Symmetry states of metalloporphyrin π-cation radicals, models for peroxidase compound I

Abstract

Oxoferryl porphyrin p-cation radical active sites of compound I intermediates which are found in enzymes such as peroxidases and catalases have been extensively modeled by oxidized synthetic metalloporphyrins. The electronic symmetry states of these compounds were initially assigned on the basis of electronic absorption data. In recent years new experimental and theoretical results have become available which have led to a re-evaluation and modification of the original assignments. A historical perspective of these developments is provided in the context of recent NMR, resonance Raman, and other spectroscopic data and theoretical calculations for the synthetic models and enzymatic systems.

Resonance Raman spectroscopy of oxoiron(IV) porphyrin π-cation radical and oxoiron(IV) hemes in peroxidase intermediates

“…It has now been realized that compound I species exhibit unique and characteristic resonance Raman signatures, as do synthetic model systems. While initial resonance Raman investigations [101,104,108] had concurred with the classic 2 A 2u assignment for HRP-I [48,49,120], later studies have advocated revising this assignment to 2 A 1u for both HRP-I and CPO-I [60,105,107,182], even though such a description might be sometimes considered to be overly simplistic [118,127,[183][184][185]. Upshifts of C b -C b modes, such as m 2 and m 11 , and downshifts of C a -C m modes for HRP-I and CPO-I relative to the non-porphyrin radical HRP-II and CPO-II (Tables 1 and 2) are consistent with an a 1u radical type for both, in accord with the b-substituent pattern of the protoheme contained in these enzymes.…”

“…Electronic absorption spectra of horseradish peroxidase and its compound I and II intermediates. [48,49,60,120,121], that are nearly degenerate. Due to their respective orbital nodal structures, each radical type, 2 A 1u or 2 A 2u , is expected to exhibit a specific pattern of shifts of vibrational frequencies relative to the frequencies of the non-radical parent.…”

“…At alkaline pH, which need be only mildly alkaline for certain isozymes, HRP turns a bright red and contains a six-coordinate low spin heme [52,53]. Peroxidase intermediates, in particular compound I, have been modeled extensively by synthetic metalloporphyrins [54][55][56][57][58][59][60]. Myoglobin [61][62][63][64][65][66][67][68][69][70] and its site-directed mutants [71,72], when oxidized to high oxidation states, are investigated as important protein models for compounds I and II.…”

High-valent transition metal centers and noninnocent ligands in metalloporphyrins and related molecules: a broad overview based on quantum chemical calculations

“…Because peroxidase compound I intermediates are at the formal Fe(V) oxidation level and because the iron center itself generally cannot sustain a +5 oxidation state, these intermediates are usually described as iron(IV)-oxo porphyrin radicals [3]. Some of the key electronic-structural questions about compound I intermediates concern the nature of the radical and the eects of dierent proximal ligands and may be articulated as follows.…”

“…iron(IV)-oxo porphyrin p-cation radicals [3]. Because a typical metalloporphyrin has near-degenerate HOMOs of a 1u and a 2u symmetry (in D 4h ) [1], an interesting question that researchers have attempted to address (at considerable pains) has involved whether various compound I intermediates and nonenzymatic metalloporphyrin radicals are A 1u -or A 2u -type radicals.…”