Recognition of α-Amino Acid Esters by Zinc Porphyrin Derivatives via Coordination and Hydrogen Bonding Interactions. Evidence for Two-Point Fixation from Thermodynamic and Induced Circular Dichroism Spectroscopic Studies

Association constants between [trans-5,15-bis(2-hydroxy-1-naphthyl)-2,3,7,8,12,13,17,18-octaethylporphyrinato]-zinc(II) (1) and a series of α-amino acid esters (RCHNH₂CO₂CH₃) were determined in chloroform by use of a UV-vis titration method. Association constants increased in the order Ala-OMe < Gly-OMe < Val-OMe < Leu-OMe, showing a preference for bulky amino acid esters. Contributions from the metal coordination and hydrogen bonding interactions to the total free energy change were estimated by use of reference compounds having no hydrogen bonding site. The free energy change for the binding of Leu-OMe to 1 in chloroform at 15 °C was −5.3 kcal/mol, which was separated into two contributions, (1) the metal coordination interaction ΔG_MC = −3.8 kcal/mol and (2) the hydrogen bonding interaction ΔG_HB = −1.5 kcal/mol. Circular dichroism (CD) induced in the porphyrin Soret band of 1 by complexation with optically active amino acid esters was of the split type for all the amino acid esters examined, whereas induced CD of a reference host, [trans-5,15-bis(2-methoxy-1-naphthyl)-2,3,7,8,12,13,17,18-octaethylporphyrinato]zinc(II) (2), was reduced in intensity and not of the split type. We found that the fixation of the carbonyl group of the guest by two-point recognition caused marked enhancements in induced CD. We suggest that the induced CD of 1 was caused by the coupling between the magnetic transition dipole moment of the carbonyl group of guest and the electric transition dipole moment of the Soret band of host.