English: Hydrogen bonding with the N-terminal residues stabilizes the peroxide enolate intermediate and orients the structure for ring closing with hydroxide displacement. The chalcone peroxide enolate is shown in green with hydrogen bonding interactions in red. The amino acid side-chains are omitted for clarity. Numbering refers to the the amino group of each amino acid in the 7-mer, beginning at the N-terminus. Image inspired by Kelly et al.