Mechano-chemical synthesis and spectroscopic characterization of hemin complexes with some amino acids

The mechano-chemical reactions of hemin with arginine (Arg), histidine (His), lysine (Lys), methionine (Met) and tryptophan (Trp) were monitored using IR and Mössbauer spectroscopies. According to IR spectra, with exception of Arg, these basic amino acids don't react at the peripheral propionic acid groups of hemin which is related with their relatively low basicity. Arg also forms a penta-coordinated complex with hemin at the iron site, as revealed by the Mössbauer spectra of the hemin-Arg milled mixtures. The hemin-Arg solid state reaction is completed in 1:3 molar ratio suggesting that this is the stoichiometry of the formed complex. The binding of Arg to the carboxylic groups of hemin and to the central Fe(III) cation inhibits any interaction between vicinal heroin molecules, which explains the solubility and stability of the Arg-hemin complex in water. His binds iron only in basic media but, in addition, hematin is also formed. It seems that the inter- and intra-molecular hydrogen bonding interactions within crystalline His limit its ability to form a complex with hemin in a solid state reaction. No complex formation at the iron site was observed with Lys, Met and Trp.