Guanine tetrads: an IRMPD spectroscopy, energy resolved SORI-CID, and computational study of M(9-ethylguanine)4+ (M = Li, Na, K, Rb, Cs) in the gas phase

The intrinsic properties of the alkali metal cationized G-tetrads, M(9eG)(4)(+) (M = Li, Na, K, Rb, Cs) composed of 9-ethylguanine (9eG), were studied by a combination of mass spectrometric techniques. The gas phase structures were probed by infrared multiple photon dissociation (IRMPD) spectroscopy in both the fingerprint region (900-1850 cm(-1)) and the N-H/C-H stretching region (2700-3800 cm(-1)). The gas phase structures were found to be similar for all five complexes and most consistent with the metal-centred G-tetrad structure. Energy-resolved CID was also used to compare the gas phase stabilities of the G-tetrads and showed that Na(9eG)(4)(+) was more stable than Li(9eG)(4)(+) followed by the K+, Rb+, and Cs+ G-tetrads in order. The experimental energy ordering was reproduced by electronic structure calculations of the energies. Furthermore, the computations also showed that the lower stability to loss of 9-ethylguanine for the Li+ complex could be due to a strong destabilization of the neutral G-tetrad due to the persistence toward maximizing the ion-dipole interactions while also maintaining hydrogen bonding interactions.