Vibrational spectroscopy and mode assignments for an analog of the green fluorescent protein chromophore

Infrared absorption (IR), Raman, and resonance Raman spectra have been obtained from 500 to 1700 cm-1 for 4-hydroxybenzylidene-2,3-dimethyl-imidazolinone (HBDI), an analog of the green-fluorescent protein (GFP) chromophore. Numerous transitions are evident in both the IR and Raman spectra, with the resonance Raman spectrum of HBDI dominated by a subset of transitions in the 1430–1700 cm-1 region. Assignment of the transitions in this frequency region to the corresponding normal coordinates is accomplished through computational studies employing density functional and Hartree–Fock theory. The computational results indicate that the vibrational transitions in this frequency range are dominated by in-plane stretching modes that are localized to the imidazolinone or tyrosine portions of the chromophore, rather than being delocalized over the entire chromophore. No evidence is obtained for significant excited-state structural evolution along the O–H stretching coordinate. The implications of these findings with respect to the excited-state proton transfer dynamics of GFP are discussed.