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

The association of various aamino acids with four new, coordinativelv 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-displacernent 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" 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.