Metalloporphyrin Core Size Resonance Raman Marker Bands Revisited: Implications for the Interpretation of Hemoglobin Photoproduct Raman Frequencies

Article abstract of DOI:10.1021/ja00247a009

Resonce Raman frequencies are examined for high-frequency skeletal modes of several metalloporphyrin derivatives.The previously noted inverse dependence on core size applies to Mn, Co, and Zn as well as to Fe and Ni species, and slightly revised coefficients are derived from the expanded data base.Systematic deviations are noted for 2-methylimidazole adducts of Mn^II, Fe^II, and Co^II and are suggested to be associated with electronic effects of porphyrin doming, or of tilting of the metal-ligand bond relative to the porphyrin plane.Positive deviations are seen for Cl2Sn^IV; evidently the correlations break down for so large a central ion (C_t-N=2.082 Angstroem).A core-size dependence is also noted for ν₄, the oxidation-state marker for heme proteins, but the slope is much smaller than for the higher frequency skeletal modes, accentuating the negative deviations seen for Fe^II, and also Mn^II and CO^II derivatives; other M^II derivatives (Ni, Zn) fall on the line, so there appears to be an electronic basis for the oxidation-state dependence, over and above the core-size dependence.Significantly, the core-size dependence is obeyed for <(OH)2Fe(PP)>^2-, a six-coordinate but high-spin Fe^II species.This observation establishes a base line for the core-size analysis of the RR frequencies of the early photolysis product of CO-hemoglobin (Hb) and their shifts relative to deoxyHb.The present results are consistent with the most recent interpretation, that the Fe is out of the heme plane in the photoproduct, but not by as much as in deoxyHb.Core-size correlations are also found for metallo TPP's, but the coefficients differ from those of metallo PP's reflecting the altered normal mode compositions.