Unexpected light emission from tyrosyl radicals as a probe for tyrosine oxidation

• Published in: Free radical biology & medicine 2020-06, Vol.153, p.12-16
• Main Authors: Ignasiak, Marta, Frackowiak, Kamil, Pedzinski, Tomasz, Davies, Michael J., Marciniak, Bronisław
• Format: Article
• Language: English
• ISSN: 0891-5849; 1873-4596
• DOI: 10.1016/j.freeradbiomed.2020.03.022

Tyrosine residues (Tyr) on proteins are a favoured site of one-electron oxidation due to their low one-electron reduction potentials. In this work, light-induced oxidation of Tyr residues was investigated using direct ionisation (via 266 nm light excitation) and sensitized photo-oxidation (by 3-carboxybenzophenone as sensitizer and 355 nm). Light emission (fluorescence) was observed at 410–440 nm as a result of Tyr oxidation. This novel light emission process is shown to be dependent on the solvent and aromatic ring substituents, however it does not depend on pH. It is proposed, that after initial formation of tyrosine phenoxyl radicals (TyrO●) by one electron-oxidation, the TyrO● absorbs a second photon to give an excited state species that undergoes subsequent light emission. The intensity of this emission depends on the Tyr concentration, and the detection of this emission can be used to identify and quantify one-electron formation of oxidized Tyr residues on proteins. [Display omitted] •Tyrosine residues on proteins are major targets in many oxidation reactions.•Tyrosyl radicals (TyrO.●) can be formed during direct or sensitized photo-oxidation•TyrO.● emit light (fluorescence) at ca. 430 nm after two-photon excitation•This fluorescence can be used to investigate Tyr oxidation in proteins.