Spectroscopic identification of reactive porphyrin motions

Abstract

Nuclear resonance vibrational spectroscopy (NRVS) reveals the vibrational dynamics of a Mössbauer probe nucleus. Here, 57 Fe NRVS measurements yield the complete spectrum of Fe vibrations in halide complexes of iron porphyrins. Iron porphine serves as a useful symmetric model for the more complex spectrum of asymmetric heme molecules that contribute to numerous essential biological processes. Quantitative comparison with the vibrational density of states (VDOS) predicted for the Fe atom by density functional theory calculations unambiguously identifies the correct sextet ground state in each case. These experimentally authenticated calculations then provide detailed normal mode descriptions for each observed vibration. All Fe-ligand vibrations are clearly identified despite the high symmetry of the Fe environment. Low frequency molecular distortions and acoustic lattice modes also contribute to the experimental signal. Correlation matrices compare vibrations between different molecules and yield a detailed picture of how heme vibrations evolve in response to (a) halide binding and (b) asymmetric placement of porphyrin side chains. The side chains strongly influence the energetics of heme doming motions that control Fe reactivity, which are easily observed in the experimental signal.

Probing Heme Vibrational Anisotropy: An Imidazole Orientation Effect?

“…Peripheral substituents can have a large effect on the dynamics of the iron, especially for in-plane motions. 61, 62 In addition, we have shown that the in-plane anisotropy is related to the orientation of the axial NO in [Fe(OEP)(NO)], 64 confirming the possibility of observable anisotropy based on the first-principle calculation of NRVS. 65 …”

“…In Figures 3 to 5, we compare modes in [Fe(OEP)] and [Fe(OEP)(2-MeHIm)] that involve similar atomic motions on the basis of vibrational correlational analysis. 62, 81, 82 In all illustrations the iron motion is exactly in the porphyrin plane, or nearly so, and the porphyrin plane is parallel to the page. We note that in [Fe(OEP)] the iron motion is exactly along the Fe–N p bond directions, as expected, but in [Fe(OEP)(2-MeHIm)] the motion deviates from these directions.…”

Nuclear Resonance Vibrational Spectra of Five-Coordinate Imidazole-Ligated Iron(II) Porphyrinates

“…A recent investigation of ferric porphyrin halides used NRVS measurements and vibrational correlation analysis of DFT predictions to demonstrate the strong influence of peripheral substituents on the vibrational dynamics of the Fe, 76 reinforcing more qualitative observations for nitrosyl complexes. 47 Similarly, comparison with the dynamics of the smaller, more symmetric compound [Fe(P)(Im)] 77 provides a useful perspective on the more complex imidazole-ligated porphyrins considered here (see Table 3).…”

“…Second, the doming mode observed for the low-spin NO derivatives is at a higher frequency than those observed for the five-coordinate imidazole species. Observation of the doming mode at 142 cm −1 for [Fe(OEP)(Cl)] 76 suggests that the spin state may be a more important factor than ligand mass.…”

“…(There is no evidence of the interaction between the ν 53 and the γ 23 modes predicted and observed for the analogous [Fe(P)(Cl)] and [Fe(P)(Br)] compounds. 76 ) Peripheral substitutions weakly influence the ν 50 modes, which remain a minor contribution to the VDOS of [Fe(TPP)(2-MeHIm)] (Figure 3) and weaken further for [Fe(OEP)(2-MeHIm)] (Figure 6). However, the ν 53 modes interact strongly with phenyl distortions in [Fe(TPP)(2-MeHIm)], resulting in splitting of the dominant in-plane vibrations to clusters of in-plane modes that account for observed peaks at 220 and 294 cm −1 .…”