We obtain the ultraviolet photodissociation spectrum of adenine complexes with K+ ion stored in a cold ion trap. The spectrum near the origin band of the S-0-S-1 transition exhibits well-resolved vibronic bands, all of which are assigned from a single isomer by UV-UV hole-burning (HB) spectroscopy. Comparing the spectrum with theoretical spectra, we determine the structure of the isomer, where K+ is bound not to 9H-adenine (A9) but to 7H-adenine (A7). We suggest that K(+)A7 ions are formed in solution through tautomerization, for which the energy barrier varies greatly depending on the binding site of the K+ ion on A9. (C) 2015 Elsevier B.V. All rights reserved.