The dilution of each compound in pyridine/NaOH solution resulted in spectra for each compound that was different from that of the parent 100 % DMSO solution. An expected decrease was observed in the intensity of the Soret band owing to the enhanced deprotonation of the axial water ligand on hematin due to the NaOH. In addition, the spectrum within the charge transfer region is a signature of μ-oxo dimer, which was significant in the case of hemin, H-Tyra and H-Tyros. However, there were no differences between the observed wide peak, centred at 574 nm in H-Styr in DMSO and that peak following dilution in pyridine/NaOH solution indicating the normal nature of this compound to be in a μoxo dimeric form. To further confirm the formation of dimers, the NMR spectra of hemin and H-Tyros was measured. In the case of hemin in NaOD/pyridine-d5/D2O solvent, the 1H-NMR spectrum was detected within the normal spectral range with less line broadening (Supplementary fig. ) than that in DMSO-d6 only (fig. ). However, in the case of H-Tyros, the peaks of phenyl hydrogens were additionally detected, with a better resolution of the peaks than in hemin (Supplementary fig. ).