Abstract
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.
Highlights
Oxidation of amino acid side chains in proteins is important in both normal human metabolism, and in a wide range of diseases [1]
In the study reported here, we describe a novel laser-induced light emission of transient species derived from the Tyrosine residues (Tyr) side chain photo-oxidation in model compounds, amino acids/peptides and proteins
A second experimental system consisting of two lasers and a spectrophotometer was constructed and employed to confirm that the light emission comes from transients derived from excited TyrO● (Supplementary Fig. 1)
Summary
Oxidation of amino acid side chains in proteins is important in both normal human metabolism, and in a wide range of diseases [1]. As a consequence of the low one-electron reduction potential of the substituted phenol ring, Tyr residues are favoured sites of one-electron oxidation and can act as ‘radical sinks’ within proteins, with this resulting in the formation of (long-lived) Tyr phenoxyl radicals, TyrO● [3]. Oxidation of Tyr residues is of potential importance in modulating the activity of tyrosine-dependent kinases and phosphatases, as alteration to these residues modulates the formation of phosphorylated (and to a lesser extent sulfated) Tyr residues [4] TyrO● are key intermediates in photosystems (e.g. photosystem II) [5], in the catalytic activity of cellular enzymes (e.g. the synthesis of 2-deoxynucleosides by ribonucleotide reductases) [6], and in the formation of structural biological polymers via radical-radical dimerization reactions to give dityrosine and higher polymers. Tyr oxidation is critical to the formation of insect cuticles [7], biological adhesives [8], the fertilization envelopes of sea urchins and oocyst walls of parasites [9]
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