Molecular recognition of amines and amino esters by zinc porphyrin receptors: binding mechanisms and solvent effects.

Abstract

Zinc porphyrin receptors bearing 12 ester groups in the meso phenyl groups (1-3) were prepared, and binding of amines and alpha-amino esters was studied with emphasis on the binding mechanisms. The X-ray crystallographic analysis of 5,10,15,20-tetrakis(2, 6-bis(carbomethoxymethoxy)-4-carbomethoxyphenyl)porphyrin (free base of 1) showed that the receptor has a binding pocket above the porphyrin plane. UV-visible titration experiments revealed that the zinc porphyrin receptors bound amines and alpha-amino esters with binding constants (K(a)) ranging from 0.5 to 52 700 M(-1) in CH(2)Cl(2) at 25 degrees C. The ester functional groups of 1 assisted the binding of aromatic alpha-amino esters (K(a) = 8 000-23 000 M(-1) in CH(2)Cl(2) at 25 degrees C) and inhibited the binding of bulky aliphatic alpha-amino esters (K(a) = 460 M(-1) for Leu-OMe in CH(2)Cl(2) at 25 degrees C), indicating that CH-pi type interactions and steric repulsions control the selectivity. The binding of amines and alpha-amino esters was tight both in a nonpolar solvent (CH(2)Cl(2)) and in a polar solvent (water) but loose in a solvent of intermediate polarity (H(2)O-MeOH (1:1)), demonstrating that two competitive driving forces are operating: (1) attractive electrostatic forces between host and guest such as coordination of the amino group to the zinc atom, and (2) entropic forces stemming from desolvation as well as enthalpic forces due to the host-guest dispersion forces. The former forces drive the binding in CH(2)Cl(2) while the latter forces drive the binding in water. The enthalpy changes in the binding in CH(2)Cl(2) and those in water range from -50 to -30 kJ mol(-1) and from -35 to 0 kJ mol(-1), respectively. The entropy changes in CH(2)Cl(2) and those in water range from -120 to -60 J K(-1) mol(-1) and from -50 to +60 J K(-1) mol(-1), respectively. Thus the binding in CH(2)Cl(2) is characterized by large negative enthalpy changes, while that in water by less negative entropy changes. These thermodynamic parameters also indicate that host-guest polar interactions (enthalpic forces) drive the binding in CH(2)Cl(2) while both host-guest dispersion interactions (an enthalpic force) and desolvation (an entropic force) drive the binding in water. Enthalpy-entropy compensation observed for the binding in water indicates that the binding of amines and amino esters in water by zinc porphyrins is associated with conformational changes as well as a high degree of dehydration. In CH(2)Cl(2), no clear compensation was observed, consistent with the mechanism that neither desolvation processes nor conformational changes contribute significantly to the binding energetics.