Photooxidation of Dipyrrinones: Reaction with Singlet Oxygen and Characterization of Reaction Intermediates

Bilirubin (BR) is a water-insoluble product of heme catabolism in mammals. Elevated blood concentrations of BR, especially in the neonatal period, are treated with blue-green light phototherapy. The major mechanism of BR elimination during phototherapy is photoisomerization, while a minor, less studied mechanism of degradation is oxidation. In this work, we studied the oxidation of the bilirubin model tetramethyl-dipyrrinone (<i>Z</i>-<b>13</b>) by singlet oxygen in methanol using UV-vis and ESI-MS spectroscopy, resulting in propentdyopents as the main oxidation products. We also identified two additional intermediates that were formed during the reaction (hydroperoxide <b>21a</b> and imine <b>17</b>). The structure of the hydroperoxide was confirmed by helium-tagging IR spectroscopy. Such reaction intermediates formed during the oxidation of BR or bilirubin models have not been described so far. We believe that this work can be used as a first step in studying the complex oxidation mechanism of BR during phototherapy.