pH-Controlled recognition of amino acids by urea derivatives of β-cyclodextrin

Abstract

Water soluble amphiphilic urea-substituted β-cyclodextrins were synthesized and applied as amino acid receptors. A great affinity towards nonpolar amino acids was observed, ranging from 2300 M⁻¹ for alanine to 54 800 M⁻¹ for tryptophan in a highly competitive environment (pH 8 phosphate-buffered water solution). Significant selectivity was observed, since carboxylates without the amino group (or having hydroxyl instead) remained not bound. Endoenthalpic effects were recorded for amino acids during titrations, pointing to a different mode of complex formation as compared to native β-cyclodextrin. We found a dramatic change in binding strength within the physiological pH range: with pH change from 8 to 6, affinity towards amino acids dropped from 54 000 to 500 M⁻¹, making the receptors potentially appropriate for biotechnological purposes.