Manipulating the monomer-dimer transformation of a heptamethine cyanine ligand: near infrared chromogenic recognition of biothiols.

A novel absorbance recovery method has been developed for the determination of biothiols with a near-infrared reagent. This method employs a two-reagent system composed of cation heptamethine cyanine (CyL) and Hg2+. The absorbance of CyL, with a maximum peak at 760 nm, was decreased due to addition of Hg2+, but recovered when biothiols were added. Under optimal conditions, the reciprocal extent of recovered absorbance was proportional to the concentration of biothiols. The calibration curves are linear over the range of (0.3-7.0) × 10-6 M for cysteine, (1.0-10.0) × 10-6 M for homocysteine and (1.0-9.0) × 10-6 M for glutathione. Because of the specific affinity of Hg2+for biothiols, there is minimal interference from other amino acids. This method has been successfully applied to the determination of homocysteine in human urine samples with satisfactory results.