Models of the Cytochromes. Axial Ligand Orientation and Complex Stability in Iron(II) Porphyrinates:  The Case of the Noninteracting d_π Orbitals

Abstract

The synthesis and characterization of seven bis-pyridine and bis-imidazole complexes of iron(II) tetramesitylporphyrinate are reported. X-ray crystal structures of three of the complexes, [Fe(TMP)(4-CNPy) 2 ], [Fe(TMP)(3-CNPy) 2 ], and [Fe(TMP)(4-MePy) 2 ], have been solved and all show parallel axial ligand orientations with nearly planar porphyrinato cores. The Mo ¨ssbauer spectra of six of the complexes, having pyridine ligands with pK a (PyH + ) ranging from ∼1.1 (4-CNPy) to 9.7 (4-NMe 2 Py), have been determined. The Mo ¨ssbauer isomer shifts at 120 K are in the range of 0.36-0.45 mm/s, and the quadrupole splittings (∆E Q ) are in the range of 1.11-1.27 mm/s. Thus, unlike the corresponding Fe(III) complexes, the X-ray structures and Mo ¨ssbauer spectroscopic parameters of these (tetramesitylporphyrinato)iron(II)-bis(pyridine) complexes are shown to be essentially independent of the basicity and π donor/acceptor properties of the axial pyridine ligands. These solid-state structural and spectroscopic properties are compared to the thermodynamic properties of the same series of complexes in solution (

pH-dependent redox and CO binding properties of chelated protoheme-l-histidine and protoheme-glycyl-l-histidine complexes

“…2, corresponding to data in Table 1 for n=3). This possibility, which is confirmed by other methods (see later), involves a third redox-linked group with a lower pK a value (Table 1), which indeed displays features expected for the protonation of the N e of the imidazole group (i.e., ox pK a3 > red pK a3 ; Table 1), as observed in other heme model systems [42].…”

Electrochemical and NMR spectroscopic studies of distal pocket mutants of nitrophorin 2: Stability, structure, and dynamics of axial ligand complexes

“…Although carboxylate groups near the heme are one factor that helps to stabilize the Fe(III) state (12,25), another may be heme ruffling (19). It has been argued that for low-spin ferrihemes ruffling of the heme should disfavor reduction from Fe(III) to Fe(II), and hence ruffled hemes have more negative reduction potentials than planar hemes (19,26,27). This is probably because low-spin Fe(II) strongly disfavors a ruffled heme core (26,28).…”

“…It has been argued that for low-spin ferrihemes ruffling of the heme should disfavor reduction from Fe(III) to Fe(II), and hence ruffled hemes have more negative reduction potentials than planar hemes (19,26,27). This is probably because low-spin Fe(II) strongly disfavors a ruffled heme core (26,28). Thus, This paper was submitted directly (Track II) to the PNAS office.…”

Conformações distorcida e planar do anel porfirínico em complexos e hemoproteínas: propriedades físico-químicas e implicações espectroscópicas

“…A influência da orientação dos planos dos ligantes axiais é efetiva sobre a configuração eletrônica e as propriedades dos sistemas ferroporfirínicos 22,23 , podendo ser um fator mais significativo que a propriedade de acidez/basicidade dos ligantes em questão 24,25 . A orientação perpendicular dos planos dos ligantes axiais imidazólicos, por exemplo, é importante na estabilização do estado eletrônico mais raro, (d xz ,d yz ) 4 (d xy ) 1 26 .…”