Synthesis and characterization of heme models and spectroelectrochemical studies of heme proteins

Abstract

To better understand the relationship between the structure and electronic properties of the iron center and the functions of heme proteins, both naturally occurring heme proteins and synthetically prepared heme models have been studied. The reduction potential (E°') and the pH dependence of E°' of nitrophorin 1 (NP1) and myoglobin (Mb) were determined by spectroelectrochemical techniques. The difference in the electrostatic interaction of the Fe(III) center with buried charged groups in the heme pocket of Mb and NP1 is the major factor that causes the 300 mV difference in E°. The pH dependence of the E°' determined between pH 5.5 and 7.5 is small for both Mb and NP1 because they have the same axial ligands. Three meso-ortho-phenyl substituted porphyrins, (o-F), (o-CF₃) and (2,6-Cl₂)(p-OCH₃)₃TPP and one meso-para-phenyl substituted porphyrin, (p-OCH₃)₄TPP, were synthesized as models of cytochrome b₅ . Cyclic voltammetry was used to measure their reduction potentials. The overall formation constants, logβ₂III and logβ₂II, have been calculated based on the reduction potentials of the iron(III)/(II) couple as a function of N-methylimidazole concentration. The values of logβ₂III are in the order of o-F > o-CF₃ > p-OCH₃ ≈ 2,6-Cl₂, indicating that the electron-donating ability is in the order of o-F < o-CF₃ < p-OCH₃ ≈ 2,6-Cl₂. The overlap and direct transfer of electron density from the halogen to the iron in the product reduces the Lewis acidity of iron(III), resulting in decreased logβ₂III. The order of logβ₂II for N-methylimidazole complexed Fe(II) porphyrinates is similar to that of the Fe(III) complexes, indicating no major difference in the Lewis acidity of Fe(II) as compared to Fe(III). Basket handle porphyrinates with covalently bound methionine and aliphatic amine model ligands (RCH₂SCH₃, RCH₂SCH₃ and RNH₂, RNH₂) were chosen as precusor of cytochrome c and f. The Fe(III) complexes were to be prepared and investigated by electrochemical and spectroscopic techniques. The precursor porphyrin was synthesized. Several schemes were investigated for the synthesis of the handles having methylthioether and aliphatic amine without success, and it was decided not to continue this project. Therefore, no final basket handle porphyrin was available for further characterization.