Cyclization of biliverdins to verdohaemochromes

Abstract

Iron(II) octaethylverdohaemochrome was obtained both by cyclization of octaethylbiliverdin in the presence of FeSO₄ and pyridine, and by coupled oxidation of octaethylhaemin and ascorbic acid with O₂ in the presence of pyridine. The compound obtained by cyclization of biliverdin IXα dimethyl ester was, however, different from that obtained by the methylation of iron(II) protoverdohaemochrome IXα, the product of coupled oxidation of the protohaem in myoglobin. The electronic absorption spectra of the pyridine and tosylmethyl isocyanide (TsCH₂NC) complexes, (14) and (15), of the product of cyclization were different from the spectra of the same complexes, (11) and (12), of the product of coupled oxidation. The product of cyclization was reduced to the product of coupled oxidation by sodium dithionite. It is postulated that compounds (14) and (15) are bis(pyridine) and bis(isocyanide) co-ordination complexes of iron (III) protoverdohaemochrome dimethyl ester. The positions of the maxima in the absorption spectra of compounds (16) and (17), the pyridine and TsCH₂NC complexes of both the product of coupled oxidation of mesohaemin dimethyl ester and the product of cyclization of mesobiliverdin dimethyl ester, indicated that these compounds are analogues, not of (11) and (12), but of (14) and (15). Iron(II) azaporphyrins (7) and (13) were prepared by treatment of the corresponding iron(II) oxaporphyrins (5) and (11) with ammonia.