Molecular recognition by natural macrocycles. I. 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‐α‐amino acids to a macrocyclic alkaloid d‐tubocurarine in aqueous solution has been studied by fluorometry, conductometry, and 1H 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‐α‐amino acids and discriminates between positional isomers of anions of hydroxy and carboxy substituted benzoic acids.