Axial coordination in nickel and vanadium porphyrins: Transient and difference raman spectroscopy

Nickel- and vanadium-porphyrin complexes and Ni(II)-reconstituted hemoglobin (/sup Ni/Hb) and myoglobin (/sup 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 ..pi..-..pi.. aggregation and complexation. Similarly, nickel porphyrins exhibit 4-, 5-, or 6-coordination depending on the molecular environment. The existence of only one axial ligand in the proteins 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 fith ligand. The effects of the T ..-->.. R change in quaternary structure on the protein environment of the Ni porphyrin are 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 (approx.10-ns duration) of the complexes formed in coordinating solvents induces the release of axial ligands, but does not induce release of themore » histidine ligand of Ni-reconstituted hemoglobin and myoglobin. The results indicate that the protein matrix controls photoinduced ligation dynamics by promoting rapid geminate recombination of the ligand. Ligand uptake is observed in /sup Ni/Hb when the 4-coordinate sites are excited.« less