Photochemistry of Biliverdin IX δ as a model for the study of the photoproducts from natural biliverdin IXγ (pterobilin)

The configurations of biliverdin-IX_γ and -IX_δ dimethyl esters 1 and 2 in solutions, have been studied using Nuclear-Overhauser-Effect (NOE) experiments. Irradiation (500-700 nm) of biliverdin IXδ gave syn-Z → anti-E isomerization of the central methine bridge and in aerated polar solutions, four new cyclized pigments were isolated for which structures 3, 4, 5 and 6 are proposed. Neobiliverdin IXδ 3 is also formed in degassed solution (Φ=4.10^?5) but pigments 4, 5 and 6 arise from photo-oxidation with O₂. Biliverdin IXδ appears to be a good model for the study of photo-reactions occurring on the vinyl groups of the natural biliverdin IXγ (pterobilin).