Unexpected Conversion of Tyrosine into a Coumaric Acid Residue at the N‐Terminal Side of an Orexin Peptide Fragment Induced by UV Irradiation

The presence of reactive species exceeding the physiological self-defense mechanisms in cells and tissues, known as oxidative stress, usually leads to damage of DNA and proteins. In this work, we analyzed the impact of UV irradiation and radical species from different sources to the Tyr¹⁷-Leu³³ segment of orexin A peptide, which is the minimal fragment with affinity for orexin 1 receptor. As a clear oxidation process, we detected the conversion of terminal Tyrosine residue into a mixture of E/Z coumaric acid derivatives. In fact, UV irradiation and nitrosyl radicals were found to selectively induce the deamination of tyrosine into the corresponding olefin derivative, which was confirmed by HPLC-MS and NMR investigations.