Pourbaix Diagram, Proton-Coupled Electron Transfer, and Decay Kinetics of a Protein Tryptophan Radical: Comparing the Redox Properties of W and Y Generated Inside the Structurally Characterized αW and αY Proteins.

Protein-based "hole" hopping typically involves spatially arranged redox-active tryptophan or tyrosine residues. Thermodynamic information is scarce for this type of process. The well-structured αW model protein was studied by protein film square wave voltammetry and transient absorption spectroscopy to obtain a comprehensive thermodynamic and kinetic description of a buried tryptophan residue. A Pourbaix diagram, correlating thermodynamic potentials (E°') with pH, is reported for W in αW and compared to equivalent data recently presented for Y in αY ( Ravichandran , K. R. ; Zong , A. B. ; Taguchi , A. T. ; Nocera , D. G. ; Stubbe , J. ; Tommos , C. J. Am. Chem. Soc. 2017 , 139 , 2994 - 3004 ). The αW Pourbaix diagram displays a pK of 3.4, a E°'(W(N/NH)) of 1293 mV, and a E°'(W(N/NH); pH 7.0) of 1095 ± 4 mV versus the normal hydrogen electrode. W(N/NH) is 109 ± 4 mV more oxidizing than Y(O/OH) at pH 5.4-10. In the voltammetry measurements, W oxidation-reduction occurs on a time scale of about 4 ms and is coupled to the release and subsequent uptake of one full proton to and from bulk. Kinetic analysis further shows that W oxidation likely involves pre-equilibrium electron transfer followed by proton transfer to a water or small water cluster as the primary acceptor. A well-resolved absorption spectrum of W is presented, and analysis of decay kinetics show that W persists ∼10 times longer than aqueous W due to significant stabilization by the protein. The redox characteristics of W and Y are discussed relative to global and local protein properties.