Semiquantitative Visual Chiral Assay with a Pseudoenantiomeric Fluorescent Sensor Pair

A new red-light-emitting fluorescent probe (R)-5 was synthesized. In the presence of Zn²⁺, this compound was found to exhibit good enantioselective fluorescence enhancement at λ = 655 nm when treated with a variety of amino acids in aqueous solution. This probe in combination with a green-light-emitting probe (S)-4 that has enantioselective fluorescence enhancement at λ = 505 nm has formed a pseudoenantiomeric sensor pair because of their opposite enantioselectivities. This sensor pair can simultaneously detect both enantiomers of a chiral amino acid at two very different wavelengths (Δ = 150 nm). It was used to visually and semiquantitatively determine the enantiomeric compositions of amino acids. For example, when a 1:1 mixture of (R)-5 and (S)-4 was treated with Zn(OAc)₂ and histidine samples of 0-100% [d-His], the color of the mixtures changed from green to yellow, orange, and red under a UV lamp (365 nm), which allowed a quick quantification of [d-His]%. This is the first example of using fluorescence to visually quantify the enantiomeric composition of chiral compounds.