CHIRAL AMINO-ACID RECOGNITION BY A PORPHYRIN-BASED ARTIFICIAL RECEPTOR

Molecular recognition of amino acid methyl esters by doubly bridged porphyrin Zn complexes was investigated. A chiral, bridged porphyrin was prepared from the alpha,alpha,alpha,alpha-atropisomer of meso-tetrakis(o-aminophenyl)porphyrin and 4-nitroisophthaloyl chloride. The similar reactions using unsubstituted isophthaloyl, 5-nitroisophthaloyl, and 4,6-dinitroisophthaloyl chlorides as the bridging reagents gave corresponding nonchiral bridged porphyrins. The Zn complex of the chiral porphyrin exhibits significant chiral recognition toward amino acids. For example, the chiral receptor porphyrin shows D-/L-selectivity of 7.5 for valine methyl ester at 293 K in CH2Cl2. Comparison of binding behavior of four types of bridged porphyrin Zn complexes and H-1 NMR investigations reveal that the present chiral recognition observed for amino acid methyl esters is caused by favorable hydrogen bond formation between the carbonyl group of the guest and the 3-carboxyamide of the 4-nitroisophthalic bridging benzene. Thermodynamic parameters of the present complexation were also determined, and two types of very strong linear isoequilibrium relationships between the observed entropies and enthalpies were found. One is the linear correlation (R = 0.993) between Delta H and T Delta S values observed for host-guest combinations of bridged porphyrins-amino acid esters, and another is that (R = 0.997) observed for combinations of tetraphenylporphyrin-amino acid esters and bridged porphyrins-3-amino-2,4-dimethylpentane. The very large slopes observed for these linear correlations indicate that the significant amount of the conformational freedom of the guests is lost even during the process of simple coordination without hydrogen bond formation.