POSSIBLE INTERMEDIATES IN BIOLOGICAL METALLOPORPHYRIN OXIDATIVE-DEGRADATION - NICKEL, COPPER, AND COBALT COMPLEXES OF OCTAETHYLFORMYBILIVERDIN AND THEIR CONVERSION TO A VERDOHEME

Formylbiliverdin and related chlorophyll-derived molecules are possibl e products of heme catabolism and other biologically important oxidati ve processes and are likely to be initially formed as metal complexes. To explore the properties of the formylbiliverdin moiety bound to tra nsition metal ions, complexes of octaethylformylbiliverdin (H2OEFB) wi th Cu(II), Ni(II), and Co(II) have been prepared, since attempts to pr epare an iron complex have produced only an unstable material. Transme talation of Mg-II(OEFB), made by photooxidation of Mg-II(octaethyl-por phyrin), with a metal(II) acetate yields the low-spin complexes: Cu-II (OEFB), Ni-II(OEFB), and Co-II(OEFB). Single-crystal X-ray diffraction of Cu-II(OEFB) reveals that it consists of a four-coordinate copper(I I) center which is bound to the four nitrogen atoms in distorted plana r coordination. The tetrapyrrole ligand has a helical geometry. The st ructure of the solid is complicated by the existence of three molecule s in the asymmetric unit and C-H ... O hydrogen bonding between pairs of these in the tab/slot arrangement seen in complexes of octaethylbil iverdin. Both Cu-II(OEFB) and Co-II(OEFB) can be converted to the verd oheme analogues, [Cu-II(OEOP)](+) and [Co-II(OEOP)](+), where OEOP is the anion of octaethyl-5-oxaporphyrin, by the addition of hydrogen per oxide. Additionally, [Cu-II(OEOP)](+) can be produced by heating a tol uene solution Cu-II(OEFB) in the presence of trifluoroacetic acid unde r dioxygen. Carbon monoxide is produced when Cu-II(OEFB) is converted to [Cu-II(OEOP)](+) by either method. [Cu-II(OEOP)](PF6) has been char acterized by single-crystal X-ray diffraction which shows that the cat ion has a planar, porphyrin-like structure. The room-temperature EPR s pectrum of this complex shows that the copper is four-coordinate with four nitrogen based ligands, but frozen solutions of [Cu-II(OEOP)](+) show a triplet EPR spectrum indicative of a dimeric species much like that in the X-ray crystal structure. The H-1 NMR spectrum of diamagnet ic Ni-II(OEFB) has been shown to be consistent with the helical struct ure through the use of lanthanide and chiral lanthanide shift reagents . The EPR spectra of Co-II(OEFB) show that it forms a low-spin adduct with pyridine and that this adduct acts as a reversible dioxygen carri er. The geometric and electronic structural properties of these comple xes of formylbiliverdin are compared to those of analogous compounds o f biliverdin and of porphyrins.