IR and UV Spectroscopy\nof Gas-Phase Monohydrated Protonated\nGuanine

We use UV and infrared photodissociation spectroscopy\nto study\nmonohydrated protonated guanine in a dual cryogenic ion trap spectrometer.\nThe monohydrated complexes are formed through helium-mediated collisions\nbetween bare electrosprayed protonated guanine and low-pressure water\nvapor in a clustering trap maintained at 180 K, before being transferred\nto a quadrupole ion trap at 10 K. The spectrum of the monohydrated\ncomplex exhibits sharp vibronic transitions at the band origin and\nbecomes broader and higher in intensity further in blue, which is\nvery similar to protonated guanine but with a notable blue shift of\n∼1850 cm<sup>–1</sup> (∼0.23 eV). The UV hole-burning\nexperiments showed that the vibronic bands recorded in the region\nof the band origin belong to a single conformer under our experimental\nconditions. The IR photodissociation spectrum in the 3000–3600\ncm<sup>–1</sup> range, with the aid of theoretical calculations\n(SCS-CC2/aug-cc-pVDZ), allowed us to assign the structure to the lowest\nenergy N<sub>7</sub>–O conformer.