The electronic and vibrational structures of iron-oxo porphyrin with a methoxide or cysteinate axial ligand

Hybrid density functional theory (DFT) calculations for the electronic and vibrational structures of compound I species with a methoxide (MeO^-) (1) or cysteinate (CysS^-) (2) axial ligand are carried out in order to elucidate the natures of a methoxide-coordinating new type of compound I species (Bull. Chem. Soc. Jpn. 71 (1998) 1343) and cysteinate-coordinating compound I species of chloroperoxidase (CPO-I) and cytochrome P450s (P450-I). DFT computations of 1 and 2 demonstrate that these 'anionic' ligands are a spin carrier; 70% (80%) of a spin density resides on the O (S) atom of the axial ligand and 30% (20%) is distributed on the porphyrin ring. These results suggest that for the generation of the compound I species, one electron is removed from the iron centers and the rest of the one electron is supplied from the oxidizable axial ligands instead of the iron centers or the porphyrin ring. Vibrational analyses demonstrate that the Fe=O bond is more strongly activated in 1 compared with 2 with the stretching mode at 849 cm^-1 (878 cm^-1) for the doublet state 1a (2a) and at 814 cm^-1 (875 cm^-1) in the quartet state 1b (2b). This reverse order of the Fe=O bond strength with respect to the axial donor strength should have relevance to the significantly oxidized character of the CysS^- axial ligand. In conjunction with the recent results of the extensive resonance Raman (RR) studies, some interpretations of unsettled RR results for compound I of chloroperoxidase (CPO-I) and a synthetic compound I species [O=Fe(IV)(TMP^.+)(alcohol)] (J. Am. Chem. Soc. 113 (1991) 6542) concerning the O=Fe stretching frequencies are discussed. Copyright (C) 2000 Elsevier Science B.V.