Ortiz Mahecha, CarlosCarlosOrtiz MahechaSchwob, LucasLucasSchwobLeroux, JulietteJulietteLerouxBari, SadiaSadiaBariMeißner, RobertRobertMeißnerBande, AnnikaAnnikaBande2025-04-232025-04-232025-03-21Physical Chemistry Chemical Physics 27 (16): 8202-8211 (2025)https://hdl.handle.net/11420/55414X-ray absorption spectroscopy (XAS) and quantum mechanical calculations bear great potential to unravel π stacking side-chain interaction properties and structure in, e.g., proteins. However, core-excited state calculations for proteins and their associated interpretation for π-π interactions are challenging due to the complexity of the non-covalent interactions involved. A theoretical analysis is developed to decompose the core-to-valence transitions into their atomic contributions in order to characterize the π stacking of aromatic amino acids as a function of their non-covalent distance change. Three models were studied as a non-covalent mixed dimers of the phenylalanine, tyrosine and tryptophan amino acids. We found that there are carbon 1s → π* charge transfer transitions associated with the non-covalently paired aromatic amino acids through their side chains. The atomic-centered contributions to the electronic transition density quantify the excited state charge transfer of the pairing amino acid models, highlighting the π stacking interactions between their aromatic side chains.en1463-90842025168202821121Royal Society of Chemistryhttps://creativecommons.org/licenses/by/3.0/de/Natural Sciences and Mathematics::539: Matter; Molecular Physics; Atomic and Nuclear physics; Radiation; Quantum PhysicsNatural Sciences and Mathematics::572: BiochemistryNatural Sciences and Mathematics::541: Physical; TheoreticalJournal Articlehttps://doi.org/10.15480/882.1509910.1039/d4cp04615c10.15480/882.15099Journal Article