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

Abstract

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.

Tyrosine‐Rich Peptides as a Platform for Assembly and Material Synthesis

“…However, a mechanistic insight into PCET can be a powerful tool for the effective design of powerful catalysts for water splitting and solar-fuel generation by artificial photosynthesis. [67] As shown in Figure 2, the formation and decay of neutral tyrosine radicals occur by a PCET process. [64] Therefore, diverse biomimetic models have been investigated as a more structurally well-defined system to define a PCET reaction mechanism.…”

“…[67] As shown in Figure 2, the formation and decay of neutral tyrosine radicals occur by a PCET process. [67] Many biomimetic model studies on PCET use a simple platform, i.e., tyrosine attached to a photosensitizer, which is usually Ru(bpy) 3 Both the surrounding pH and specific noncovalent interactions can control the tyrosine decay rate.…”

“…[66] Therefore, the mechanism of PCET strongly depends on noncovalent interactions in proteins that can modify the overall geometry. [65,67] The importance of hydrogen bonding in tyrosine for the PCET process can also be observed in blue-light receptor proteins. In both cases, hydrogen bonding is key to determining the decay rate of tyrosine radicals, and if hydrogen bonding is either changed or disrupted, the nature of electron transfer changes significantly.…”

“…[65] Hydrogen bonding of tyrosine residues with surround residues is key to proton transfer in many biological systems, and the overlap between the donor and acceptor proton vibrational wavefunctions strongly depends on the proton-transfer distance. [67] PCET reactions mediated by tyrosine exist in biological systems, such as in PS2 and blue-light receptor proteins. [67] PCET reactions mediated by tyrosine exist in biological systems, such as in PS2 and blue-light receptor proteins.…”

Mechanisms of L‐lysine extraction with sec‐octylphenoxy acetic acid in sulfonated kerosene

“…There are four theories on the extraction mechanisms of amino acids including electrostatic interaction, Lewis acid–base interaction, proton transfer reaction and complexation extraction . The electrostatic interaction was involved in many amino acids extraction either using acidic extractant (e.g.…”