STRUCTURAL STUDIES OF METALLOPORPHYRINS .11. COMPLEXES OF WATER-SOLUBLE ZINC(II) PORPHYRINS WITH AMINO-ACIDS - INFLUENCE OF LIGAND-LIGAND INTERACTIONS ON THE STABILITY OF THE COMPLEXES

The stability constants of a series of complexes of the cationic water-soluble porphyrin ZnTMPyP with various amino acids have been determined by 1H NMR spectroscopy at pH 10.5. The following stability order has been observed: Tyr > Phe, Glu > Asp > Ile > Val > Gly. These results can be best rationalized by invoking complex stabilization due to ligand-ligand (e.g., stacking or electrostatic) interactions. Evidence for stacking interactions between the porphyrin ring and the aromatic ring of phenylalanine, tyrosine, and tryptophan was further provided by study of the complexation of these amino acids with the free-base porphyrin TMPyPH2. In this case, complexation constants increased in the order: Phe < Tyr < Trp. Attempts to form complexes of the amino acids with the anionic porphyrin ZnTCPP proved unsuccessful, indicating that electrostatic interactions play a major role in the stability of the zinc porphyrin-amino acids complexes. © 1990.