Resonant Inelastic X‑ray Scattering on Ferrous and Ferric Bis-imidazole Porphyrin and Cytochrome <i>c</i>: Nature and Role of the Axial Methionine–Fe
Bond
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Abstract
Axial
Cu–S(Met) bonds in electron transfer (ET) active sites are
generally found to lower their reduction potentials. An axial S(Met)
bond is also present in cytochrome <i>c</i> (cyt <i>c</i>) and is generally thought to increase the reduction potential.
The highly covalent nature of the porphyrin environment in heme proteins
precludes using many spectroscopic approaches to directly study the
Fe site to experimentally quantify this bond. Alternatively, L-edge
X-ray absorption spectroscopy (XAS) enables one to directly focus
on the 3d-orbitals in a highly covalent environment and has previously
been successfully applied to porphyrin model complexes. However, this
technique cannot be extended to metalloproteins in solution. Here,
we use metal K-edge XAS to obtain L-edge like data through 1s2p resonance
inelastic X-ray scattering (RIXS). It has been applied here to a bis-imidazole
porphyrin model complex and cyt <i>c</i>. The RIXS data
on the model complex are directly correlated to L-edge XAS data to
develop the complementary nature of these two spectroscopic methods.
Comparison between the bis-imidazole model complex and cyt <i>c</i> in ferrous and ferric oxidation states show quantitative
differences that reflect differences in axial ligand covalency. The
data reveal an increased covalency for the S(Met) relative to N(His)
axial ligand and a higher degree of covalency for the ferric states
relative to the ferrous states. These results are reproduced by DFT
calculations, which are used to evaluate the thermodynamics of the
Fe–S(Met) bond and its dependence on redox state. These results
provide insight into a number of previous chemical and physical results
on cyt <i>c</i>