Haem–peptide complexes. Synthesis and stereoselective oxidations by deuterohaemin-L-phenylalanyl-poly-L-alanine complexes

Abstract

Deuterohaemin-L-histidine methyl ester—peptide complexes have been obtained by covalently linking L-histidine methyl ester and the peptides Ala-Ala-Phe-Ala-Ala-Ala-Ala-Ala-Ala-Ala (compound 3) or Ala-Ala-Ala-Phe-Ala-Ala-Ala-Ala-Ala-Ala (compound 4) on the propionic acid side chains. The spectroscopic properties of 3 and 4 and the imidazole binding equilibria indicate that folding of the peptide chain on the opposite part of the porphyrin plane with respect to that occupied by the bound histidine side-arm reduces in the order 3 > 4 the accessibility of exogenous ligands to the sixth co-ordination position of the iron atom, probably through some stacking interactions between the phenylalanine residue of the peptide and the porphyrin ring in the case of 3. This arrangement has marked consequences on the stereoselectivity observed in a model peroxidase reaction using L- or D-tyrosine methyl ester as substrates and tert-butyl hydroperoxide as oxidant in dichloromethane–trifluoroethanol (9:1), that have been interpreted in terms of the interaction between the chiral substrates and the peptide chains of the deuterohaemin complexes.