Five‐Coordinate Iron‐Porphyrin as a Model for the Active Site of Hemoproteins

• Published in: European Journal of Biochemistry
• Main Authors: MOMENTEAU, Michel, ROUGÉE, Michel, LOOCK, Bernard
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1976
• Subjects: Carbonic acid
• Online Access: http://dx.doi.org/10.1111/j.1432-1033.1976.tb11090.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1432-1033.1976.tb11090.x; https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1976.tb11090.x

Preparation of iron(III)‐deuteroporphyrin 6(7)‐methyl ester, 7(6)‐(histidine methyl ester) by coupling histidine methyl ester to deuterohemin has been performed using the mixed carboxylic/carbonic‐acid‐anhydride method. This compound, which is very soluble in various organic solvents, can be considered as a prosthetic group model for the active site of five‐coordinate hemoproteins. In the oxidized state a basic, a neutral or an acid form can be isolated. The basic and acid forms are monomeric at all concentrations. The neutral form is found dimeric in concentrated solutions while it is monomeric at low concentrations. The coordination state of iron in these various species is investigated. The neutral form reacts with ligands, such as nitrogenous organic bases, leading to six‐coordinate well‐known hemichromes which exhibit low‐spin electron spin resonance (ESR) spectra. The reaction of anionic ligands, such as F − , CN − , NO 2 − and N 3 − , with the neutral form model leads to unsymmetrical six‐coordinate complexes whose optical and ESR spectra are similar to those of synthetic deuteromyoglobin. In benzene, toluene or dichloromethane solutions iron(II)‐deuteroporphyrin 6(7)‐methyl ester, 7(6)‐histidine methyl ester), obtained from ferric forms by heterogeneous reduction with aqueous dithionite, exhibits an optical spectrum characteristic of a five‐coordinate high‐spin ferrous complex. At low temperature important spectral modifications are observed indicating a dimeric association. At room temperature it binds one nitrogenous base molecule leading to the well‐known hemochrome. It reacts also with carbon monoxide with a very high affinity constant (4.5 × 10 8 M −1 ), comparable to that of the isolated human hemoglobin α and β chains, but much higher than the values reported for other various models, suggesting that the side‐chain length bearing the fifth ligand may have an important influence upon the reactivity of the sixth position of the iron ion. At low temperature in toluene or dichloromethane, this ...