Building up water-wire clusters: Isomer-selective ultraviolet and infrared spectra of jet-cooled 2-aminopurine (H2O)n, n = 2 and 3

2-Aminopurine (2AP) is an adenine analogue with a high fluorescence quantum yield in water solution, which renders it a useful real-time probe of DNA structure. We report the ultraviolet (UV) and infrared (IR) spectra of size-selected and jet-cooled 9H-2AP(H₂O)_n clusters with n = 2 and 3. Mass- and species-specific UV/UV holeburning spectroscopy allows to separate the UV spectra of four cluster isomers in the 31200-33000 cm ^-1 spectral region with electronic band origins at 31339, 31450, 31891, and 32163 cm^-1. Using IR/UV depletion spectroscopy in combination with B3LYP calculated harmonic vibrational frequencies, the H-bonding topologies of two isomers of the n = 2 and of two isomers of the n = 3 cluster are identified. One n = 2 isomer (denoted 2A) forms a water dimer chain between the N₉H and N₃ atoms at the sugar-edge site, the other isomer (denoted 2D) binds one H₂O at the sugar-edge site and the other at the trans-amino site between the N₁ atom and the NH ₂ group. For 2-aminopurine(H₂O)₃, one isomer (denoted 3A) forms an H-bonded water wire at the sugar-edge site, while isomer 3B accommodates two H₂O molecules at the sugar-edge and one at the trans-amino site. The approximate second-order coupled cluster (CC2) method predicts the adiabatic S₁ ← S₀ transitions of 9H-2-aminopurine and six water cluster isomers with n = 1-3 in very good agreement with the experimental 000 frequencies, with differences of <0.6%. The stabilization of the S₁(ππ*) state of 2-aminopurine by water clusters is highly regiospecific: Isomers with one or two H ₂O molecules H-bonded in the trans-amino position induce large spectra red shifts, corresponding to ¹ππ* state stabilization of 10-12 kJ/mol, while water-wire cluster solvation at the sugar-edge leads to much smaller stabilization. The evolution of the IR spectra of the water-wire clusters with n = 1-3 that are H-bonded to the sugar-edge site is discussed. Qualitatively different regions (denoted I to IV) can be attributed to the different free and H-bonded OH, NH, NH₂ and OH···OH water-wire stretch vibrations.