Cryogenic ion spectroscopy of adenine complexes containing alkali metal cations

Cryogenic ion spectroscopy was used to characterize adenine complexes containing alkali metal cations (M⁺A, M = Cs, Rb, K, Na, and Li) produced by electrospray ionization. The ultraviolet (UV) photodissociation spectra of the complexes stored in a cryogenic ion trap exhibited well-resolved vibronic bands near their origin bands of the S₀-S₁ transition. The UV-UV hole-burning and infrared ion-dip spectra showed that all the M⁺A ions in the ion trap were single isomers of M⁺A7a, where the M⁺ ion was not bound to canonical 9H-adenine (A9) but bound to a rare tautomer, 7H-adenine (A7). Density functional theory calculations showed lower tautomerization barriers for M⁺A9 than for bare A9 in aqueous solution. We suggest that M⁺ ions not only play a catalytic role in the tautomerization of A9 to A7 but also increase the tautomerization yield by forming stable M⁺A7a isomers.