The m/z calculated for hemin ([C34H32FeN4O4] + ) at 616.18 was observed at 616.1 using ESI-MS, in addition to the other isotopic peaks (Supplementary fig. ). The fragmentation of hemin was found to take place mainly via the degradation of the two peripheral -CH2COOH groups leaving intact tetrapyrrolic macrocycle [C30H26FeN4] + (m/z 498.1) with only -CH3 and -CH=CH2 side groups (fig. ).