Vibronic spectra of jet-cooled 2-Aminopurine · H₂O clusters studied by UV resonant two-photon ionization spectroscopy and quantum chemical calculations

For understanding the major- and minor-groove hydration patterns of DNAs and RNAs, it is important to understand the local solvation of individual nucleobases at the molecular level. We have investigated the 2-aminopurine 3H₂O monohydrate by two-color resonant two-photon ionization and UV/UV hole-burning spectroscopies, which reveal two isomers, denoted A and B. The electronic spectral shift δν of the S₁←S ₀ transition relative to bare 9H-2-aminopurine (9H-2AP) is small for isomer A (-70 cm^-1), while that of isomer B is much larger (δν =-889 cm^-1). B3LYP geometry optimizations with the TZVP basis set predict four cluster isomers, of which three are doubly H-bonded, with H2O acting as an acceptor to a N-H or -NH₂ group and as a donor to either of the pyrimidine N sites. The "sugar-edge" isomer A is calculated to be the most stable form with binding energy De = 56.4 kJ/mol. Isomers B and C areH-bonded between the -NH₂ group and pyrimidine moieties and are 2.5 and 6.9 kJ/mol less stable, respectively. Time-dependent (TD) B3LYP/TZVP calculations predict the adiabatic energies of the lowest ¹ππ^* states of A and B in excellent agreement with the observed 0⁰₀ bands; also, the relative intensities of the A and B origin bands agree well with the calculated S0 state relative energies. This allows unequivocal identification of the isomers. The R2PI spectra of 9H-2AP and of isomer A exhibit intense low-frequency out-of-plane overtone and combination bands, which is interpreted as a coupling of the optically excited 1ππ^* state to the lower-lying ¹nπ^* dark state. In contrast, these overtone and combination bands are much weaker for isomer B, implying that the 1ππ^* state of B is planar and decoupled from the ¹nπ^* state. These observations agree with the calculations, which predict the ¹nπ^* above the 1ππ^* state for isomer B but below the 1ππ^* for both 9H-2AP and isomer A.