Synthesis of Chiral Amino Acid-Linked Porphyrin Protein Supramolecular Complexes and Their Novel Induced Circular Dichroism

Abstract

A chiral amino acid has been covalently linked to (5-(2-carboxylphenyl)-10,15,20-triphenylporphinato)zinc(II) via amide formation. As a heme protein model, the resulting chiral amino acid-linked porphinatozinc(II) complex, AA-COTPPZn (AA = amino acid residue), was designed to investigate the relationship of the induced circular dichroism (ICD) of heme protein and the intramolecular interaction. The synthesized porphinatozinc(II) complexes exhibited split ICD in the Soret region. The ICD of AA-COTPPZn in the Soret region also indicates that an amino acid residue interacts with a porphyrin plane and the intramolecular interaction makes the conformation of an amino acid residue relatively fixed. Electronic absorption spectra demonstrate that a hydroxyl group of the amino acid residue coordinates to a Zn atom of the porphinatozinc(II) complex in Thr-COTPPZn (Thr = threonine) and Ser-COTPPZn (Ser = serine) and the coordination interaction between the amino acid residue and a porphyrin moiety is weakin Leu-COTPPZn (Leu = leucine). ICD studies indicate that the stronger intramolecular interaction increases the ICD intensity of these chiral porphinatozinc(II) complexes and that the different direction of intramolecular interaction would make the disposition of the ICD different. AA-COTPPZn represents a unique system which allows us to investigate the CD (CD = circular dichroism) features which arise from the heme moiety directly rather than from heme protein model compounds, as was done previously.