To examine the protonation state and excitation dynamics of green fluorescent protein (GFP), the electronic, resonance Raman, and Fourier transform infrared spectra of GFP were compared to the corresponding spectra of an analogue of the GFP chromophore (4-hydroxybenzylidene-2,3-dimethyl-imidazolinone, HEDI). Spectra were measured with GFP in both H2O and D2O and with HBDI in ordinary or deuterated ethanol under neutral, acidic, or basic conditions. The vibrational transitions were assigned with the aid of ab initio calculations using density functional theory methods. The results indicate that the main absorption band of GFP at 400 nm represents the neutral chromophore and the minor band at 475 nm represents the anion. The resonance Raman spectra of both GFP and neutral HBDI are dominated by a band in the region of 1565 cm(-1). We assign this band to the "C=N stretch" mode, which involves stretching of the imidazolinone C=N bond and the central C=C bond of the chromophore. The absence of resonance Raman intensity along any coordinates having substantial phenolic OH stretching character indicates that stretching of the O-H bond is not coupled strongly to the optical transition. However, substantial differences are observed between the resonance Raman spectra of GFP and HBDI, suggesting that the protein does influence the structural evolution of the excited state.