Possible Intermediates in Biological Metalloporphyrin Oxidative Degradation. Nickel, Copper, and Cobalt Complexes of Octaethylformybiliverdin and Their Conversion to a Verdoheme

Abstract

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 MgII(OEFB), made by photooxidation of MgII(octaethyl-porphyrin), with a metal(II) acetate yields the low-spin complexes: CuII(OEFB), NiII(OEFB), and CoII(OEFB). Single-crystal X-ray diffraction of CuII(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 CuII(OEFB) and CoII(OEFB) can be converted to the verdoheme analogues, [CuII(OEOP)]+ and [CoII(OEOP)]+, where OEOP is the anion of octaethyl-5-oxaporphyrin, by the addition of hydrogen peroxide. Additionally, [CuII(OEOP)]+ can be produced by heating a toluene solution CuII(OEFB) in the presence of trifluoroacetic acid under dioxygen. Carbon monoxide is produced when CuII(OEFB) is converted to [CuII(OEOP)]+ by either method. [CuII(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 [CuII(OEOP)]+ show a triplet EPR spectrum indicative of a dimeric species much like that in the X-ray crystal structure. The 1H NMR spectrum of diamagnetic NiII(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 CoII(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.

Comparative study of catalytic activity of some biomimetic models of cytochrome P450 in oxidation of olefins with tetra-n-butylammonium periodate: Electron-rich Mn-porphyrins versus the electron-deficient ones

“…In other words, the observed pattern of catalytic activity of the used Mn-pors in oxidation of a given olefin cannot be generalized to the rest. In contrast to the previous reports on the oxidation of olefins with TBAP catalyzed by electron-rich and electron-deficient Mn-pors [7,9], the results of this study indicate that the relative catalytic activity of the two classes of Mn-pors changes when going from an olefin to another one.…”

“…The positions with highest electron densities i.e. the α and meso ones are expected to be involved in oxidative degradation of metalloporphyrins [7]. The introduction of four NO 2 groups at the para-positions of the Mn(III) complex of meso-tetraphenylporphyrin, Mn(TPP)(OAc), is expected to decrease the total electron density on the porphyrin core of MnT(4-NO 2 P)P(OAc) with respect to that of Mn(TPP)(OAc).…”

Structural and Spectroelectrochemical Studies on (2,2′-Bidipyrrinato)copper(II) and -palladium(II) Complexes

“…ϪC(10)Ϫ C(9) as well as those at the meso carbon atoms C(4)ϪC(5)ϪC(6) and C(13)ϪC(14)ϪC(15)became larger. In summary, the changes result in reduced overlap of the terminal methyl groups, and thus in diminished helicity of the ligand and enhanced planarity of the metal centre {rms deviation of MN 4 fragments from planarity: Pd 0.151 Å , Ni 0.203 Å ; angle between the planes [N(2),N(3),M] and [N(1),N(4),M]: for 8 14.0°, for 10 19.9°; angles N(1)ϪMϪN(3) and N(2)ϪMϪN(4): Pd 164.02(11) and 162.69(11)°, Ni 161.72…”

Iron Chelates of 2,2′‐Bidipyrrin: Stable Analogues of the Labile Iron Bilins

“…[18,19,71] The reactivity of the terminal ligand positions has long and often been employed for metal-templated macrocyclization reactions in the synthesis of porphyrins, [72] corrins, [73][74][75] corroles, [76] and other porphyrinoids [77][78][79] from transition-metal bilinoids. In the case of the related verdoheme derivatives, several nucleophilic ringopening reactions have been described to occur at this position with a range of nucleophiles.…”