Helically fixed chiral bilirubins and biliverdins: a new insight into the conformational, associative and dynamic features of linear tetrapyrrols

Abstract

The 2,18-bridged biliverdins 1, 2 and 5 and bilirubins 3, 4 and 6 each forced into a helical conformation have been prepared and investigated by UV–VIS spectroscopy, ¹H NMR spectroscopy techniques and vapour pressure osmometry. Barriers to interconversion of the tetrapyrrole moieties of M and P chirality were accomplished by kinetic measurements of partially resolved materials using CD spectroscopy. These results were compared with those obtained for non-bridged analogues reported here and in the literature. Based on the above, the structural and environmental factors controlling the dynamic, associative and conformational behaviour of biliverdins and bilirubins have been established. In this connection the complete conformational and associative pattern of bilirubin-IXα-dimethyl ester (10) is presented.