MAGNETIC CIRCULAR-DICHROISM SPECTROSCOPIC STUDIES ON THE STEREOCHEMISTRY AND COORDINATION BEHAVIOR OF NICKEL PORPHYRINS

The electronic structures and the ligand affinities of several nickel porphyrins with different types and degrees of macrocycle deformations have been studied by magnetic circular dichroism (MCD) spectroscopy. The nickel porphyrins studied in this work are nickel(II) octaethylpor phyrin (NiOEP) with a mixture of planar and ruffled porphyrin ring con formations, nickel(II) tetracyclopentenyltetrapentylporphyrin (NiTC5TC 5P) with a ruffled macrocycle, nickel(II) octaethyltetraphenylporphyri n (NiOETPP) with a saddle-shaped macrocycle, and nickel(II) protoporph yrin IX (NiPP) reconstituted hemoglobin ((Ni)Hb) and myoglobin ((Ni)Mb ). The MCD spectra of all three compounds, NiOEP, NiTC5TC5P, and NiOET PP, consist of mainly normal A-term peaks. This indicates that neither the ruffled nor the saddle-shaped conformation lifts the excited stat e degeneracies, and the D2d symmetry in both cases is retained. The MC D spectra of (Ni)Mb consists of normal A-term peaks, showing no protei n effect on the electronic energy level of nickel porphyrin. There see ms to be a correlation between the coordination states of various nick el porphyrins studied in this work and the relative intensity of the S oret MCD band (I(S)) to the Q MCD band (I(Q)), I(S)/I(Q). I(S)/I(Q) is less than 0.5 for 4-coordinate, approximately 1 for 5-coordinate, and greater than 1.5 for 6-coordinate Ni(II) species. I(S)/I(Q) of NiTC5T C5P in pyrrolidine below 100 K is about 3.3, suggesting that pyrrolidi ne biaxially binds to NiTC5TC5P. No evidence for axial ligation of pyr rolidine to NiOETPP has been detected down to 2 K. The affinity of NiP P in (Ni)Hb or (NI)Mb for exogenous ligands such as 1-methylimidazole, cyanide, azide, and fluoride has been monitored by MCD. None of these ligands bind to NiPP in (Ni)Hb or (Ni)Mb down to 2 K.