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. (C) 2001 Elsevier Science B.V. All rights reserved.