Probing the Microsolvation Environment of the Green Fluorescent Protein Chromophore in Vacuo.

We present vibrational and electronic photodissociation spectra of a model chromophore of the green fluorescent protein in complexes with up to two water molecules, prepared in a cryogenic ion trap at 160 - 180 K. We find the band origin of the singly hydrated chromophore at 20985 cm-1 (476.5 nm), and we observe partially resolved vibrational signatures. While a single water molecule induces only a small shift of the S1 electronic band of the chromophore, without significant change of the Franck-Condon envelope, the spectrum of the dihydrate shows significant broadening and a greater blue shift of the band edge. Comparison of the vibrational spectra with predicted infrared spectra from density functional theory indicate that water molecules can interact with the oxygen atom on the phenolate group or on the imidazole moiety, respectively.