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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 α 3 W 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 32 in α 3 W and compared to equivalent data recently presented for Y 32 in α 3 Y ( 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 α 3 W Pourbaix diagram displays a pK OX of 3.4, a E°'(W 32 (N •+ /NH)) of 1293 mV, and a E°'(W 32 (N • /NH); pH 7.0) of 1095 ± 4 mV versus the normal hydrogen electrode. W 32 (N • /NH) is 109 ± 4 mV more oxidizing than Y 32 (O • /OH) at pH 5.4-10. In the voltammetry measurements, W 32 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 32 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 32  • is presented, and analysis of decay kinetics show that W 32  • persists ∼10 4 times longer than aqueous W • due to significant stabilization by the protein. The redox characteristics of W 32 and Y 32 are discussed relative to global and local protein properties.

journal of the american chemical society

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