Axial Coordination in Nickel and Vanadium Porphyrins: Transient and Difference Raman Spectroscopy

Abstract

Nickel-and vanadium-porphyrin complexes and Ni(II)-reconstituted hemoglobin ( Ni Hb) and myoglobin ( Ni Mb) have been investigated using cw and transient resonance Raman spectroscopy.The state of axial coordination at the metal in these materials can be determined using the characteristic frequencies of the Raman marker lines arising from the porphyrin moiety.The state of axial ligation is highly sensitive to the molecular environment of these geoporphyrin analogs.Whether vanadium porphyrins are 5-or 6-coordinate depends on solvent and π-π aggregation and complexation. Similarly, nickel porphyrins exhibit 4-, 5-, or 6-coordination depending on the molecular environment.Nickel -reconstituted proteins provide well characterized models of hydrophobic cavities such as might exist in a heterogeneous petroleum environment.In the protein cavity a novel 5-coordinate Ni-porphyrin complex is observed. The existence of only one axial ligand is supported by identification of the axial ligand-Ni stretching vibration by isotopic substitution.The frequency of the Ni-ligand mode is consistent with histidine as the fifth ligand. The effects of the T->R change in quaternary structure of the protein environment of the Ni porphyrin are also observed in the Raman spectrum. The photoinitiated ligand-release and ligand-uptake processes in excited states of the Ni porphyrins and Ni-reconstituted proteins were also investigated. Laser excitation (~10-ns duration) of the complexes formed in coordinating solvents induces the release of axial ligands, but does not induce release of the histidine ligand of Ni-reconstituted hemoglobin and myoglobin. The results indicate that the protein matrix controls ligation dynamics by promoting rapid geminate recombination of the ligand.

Chemistry and Association of Vanadium Compounds in Heavy Oil and Bitumen, and Implications for Their Selective Removal

“…The prospect ligand must be capable of penetrating the microwell created by the elevated vanadium atom in order to create the coordinating bond. Shelnutt et al observed similar tendencies and found that pyrrolidine coordinated much more readily with VOOEP than did pyridine, and this coordination caused a red shift in the Soret band from 407 to 425 nm.…”