“Naked‐Eye” Detection of Histidine by Regulation of CuII Coordination Modes

Abstract

The association of various α‐amino acids with four new, coordinatively unsaturated metal complexes ([Cu(5)]²⁺, [Cu(6)]²⁺, [Cu(7)]²⁺, and [Zn(8)]²⁺) was examined. The receptors [Cu(5)]²⁺ and [Cu(7)]²⁺ were found to discriminate histidine (His) from other zwitterionic α‐amino acids by means of indicator‐displacement assays (IDAs) using 5(6)‐carboxyfluorescein as an indicator in buffered methanol/water (3:1) solvent. The colorimetric detection of His was achieved by using this IDA method, which appears to owe its selectivity to a unique process involving disruption of the host complex to form a 2:1 His/Cu^II complex rather than simple indicator displacement. The occurrence of distinct intermolecular coordination processes in response to the introduction of a different amino acid is observed. X‐ray crystal structures of the host metal complexes were obtained and exhibit the adoption of a variety of coordination geometries about the metal center.