title: Quantum mechanical investigations of heme structure and vibrational spectra: effects of conformation, oxidation state, and electric field.
abstract: Three conformers of a complex consisting of a Fe-porphyrin ring with axial imidazole groups FeP(Im)(2) that differed by orientation of axial imidazole groups were studied with quantum mechanical calculations in both reduced (neutral) and oxidized (cationic) states. In the reduced state, all three conformers correspond to local minima. On the other hand, in the oxidized state the conformer with the imidazole groups perpendicular to one another does not have a local minimum, which suggests that there could be changes in the structure of heme groups depending on oxidation state. The relative energy differences between conformers are small as well the differences of orbital energies. However, the populations of Fe 3d orbitals, and hence charge distributions, are predicted to change significantly between conformations. The orientation of heme groups can affect the kinetics of interheme electron transfer, so this electronic population redistribution will affect electron transfer kinetics. A comparison of calculated Raman spectra with measured surface-enhanced Raman spectra (Biju et al. Langmuir 2007, 23, 1333) shows excellent agreement for frequencies, but correlations of Raman intensities are less satisfactory. Possible explanations of the observed discrepancies could arise due to a problem in assignments of vibration bands in the experiment, a dependence of Raman spectra on heme complex on oxidation state, long-range protein structure, or membrane electric field are discussed. In particular, it was shown that the oxidation state does not dramatically alter the calculated Raman spectrum. However, the cell membrane electric field can significantly modify Raman spectra. An interpretation of the experimental oxidized cytochrome surface-enhanced Raman spectra is discussed.
authors.affiliation: The Center for Environmental Kinetics Analysis, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.
authors.forename: Alexander V
authors.initials: AV
authors.lastname: Mitin
authors.forename: James D
authors.initials: JD
authors.lastname: Kubicki
mesh_terms: D004560:Electricity; D006418:Heme / Q000737:chemistry*; D008958:Models, Molecular; D010084:Oxidation-Reduction; D011487:Protein Conformation; D011789:Quantum Theory; D013059:Spectrum Analysis, Raman; D014732:Vibration
chemical_list: D006418:Heme