A SYNTHETIC BILIRUBIN IN A PORPHYRIN-LIKE CONFORMATION

An analog of bilirubin is held in a porphyrin-like shape by an N21,N24 -methano-bridge. In this conformation, the two propionic acid groups c annot participate in intramolecular hydrogen bonding, and the pigment is thus much more polar and hydrophilic than bilirubin. The uv-visible spectra (epsilon(max) 20,400, lambda(max) 381 nm in benzene and epsil on(max) 23,800, lambda(max) 385 nm) are nearly invariant over the solv ent polarity range. Circular dichroism of the pigment complex with hum an serum albumin gives a weak bisignate Cotton effect: DELTAepsilon422 max = -4.7, DELTAepsilon366max = +3.7 in pH 7.4 buffer.