Title

Magnetic Circular Dichroism Spectroscopic Studies on the Stereochemistry and Coordination Behavior of Nickel Porphyrins

Document Type

Article

Publication Date

8-1-1994

Abstract

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) octaethylporphyrin (NiOEP) with a mixture of planar and ruffled porphyrin ring conformations, nickel(II) tetracyclopentenyltetrapentylporphyrin (NiTC5TC5P) with a ruffled macrocycle, nickel(II) octaethyltetraphenylporphyrin (NiOETPP) with a saddle-shaped macrocycle, and nickel(II) protoporphyrin IX (NiPP) reconstituted hemoglobin (NiHb) and myoglobin (NiMb). The MCD spectra of all three compounds, NiOEP, NiTC5TC5P, and NiOETPP, consist of mainly normal A-term peaks. This indicates that neither the ruffled nor the saddle-shaped conformation lifts the excited state degeneracies, and the D2d symmetry in both cases is retained. The MCD spectra of NiHb and NiMb consist of normal A-term peaks, showing no protein effect on the electronic energy level of nickel porphyrin. There seems to be a correlation between the coordination states of various nickel porphyrins studied in this work and the relative intensity of the Soret MCD band (IS) to the Q MCD band (IQ), IS/IQ. IS/IQ is less than 0.5 for 4-coordinate, approximately 1 for 5-coordinate, and greater than 1.5 for 6-coordinate Ni(II) species. IS/IQ of NiTC5TC5P in pyrrolidine below 100 K is about 3.3, suggesting that pyrrolidine biaxially binds to NiTC5TC5P. No evidence for axial ligation of pyrrolidine to NiOETPP has been detected down to 2 K. The affinity of NiPP in NiHb or NiMb for exogenous ligands such as 1-methylimidazole, cyanide, azide, and fluoride has been monitored by MCD. None of these ligands bind to NiPP in NiHb or NiMb down to 2 K. © 1994, American Chemical Society. All rights reserved.

Publication Source (Journal or Book title)

Inorganic Chemistry

First Page

3873

Last Page

3876

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