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Effect of a charge relay on the vibrational frequencies of carbonmonoxy iron porphine adducts: the coupling of changes in axial ligand bond strength and porphine core size.

The effect of a charge relay involving Asp-His-Fe in peroxidase enzymes is explored using density functional theory (DFT) calculations of vibrational spectra and potential energy surfaces of carbonmonoxy model systems. The series of models consists of a carbonmonoxy iron porphine molecule with a trans imidazole ligand hydrogen-bonded to six different partners at the Ndelta position. Calculations on the oxy system and on models of the Asp-His-Ser catalytic triad of serine proteases were also performed to obtain an understanding of how the redistribution of charge in these systems may contribute to enzymatic function. The goal of the study is to relate the experimental frequencies in resonance Raman and Fourier transform infrared studies to bonding that is important for the function of heme enzymes. Calculations of both axial and in-plane modes exhibit trends that agree with experimental data. Comparisons of the charge distribution on the different models show that polarization of iron carbonomonoxy bonds consistent with the mechanism for peroxidase function leads to a frequency reduction in the C-O stretching mode nuCO. The combination of axial trans sigma-bonding and pi-bonding effects that include expansion of the porphine core result in little change in the Fe-C stretching frequency nuFe-CO in the series of molecules studied with different Ndelta-H hydrogen bonding. A particular role for the core size is discussed that demonstrates the applicability of trends observed in vibrational spectroscopy of hemes to the charge relay mechanism and other axial ligation effects. The bonding interactions described account for the increase in electron density on bound diatomic ligands, which is required for peroxidase function.