Fluorescently Labeling Amino Acids in a Deep Eutectic Solvent

The increased use of deep eutectic solvents (DESs) in recent years has been significant and provides new approaches to sample collection and preparation. At the same time, the use of these new solvents to prepare samples can present challenges for subsequent analyses. Common analytical approaches, such as fluorescent labeling, may not be compatible with the solvents. In this work, we explore how effective three traditional fluorescent labels can be at derivatizing amino acids in the most common DESs, formed from choline chloride and ethylene glycol. We demonstrate that the unique solvent characteristics of the DESs still allow for two of the fluorophores, fluorescein isothiocyanate and 5-carboxyfluorescein succinimidyl ester, to effectively label amino acids. Initial optimizations of the reaction conditions demonstrate that we can effectively label both d- and l-amino acids, in solution with concentrations of amino acids down to 4 μM. Capillary electrophoretic separations following this preparation can detect as little as 50 nM. This is possible without removal of any DES from the sample matrix. These results represent the first complete fluorescent labeling reaction in a DES and subsequent capillary electrophoretic separation of the analytes.