We present an ab initio molecular dynamics study of the chromophore of green fluorescent protein (GFP) in the four relevant protonation states. Ground-state geometries, electronic structures, and vibrational spectra are calculated. Patterns of vibrations are assigned to recently detected Raman bands. These results provide insight into the correlation between vibrational frequencies and optical absorption wavelengths of the GFP chromophore, observed upon mutations of its environment. An intramolecular mode-coupling mechanism is suggested, which rationalizes the coherent dynamics recently revealed by ultrafast pump-and-probe optical spectroscopy. This mechanism is tested by a simulation mimicking the dynamics after the electronic excitation, which shows a coupling between high and low-frequency modes.