Structural Correlations and Vinyl Influences in Resonance Raman Spectra of Protoheme Complexes and Proteins

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Resonance Raman spectra obtained with B (Soret) and Q1 (β) band excitations are compared for protoheme complexes displaying several oxidation, spin, and ligation states: Im2FeIIIPP+ (Im = imidazole, PP = protoporphyrin IX), Im2FeIIPP, ClFeIIIPP, (Me2SO)2FeIIIPP+ (Me2SO = dimethyl sulfoxide), and (2-MeIm)FeIIPP (2-MeIm = 2-methylimidazole). The effects of Cα and Cβ vinyl deuteration reveal the same extensive pattern of vinyl modes as seen in NiPP (see preceding paper). Vinyl influences on the porphyrin skeletal frequencies appear to include both kinematic and conjugative effects. Previously observed correlations with porphyrin core size have been augmented; all the skeletal modes above 1450 cm−1 show a negative linear dependence on Ct-N (center-to-nitrogen distance), with slopes that depend on the extent of methine bridge bond involvement in the modes. The complex Im2FeIIPP shows large deviations from these correlations, associated with dπ back-bonding. The deviations depend on the vibrational mode symmetries in a manner plausibly related to the nodal pattern of the porphyrin eg* acceptor orbitals. Comparison of the complexes with deoxyMb (Mb = myoglobin), MbIIIF, and HbIIIF (Hb = hemoglobin) shows protein influences to be small for modes above 1300 cm−1 but substantial for modes in the 900–1300-cm−1 region, which have appreciable contributions from pyrrole-substituent stretching. In particular deoxyMb and MbIIIF, but not HbIIIF, show a new band at ∼ 1120 cm−1, which shifts up on Cα deuteration and is assignable to pyrrole-vinyl stretching. This band correlates with an IR band of NiPP at 1118 cm−1; it is evidently activated by a specific symmetry-lowering effect of Mb, which is suggested to be an electrostatic field near the vinyl groups. © 1982, American Chemical Society. All rights reserved.

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Journal of the American Chemical Society

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