Redox Effects on the Coordination Geometry and Heme Conformation of Bis(N-methylimidazole) Complexes of Superstructured Fe-Porphyrins. A Spectroscopic Study

Abstract

Electronic absorption, electron paramagnetic resonance (EPR), and Soret-excited resonance Raman (RR) spectra are reported for bis(N-alkylimidazole) complexes of various iron(III)-"basket-handle" (Fe(III)BHP(+)) and "picket-fence" (Fe(III)PFP(+)) porphyrins in methylene chloride. The Fe(III)BHP(+) derivatives consist of four cross-trans (CT) and two adjacent-cis (AC) -linked in which the composition and the length of the handles are variable (CT Fe(III)[(C(11)Im)(2)(+)], CT and AC Fe(III)[((C(4))(2)phi)(2)](+), CT Fe(III)[((C(3))(2)phi)(C(12))](+), CT and AC Fe(III)[((C(3))(2)phi)(2)](+)). The meso-alphaalpha betabeta and meso-alphabeta alphabeta atropisomers of Fe(III)-tetrakis(o-pivalamidophenyl)-porphyrins represents the Fe(III)PFP(+) derivatives (Fe(III)alphaalpha betabeta-T(piv)PP(+) and Fe(III)alphabeta alphabeta-T(piv)PP(+), respectively). The absorption and RR data obtained for these ferric compounds were compared to those previously published for the homologous ferrous complexes (Picaud, T., Le Moigne, C., Loock, B., Momenteau, M. and Desbois, A. J. Am. Chem. Soc. 2003, 125, 11616 and Le Moigne, C., Picaud, T., Boussac, A., Loock, B., Momenteau, M. and Desbois, A. Inorg. Chem. 2003, 42, 6081). The Soret band position of the eight investigated ferric compounds is observed between 417 and 424 nm, indicating that none of the complexes possesses a planar heme. The EPR spectra show that most of the Fe(III)BHP(+) complexes and all the Fe(III)PFP(+) complexes are rhombic B-type hemichromes (g(max) = 2.86-2.96). Notable exceptions concern the bis(N-methylimidazole) complexes of two CT Fe(III)BHP(+). The Fe(III)BHP(+) with the shortest handles (Fe(III)[((C(3))(2)phi)(2)](+)) exhibits a g value at 2.80. When the handles are lengthened by two methylene units (Fe(III)[((C(3))(2)phi)(2)](+)), the EPR spectrum corresponds to a mixture of two "highly anisotropic low-spin " or "large g(max)" type I EPR signals, a major species at g = 3.17 and a minor species at g = 3.77. All these EPR data were converted in terms of dihedral angle formed by the rings of the axial ligands. The RR spectra of the Fe(III)BHP(+) and Fe(III)PFP(+) complexes exhibited variable frequencies for the structure-sensitive nu(2) and nu(8) lines (1558-1563 cm(-1) and 386-401 cm(-1), respectively). In considering the ability of the different superstructures to stabilize particular out-of-plane distortions, this vibrational information was analyzed in terms of heme structure through changes in core size and Fe-N(pyrrole) bond length, in relation to changes in coordination geometry. The bis(N-methylimidazole) complex of Fe(III)[((C(3))(2)phi)(2)](+) was found to be the most distorted with a strongly ruffled tetrapyrrole. Because of a handle asymmetry, the heme conformation of the bis(N-methylimidazole) complex of Fe(III)[((C(3))(2)phi)(C(12))](+) was deduced to be a composition of ruffled and domed structures. The heme structure of the other complexes is a mixture of ruffled and saddled or ruffled and waved conformations. Taking into account our previous ...

Resonance Raman optical activity of the imidazole–Myoglobin complex: Titrating enhancement

“…The use of a laser excitation (532 nm) in coincidence with the Q α band led to the selective enhancement of the haem vibrational modes; hence, all bands observed in the rRaman (and rROA) spectral region 1,750–600 cm −1 were assigned to porphyrin vibrations (Figure and Table ) . The spectral region between 1,700 and 1,100 cm −1 is dominated by the enhanced in‐plane vibrational modes of the haem moiety, giving rise to the strongest rRaman signals (Figure ) . In particular, in this region, so‐called marker bands can be found, indicating the oxidation state of the haem iron (ν 4 ), the coordination of the metal centre (ν 2 and ν 10 ), and the ligation state of the iron (ν 3 ); therefore, they can be used to characterize the active form of the prosthetic group in Mb …”

“…[6,9,32] The spectral region between 1,700 and 1,100 cm −1 is dominated by the enhanced in-plane vibrational modes of the haem moiety, giving rise to the strongest rRaman signals (Figure 2). [33] In particular, in this region, so-called marker bands can be found, indicating the oxidation state of the haem iron (ν 4 ), the coordination of the metal centre (ν 2 and ν 10 ), and the ligation state of the iron (ν 3 ); therefore, they can be used to characterize the active form of the prosthetic group in Mb. [1] The rRaman/rROA spectra of the globin after incubation with imidazole revealed substantial changes in the marker bands region, especially in the range 1,700-1,500 cm −1 , indicating the gradual formation of a new species in solution, showing different spectroscopic features with respect to the former met-Mb.…”

Out of Plane Distortions of the Heme b of Escherichia coli Succinate Dehydrogenase

“…The positions of these altered amino acids and their consequences on the EPR signals provide indirect evidence that allow us to speculate that the different EPR signals are a consequence of out-of-plane distortions of the porphyrin ring. Although energetically unfavorable, distortion of the heme away from planarity is common in Fe-porphyrins [26] and is heavily influenced by the surrounding protein milieu [27], [28].…”