Design, Synthesis, and High-throughput Applications of Small Molecule Chiroptical Probes

Abstract

This work showcases several chirality sensors that undergo covalent bond formation with small amine-containing molecules and thus produce a circular dichroism (CD) signal that can be used for chiroptical analysis of the absolute configuration and enantiomeric composition of the substrate. Pyridoxal-5’-phosphate (PLP) was introduced for simultaneous absolute configuration, ee and concentration analysis of unprotected amino acids, amino alcohols and amines. Fast imine metathesis with a PLP aryl imine probe was shown to capture analytes and to generate characteristic Cotton effects for suitable %ee determination. Synchronous release of an aromatic indicator moiety results in a non-enantioselective off-on fluorescence response independent of the enantiomeric sample composition which in turn allows for determination of the analyte concentration. N-aminobenzaldehyde sensors were prepared and tested. Schiff base formation with amines generates a conformational bias in the chromophoric sensor moiety resulting in characteristic CD signals. Computational analysis revealed that CD prediction of the sign of the Cotton effect and subsequent determination of absolute configuration becomes practical with a sterically crowded probe design due to the limited number of relevant conformations as the chiroptical response is less sensitive to conformational equilibria. At the same time, experimental CD amplitudes were successfully used for quantitative ee analysis. A novel approach to chiral recognition of small molecules using the classical ninhydrin agent was introduced. Well-defined dynamic covalent chemistry (DCC) with amines and amino alcohols was developed and applied to quantitative ee sensing with good accuracy using a straightforward mixing protocol and subsequent CD measurements. This assay is fast, broadly useful, practical and repurposes an inexpensive reagent known for 100 years in a new application. The utility of using the EKKO CD plate-reader and a coumarin reagent for the ee determination of a diverse set of chiral amines was examined. Linear relationships between the ee and the induced CD responses were obtained, minimum detectable and quantifiable ee values were calculated, an asymmetric hydrogenation reaction was analyzed, and the effects of interferents often found in crude samples were evaluated. The automated CD assay was found to be suitable for high-throughput experimentation applications.