The Peptide Bond: Resonance Increases Bond Order and Complicates Fragmentation

Abstract The enhancement of the peptide bond order by a resonance in the lone pair of N and the π‐bond of CO is analyzed. A decomposition of the bond order in terms of localized molecular orbitals is developed and applied to the peptide bond. A combination of two rotations of hybrid orbitals is proposed to improve the boundary treatment in the fragment molecular orbital method. The developed approach is applied to peptide bonds, and it is found crucial to retain the π orbital in the variational space of both fragments across the boundary. The interaction energies between conventional amino acid residues in Trp‐cage (1L2Y) are discussed.