A proton switch in GFP

Emission from green fluorescent protein (GFP) is one of the most widely used methods of molecular marking in cell biology, since GFP can be prepared as a fusion protein with just about any other gene product. But the fluorescence behaviour is curious and hasn�t been fully explained. In particular, it shows a t**-3/2 time dependence in the long-time tail at room temperature, but switches to a t**-1/2 dependence below 230 K. Fluorescence involves a photoexcited proton transfer from the chromophore, which is thought to occur along a hydrogen-bonded chain involving various residues and bound water molecules. Noam Agmon has modelled this process, considering the �proton wire� to be rather longer than is normally thought and to have within it a switch at a threonine residue (Thr203) with a large activation energy for proton migration (J. Phys. Chem. B 111, 7870; 2007). The rapid migration of the proton is held up at this switch point for typically 300 ps at room temperature. This model can explain both the t**-3/2 behaviour at room temperature and the changeover to different asymptotics at 230 K. Here�s another example of water and hydrogen-bonding residues collaborating to engineer biological function.