Pourbaix Diagram, Proton-Coupled Electron Transfer, and Decay Kinetics of a Protein Tryptophan Radical: Comparing the Redox Properties of W₃₂^• and Y₃₂^• Generated Inside the Structurally Characterized α₃W and α₃Y 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 α₃W 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 W₃₂ in α₃W and compared to equivalent data recently presented for Y₃₂ in α₃Y (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 α₃W Pourbaix diagram displays a pK_OX of 3.4, a E°′(W₃₂(N^•+/NH)) of 1293 mV, and a E°′(W₃₂(N^•/NH); pH 7.0) of 1095 ± 4 mV versus the normal hydrogen electrode. W₃₂(N^•/NH) is 109 ± 4 mV more oxidizing than Y₃₂(O^•/OH) at pH 5.4-10. In the voltammetry measurements, W₃₂ 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 W₃₂ 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 W₃₂^• is presented, and analysis of decay kinetics show that W₃₂^• persists ∼10⁴ times longer than aqueous W^• due to significant stabilization by the protein. The redox characteristics of W₃₂ and Y₃₂ are discussed relative to global and local protein properties.