The meso-reactivity of porphyrins and related compounds. Part VI. Oxidative cleavage of the haem system. The four isomeric biliverdins of the IX series

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Abstract
The oxidative cleavage of haemin in aqueous pyridine under various conditions (ascorbic acid–oxygen; hydrazine–oxygen; and ascorbic acid–hydrogen peroxide) gives four isomeric biliverdins of the IX series which have been isolated (from a much improved ascorbic acid–oxygen reaction) as their crystalline dimethyl esters. The properties of the four isomers are described. On the basis of mass and n.m.r. spectra, structures are assigned to all four isomers for the first time. A key observation in the n.m.r. spectra of these compounds is the chemical shift of the heteroaryl methyl group. The implications of these results are discussed.