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 possible products of heme catabolism and other biologically important oxidative processes and are likely to be initially formed as metal complexes. To explore the properties of the formylbiliverdin moiety bound to transition metal ions, complexes of octaethylformylbiliverdin (H2OEFB) with Cu(II), Ni(II), and Co(II) have been prepared, since attempts to prepare an iron complex have produced only an unstable material. Transmetalation of Mg-II(OEFB), made by photooxidation of Mg-II(octaethyl-porphyrin), 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(II) center which is bound to the four nitrogen atoms in distorted planar coordination. The tetrapyrrole ligand has a helical geometry. The structure of the solid is complicated by the existence of three molecules in the asymmetric unit and C-H ... O hydrogen bonding between pairs of these in the tab/slot arrangement seen in complexes of octaethylbiliverdin. Both Cu-II(OEFB) and Co-II(OEFB) can be converted to the verdoheme analogues, [Cu-II(OEOP)](+) and [Co-II(OEOP)](+), where OEOP is the anion of octaethyl-5-oxaporphyrin, by the addition of hydrogen peroxide. Additionally, [Cu-II(OEOP)](+) can be produced by heating a toluene solution Cu-II(OEFB) in the presence of trifluoroacetic acid under dioxygen. Carbon monoxide is produced when Cu-II(OEFB) is converted to [Cu-II(OEOP)](+) by either method. [Cu-II(OEOP)](PF6) has been characterized by single-crystal X-ray diffraction which shows that the cation has a planar, porphyrin-like structure. The room-temperature EPR spectrum 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 diamagnetic Ni-II(OEFB) has been shown to be consistent with the helical structure 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 carrier. The geometric and electronic structural properties of these complexes of formylbiliverdin are compared to those of analogous compounds of biliverdin and of porphyrins.