"Naked-eye" detection of histidine by regulation of Cu(II) coordination modes

The association of various alpha-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 alpha-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.