Catalytic DNA-based fluorescence polarization chiral sensing platform for L-histidine detection at trace level

This paper reports a novel fluorescence polarization (FP) chiral sensor approach based on a catalytic DNA. This platform involves an enzyme module (E), which was able to trigger the L-histidine-dependent cleavage of an RNA phosphoester bond of a substrate domain (S), whereas it did not accept the D-enantiomer as cofactor. Two assay formats were proposed, based on bi- and unimolecular strategies. The bimolecular design was related to the use of separate E and fluorescently labelled S* sequences. The two oligonucleotide strands were pre-assembled via complementary regions at their extremities. As the result of the large molecular volume of the formed assembly, the S* probe displayed a high fluorescence anisotropy signal. Upon addition of the L-histidine, the DNAzyme cleaved the phosphoester bond of the S* component, leading to the loss of stem stability and the release of single-stranded products of lower size. This was accompanied by a significant decrease in the fluorescence anisotropy response. As a simpler alternative, the unimolecular design, where E and S sequences are linked together through a loop to form a single fluorescent probe E-S*, was also investigated. It was found that the unimolecular approach provided an improved FP response relative to the bimolecular one. Under optimized operating conditions, such a chiral sensing platform allowed the detection of as low as 0.05% of the L-histidine enantiomer in a non-racemic mixture.