Liquid-microjet photoelectron spectroscopy of the green fluorescent protein chromophore

Green fluorescent protein (GFP), the most widely used fluorescent protein for in vivo monitoring of biological processes, is known to undergo photooxidation reactions. However, the most fundamental property underpinning photooxidation, the electron detachment energy, has only been measured for the deprotonated GFP chromophore in the gas phase. Here, we use multiphoton ultraviolet photoelectron spectroscopy in a liquid-microjet and high-level quantum chemistry calculations to determine the electron detachment energy of the GFP chromophore in aqueous solution. The aqueous environment is found to raise the detachment energy by around 4 eV compared to the gas phase, similar to calculations of the chromophore in its native protein environment. In most cases, electron detachment is found to occur resonantly through electronically excited states of the chromophore, highlighting their importance in photo-induced electron transfer processes in the condensed phase. Our results suggest that the photooxidation properties of the GFP chromophore in an aqueous environment will be similar to those in the protein. The electronic structures of photoactive proteins underlie many natural photoinduced processes. The authors, using UV liquid-microjet photoelectron spectroscopy and quantum chemistry calculations, determine electron detachment energies of the green fluorescent protein chromophore in aqueous solution, approaching conditions of the protein environment.

Automated summary

This study determines the electron detachment energy of the green fluorescent protein chromophore in aqueous solution using UV liquid-microjet photoelectron spectroscopy and quantum chemistry calculations. The research highlights the importance of electronically excited states of the chromophore in photo-induced electron transfer processes in the condensed phase.