Redox-Induced Conformational Switching in Photosystem-II-Inspired Biomimetic Peptides: A UV Resonance Raman Study

Abstract

Long-distance electron transfer (ET) plays a critical role in solar energy conversion, DNA synthesis, and mitochondrial respiration. Tyrosine (Y) side chains can function as intermediates in these reactions. The oxidized form of tyrosine deprotonates to form a neutral tyrosyl radical, Y^•, a powerful oxidant. In photosystem II (PSII) and ribonucleotide reductase, redox-active tyrosines are involved in the proton-coupled electron transfer (PCET) reactions, which are key in catalysis. In these proteins, redox-linked structural dynamics may play a role in controlling the radical’s extraordinary oxidizing power. To define these dynamics in a structurally tractable system, we have constructed biomimetic peptide maquettes, which are inspired by PSII. UV resonance Raman studies were conducted of ET and PCET reactions in these β-hairpins, which contain a single tyrosine residue. At pH 11, UV photolysis induces ET from the deprotonated phenolate side chain to solvent. At pH 8.5, interstrand proton transfer to a π-stacked histidine accompanies the Y oxidation reaction. The UV resonance Raman difference spectrum, associated with Y oxidation, was obtained from the peptide maquettes in D₂O buffers. The difference spectra exhibited bands at 1441 and 1472 cm^–1, which are assigned to the amide II′ (CN) vibration of the β-hairpin. This amide II′ spectral change was attributed to substantial alterations in amide hydrogen bonding, which are coupled with the Y/Y^• redox reaction and are reversible. These experiments show that ET and PCET reactions can create new minima in the protein conformational landscape. This work suggests that charge-coupled conformational changes can occur in complex proteins that contain redox-active tyrosines. These redox-linked dynamics could play an important role in control of PCET in biological oxygen evolution, respiration, and DNA synthesis