Molecular recognition by natural macrocycles .1. d-tubocurarine as a host molecule for organic anions

Abstract

The binding of 8-anilino-1-naphthalenesulfonate and 15 anions of substituted benzoic. aliphatic dicarboxylic, and N-acetyl-alpha-amino acids to a macrocyclic alkaloid d-tubocurarine in aqueous solution has been studied by fluorometry, conductometry, and H-1 NMR. The binding constants vary from ca. 50 to 3300 m (1)depending on the guest structure, charge and hydrophobicity. The results of fluorescence and NMR studies show that the host-guest complexation of the anions of aromatic acids involves the formation of a salt bridge between the quaternary nitrogen of the alkaloid and the anionic group of the guest as well as hydrophobic/Van der waals interactions between the guest and host aromatic moieties. The binding of dianions of aliphatic dicarboxylic acids most probably is purely electrostatic. In general, d-tubocurarine possesses binding ability comparable to that of synthetic cyclophanes, It binds enantiospecifically anions of N-acetyl-alpha-amino acids and discriminates between positional isomers of anions of hydroxy and carboxy substituted benzoic acids.