Abstract
The bright fluorescence of green fluorescent protein (GFP) from Aequorea victoria arises from the excited state proton transfer (ESPT) of its p-hydroxybenzylidene imidazolinone chromophore in the binding pocket. Recent ultrafast Raman studies have observed vibrational coupling between a low frequency mode (∼100 cm−1) and the phenolic CH bend (∼1140 cm−1) of the chromophore in the ESPT, spurring a debate over the assignment of the low frequency mode, its importance in the reaction, and the vibrational coupling mechanism. We performed a hybrid quantum mechanics/molecular mechanics calculation to analyze the vibrational modes of GFP below 300 cm−1. We identified one low-frequency mode (128 cm−1) with a strong anharmonic coupling that can modulate the resonance Raman intensity of the phenolic CH bend. The low frequency mode corresponds to phenolic twisting, which is unlikely to promote ESPT directly.