Leroux, JulietteJulietteLerouxOrtiz Mahecha, CarlosCarlosOrtiz MahechaSchubert, KajaKajaSchubertTrinter, FlorianFlorianTrinterUnger, IsaakIsaakUngerSchwob, LucasLucasSchwobBari, SadiaSadiaBari2025-04-232025-04-232025-04-04Physical Chemistry Chemical Physics 27 (16): 8320-8326 (2025)https://hdl.handle.net/11420/55418The structure and reactivity of peptides can be influenced by their protonation state. Notably, protonation of the backbone can induce structural changes, such as tautomerism, shifting from the stable keto form to the enol form. This phenomenon, particularly in the backbone protonated peptide acetyl-pentaglycine, was examined using a combination of soft X-ray action spectroscopy at the nitrogen K-edge and theoretical calculations based on density functional theory (DFT). We identified a resonance at 400 eV that can be clearly attributed to π*(C=N) transitions, linked exclusively to the enol form, as no keto form structures could replicate this resonance. These findings enhanced our understanding of the effect of protonation on the structure of peptides and could be employed for future dynamic studies.en1463-908420251683208326Royal Society of Chemistryhttps://creativecommons.org/licenses/by/3.0/Natural Sciences and Mathematics::572: BiochemistryNatural Sciences and Mathematics::541: Physical; TheoreticalNatural Sciences and Mathematics::539: Matter; Molecular Physics; Atomic and Nuclear physics; Radiation; Quantum PhysicsNatural Sciences and Mathematics::519: Applied Mathematics, ProbabilitiesJournal Articlehttps://doi.org/10.15480/882.1510310.1039/d5cp00506j10.15480/882.15103Journal Article