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.

In the presence of dioxygen, four cobalt complexes of linear tetrapyrroles, Co(OEB) (OEB is the trianion of octaethylbiliverdin), [CoII(OEBOx)]I3 (OEBOx is the monoanion of the oxidized form of octaethylbiliverdin), CoII(OEFB) (OEFB is the octaethylformylbiliverdin dianion), and CoII(OEBOMe) (OEBOMe is the dianion of octaethylmethoxybiliverdin) form the bischelate cobalt(II) complex of the anionic dipyrrole, tetraethylpropentdyopent anion, CoII(TEPD)2. Both carbon monoxide and carbon dioxide are formed when CoII(TEPD)2 is produced by the thermal reactions between CoII(OEFB) or Co(OEB) and O2. Three different crystal morphologies of CoII(TEPD)2 have been analyzed by single-crystal X-ray diffraction. In each case the cobalt atom is coordinated by four nitrogen atoms of the two propentdyopent ligands in a pseudotetrahedral fashion. In each morphology, a common tab/slot intermolecular C−H···O hydrogen bond interaction that involves the methine C−H unit and two adjacent C−H units of the flanking ethyl groups and the lactam oxygen atom is found. These interactions connect pairs of molecules into extended chains. CoII(TEPD)2 has been characterized spectroscopically (UV−vis and NMR) and electrochemically.

SUMMARYIn fistulated sheep (50)(51)(52)(53)(54)(55)(56)(57)(58)(59)(60) kg live weight) the absorption of ammonia from the reticulo-rumen in vivo was studied applying the technique of the temporarily isolated and washed reticulorumen. It was found that, at ammonia concentrations between 3 and 18 mm, ammonia efflux and ammonia net absorption were linearly related to the ammonia concentration in the artificial rumen fluid, whereas influx of ammonia nitrogen from endogenous sources remained almost constant. When the concentration of unionized NH3 was changed at the ratio 1: 10:76 by varying the pH from 5-8 to 6-8 and 77, ammonia net absorption did not reflect the concentration ratio of unionized NH3, indicating either flux of NH4+ ions or titration of NH4 + to NH3 at the absorptive surface. In the experiments with buffer solutions without ammonium salts and extended over 2 h, ammonia concentrations in the artificial rumen fluid increased due to endogenous nitrogen influx and reached levels far beyond the expected plateau concentration of about 2 mm. Labelling of the N pool in the isolated organ by 15N showed that ammonia efflux had almost ceased in these experiments. It is argued that as yet unidentified changes have taken place in the artificial rumen fluid during the experiment, but there is some reason to believe that volatile fatty acid (VFA) absorption was affecting ammonia absorption.

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