G-(2df,2p) level are more reliable.