Abstract: The development of artificial receptors for the selective recognition of biologically important species has been one of the best concerned topic in the field of supramolecular chemistry. However, compared to the large number of chromo/fluororeceptors for cations or anions, the development of artificial receptors for amino acids is quite limited. The effective and selective molecular recognition or sensing of unprotected amino acids in aqueous solution is still a challenging problem due to their highly hydrophilic character. In this paper, a series of 3,3′-bis(indolyl)methene receptors were designed and synthesized, and the recognition properties of the receptors for 20 natural amino acids have been studied using UV-Vis spectroscopic techniques. The result indicated that the receptors exhibited selective colorimetric sensing for amino acids by protonated/deprotonated process of the hydrogen bond site of the receptors in CH3CN/H2O (1:1, v/v). The introduction of methoxy group into indole ring, which increased the basicity of the hydrogen bond acceptor unit, can promote the occurrence of the protonated process and increase the sensing sensitivity for acidic amino acids. The receptors with nitro group exhibited obviously colorimetric sensing for basic amino acids (arginine, lysine) due to the deprotonation of the hydrogen bond donor unit of receptors. In CH3CN/H2O (1:1, v/v; 0.01 M HEPES, pH=7.4) neutral buffered solution, the receptors without nitro group showed negligible spectral responses for various amino acids, but the nitro-substituted receptors exhibited high selective colorimetric sensing for cysteine (Cys) and homocysteine (Hcy) based on the nucleophilic addition reaction between the sulphydryl group of Cys/Hcy and the meso carbon–carbon double bond of receptors, leading to clear color change from yellow to colorless.

Key words: Bis(indolyl)methene receptor, Amino acid, Colorimetric recognition