Gas-phase infrared spectrum of the anionic GFP-chromophore

The gas-phase IR spectrum of the anionic chromophore of the green fluorescent protein (p-hydroxy-benzylidene-2,3-dimethylimidazolidinone, HBDI) is recorded in the 800–1800 cm−1 frequency range using the free electron laser FELIX in combination with an electrospray ionization (ESI) Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. The spectrum is substantially different from IR spectra of the anion recorded previously in solution, which were found to be difficult to interpret on the basis of electronic structure calculations involving the polarisable continuum model (PCM) method. In contrast, the IR spectrum of the isolated anion recorded here matches favourably with its DFT calculated counterpart if diffuse functions are included in the basis set. IR photo-fragmentation of the HBDI anion proceeds via loss of a methyl radical (CH3) resulting in an odd-electron product anion. The IR spectrum of this radical anion photoproduct is also recorded, which indicates that the radical site resides on the imidazolinone nitrogen atom where the methyl group is detached.