Fig. 5: Effect of Fe²⁺ on the stability of biliverdin (BV) and bilirubin (BR).
From: Systemic engineering and global regulation enabling high-level bilirubin biosynthesis
a HPLC chromatograms showing the degradation of BV upon incubation with Fe²⁺, indicating stability loss. b HPLC chromatograms of BR incubated with Fe²⁺, demonstrating similar instability and degradation. c DFT-calculated free energies of Fe²⁺ coordination with deprotonated BV (deBV) and axial ligand (O₂/OH⁻) chelation in different spin states. d DFT-calculated free energies of Fe²⁺ coordination with deprotonated BR (deBR) and axial ligand (O₂) chelation in different spin states. e Frontier molecular orbitals (HOMO/LUMO) of deBV, deBV-Fe²⁺, and the case with axial ligands (isovalue = 0.02), highlighting the electronic characteristics on different atoms, green and blue represent electron probability density isosurfaces with opposite phases, respectively. f Frontier molecular orbitals (HOMO/LUMO) of deBR-Fe²⁺ with axial ligands (isovalue = 0.02), showing similar electronic characteristics to the series of deBV complexes, green and blue represent electron probability density isosurfaces with opposite phases, respectively. g, Proposed mechanism of Fe²⁺-induced oxidative ring-opening degradation of BV and BR, outlining the critical steps in degradation triggered by Fe²⁺ coordination. Source data are provided as a Source Data file.
