Optical chemosensors for organic and inorganic anions based on an amino acid core functionalised with benzimidazole

Abstract

Anion coordination by natural amino acids can arise from side chain OH and NH groups (in serine, threonine, tyrosine or arginine and tryptophan, respectively), as well from additional binding with amide NH groups from the backbone, which act as hydrogen-bond donors to the anion.1 Incorporation of extra anion recognition sites through functionalisation at the side chain should result in amino acid and peptidic frameworks that may play an innovative role in molecular recognition since they mimic natural mechanisms of organisation. Benzimidazole and some derivatives have been studied in anion recognition systems that display color changes or fluorescence quenching or enhancement upon binding.2 Following our research interests that include the synthesis and application of colorimetric and fluorimetric probes for anions and cations based on heterocycles and amino acids,3 we now report the synthesis of unnatural phenylalanine derivatives, starting from tyrosine, by introduction of a benzimidazole as coordinating/reporting unit, substituted with electron acceptor groups (cyano and nitro), and their evaluation as optical chemosensors for recognition of organic and inorganic anions such as halides, CN-, AcO-, BzO-, NO3 -, ClO4 -, HSO4 -, H2PO4 - and OH-. The substituent at the benzimidazole resulted in a fine tuning of the sensory properties and signalling mode, as seen by spectrofotometric and spectrofluorimetric titrations which showed that the cyano derivative 1 is a fluorimetric chemosensor while the nitro derivative 2 behaves as a colorimetric chemosensor for F-, CN-, AcO-, and OH-.