Unveiling structures and bonding mechanisms in deprotonated methionine dipeptide anions: Insights from photoelectron spectroscopy and theoretical investigation

The structural and bonding mechanisms of the deprotonated methionine dipeptide anion were investigated in detail through anion photoelectron spectroscopy coupled with theoretical calculations. By comparing calculated vertical detachment energies with experimental photoelectron spectra, the most stable structures corresponding to the distinct peaks at 4.0 and 4.4 eV were identified. It is found that the formation of intramolecular hydrogen bonds contributes to the stability of the structures. The analysis of natural population analysis charge and energy decomposition, as well as extended transition state natural orbitals for chemical valence, reveals how electron delocalization effects and fragment orbital interactions synergistically govern the stability of the deprotonated methionine dipeptide. This study offers insights into the electronic structure and conformational predispositions within biologically significant peptide systems.