Biophysical Society Conference | Tahoe 2023

Proton Reactions: From Basic Science to Biomedical Applications

Monday Speaker Abstracts

QUANTUM MECHANICAL TUNNELING IN PROTON WIRES Paul M Champion 1 ; 1 Northeastern University, Physics, Boston, MA, USA The importance of quantum mechanical proton tunneling in biological proton transport chains (or “wires”) is sometimes overlooked. Here we examine the short proton wire in the green fluorescent protein (GFP) and experimentally monitor the vibrationally equilibrated proton back transfer reaction that takes place in the ground electronic state. The back transport chain involves the chromophore phenolate group H-bonded with an associated water-serine-glutamic acid triad. Back proton transfer takes place following a short laser pulse that initiates forward transport in the excited state on a 1-10 ps timescale. The time-dependent populations of the entire photocycle, involving forward transfer, green fluorescence emission, and back transfer are monitored with optical pump-probe techniques. It is found that the rate limiting step of the back transport process involves asynchronous concerted proton tunneling from the ionization resistant serine hydroxyl. The observed tunneling kinetics reveal a kinetic isotope effect (KIE) that varies between 75-35 over the temperature range 80-300K. The rapid ~400 ps timescale observed for tunneling near room temperature suggests a functional role for proton tunneling that goes beyond the short proton wire in GFP. In longer proton wires, which involve multiple waters within a protein environment, serine residues are often considered only as structural stabilizers of the water molecules. However, as demonstrated by GFP, serine (and presumably threonine) can also directly participate in the transport process via tunneling. Importantly, tunneling can also kinetically bias the proton flow direction by a dynamic (and super-exponential) trapping mechanism that is based on slight differences in the tunneling distances between the hydrogen bonded proton acceptors that lie on opposite sides of the serine residue. Slower ( > 400 ps) correlated H-bond reorientations of the serine and water wire network segments that are required for continuous proton pumping can “mask” the KIE and the underlying role of proton tunneling in such systems.

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