A simple yet highly sensitive diastereoselective fluorescent assay was developed, which can differentiate diastereomeric 1,2-aminoalcohols such as ephedrine, pseudoephedrine, and methylephedrine using commercially available xanthene dyes. The assay is based on the rhodamine B lactone–zwitterion equilibrium, which is highly sensitive to the presence and structure of 1,2-aminoalcohols. This is in contrast to the majority of rhodamine sensors, which are based on the pH sensitive carboxylate–lactone equilibrium. A model is proposed in which the diastereoselectivity arises from the differences in the strength of an intermolecular lone pair-π interaction within the analyte–rhodamine zwitterion complex. An assay was developed based on this sensing mechanism in which the structure and concentration of a 1,2-aminoalcohol solution can be determined using a mixture of rhodamine B and fluorescein. The selectivity and accuracy of this two-dye system was then demonstrated by the ability to measure the diastereomeric excess of samples of methylephedrine with 98% accuracy.
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