Proton-Coupled Electron Transfer in Tyrosine and a β‑Hairpin Maquette: Reaction Dynamics on the Picosecond Time Scale

In proteins, proton-coupled electron transfer (PCET) can involve the transient oxidation and reduction of the aromatic amino acid tyrosine. Due to the short life time of tyrosyl radical intermediates, transient absorption spectroscopy provides an important tool in deciphering electron-transfer mechanisms. In this report, the photoionization of solution tyrosine and tyrosinate was investigated using transient, picosecond absorption spectroscopy. The results were compared to data acquired from a tyrosine-containing β-hairpin peptide. This maquette, peptide A, is an 18-mer that exhibits π–π interaction between tyrosine (Y5) and histidine (H14). Y5 and H14 carry out an orthogonal PCET reaction when Y5 is oxidized in the mid-pH range. Photolysis of all samples (280 nm, instrument response: 360 fs) generated a solvated electron signal within 3 ps. A signal from the S₁ state and a 410 nm signal from the neutral tyrosyl radical were also formed in 3 ps. Fits to S₁ and tyrosyl radical decay profiles revealed biphasic kinetics with time constants of 10–50 and 400–1300 ps. The PCET reaction at pH 9 was associated with a significant decrease in the rate of tyrosyl radical and S₁ decay compared to electron transfer (ET) alone (pH 11). This pH dependence was observed both in solution and peptide samples. The pH 9 reaction may occur with a sequential electron-transfer, proton-transfer (ETPT) mechanism. Alternatively, the pH 9 reaction may occur by coupled proton and electron transfer (CPET). CPET would be associated with a reorganization energy larger than that of the pH 11 reaction. Significantly, the decay kinetics of S₁ and the tyrosyl radical were accelerated in peptide A compared to solution samples at both pH values. These data suggest either an increase in electronic coupling or a specific, sequence-dependent interaction, which facilitates ET and PCET in the β hairpin.