A new water soluble Zn-salophen derivative as a receptor for alpha-aminoacids: unexpected chiral discrimination.

A new water soluble zinc-salophen complex with appended D-glucose moieties was synthesized and characterized. Its binding properties toward six aminoacids were investigated by UV-vis spectrophotometric titrations. Association constants showed to be unfavorably affected by increasing steric hindrance of the side chain. Quite surprisingly, different association constants were measured for the L and D enantiomers. These data suggest that aminoacids are bound via two interactions, zinc-carboxylate coordination and two hydrogen bonds between the ammonium group of the aminoacid and two oxygen atoms of one D-glucose moiety. Such conclusion is supported by the result of semiempirical (PM3) calculations. Notably, the K(L)/K(D) value of 9.6 observed for the association of phenylalanine rivals with the highest values found for the chiral recognition of aminoacids in water.