Weighing Photon Energies with Mass Spectrometry: Effects of Water on Ion Fluorescence

We report a new, highly sensitive method for indirectly measuring fluorescence form ions with a discrete number of water molecules attached. Absorption of a 248 nm photon by hydrated protonated proflavine, PH*(H2O)(n) (n = 13-50), results in two resolved product ion distributions that correspond to full internal conversion of the photon energy (loss of similar to 11 water molecules) and to partial internal conversion of the photon energy and emission of a lower energy photon (loss of similar to 6 water molecules). In addition to fluorescence, a long-lived triplet state with a half-life of similar to 0.5 s (for n = 50) is formed. The energy of the emitted photon can be obtained from the number of water molecules lost form the precursor to form each distribution. The photon energies generally red shift form similar to 450 to 580 nm with increasing cluster size (the onset of the PH*(aq) fluorescence spectrum is 600 nm and the maximum is 518 nm) consistent with preferential stabilization of the first excited singlet state versus the ground state. The fluorescence quantum yield of PH*(H2O)(n) for n >= 30 is 0.36 +/- 0.02, the same as that in bulk solution, and increases dramatically with decreasing cluster sizes, due to less efficient conversion of electronic-to-vibrational energy. The high sensitivity of this method should make it possible to perform Forster resonance energy transfer experiments with gas-phase biomolecules in a microsolvated environment to investigate how a controlled number of water molecules facilitates dynamical motions in proteins or other molecules of interest.