Homochiral MOF as Circular Dichroism Sensor for Enantioselective Recognition on Nature and Chirality of Unmodified Amino Acids

Self-assembly of zinc salt with rationally
designed chiral ligand,
(1R,2R)-2-(pyridine-4-ylcarbamoyl)
cyclohexanecarboxylic acid (RR-PCCHC) generated 2D homochiral metal–organic
framework [Zn­(RR-PCCHC)2] (HMOF-1) that is
composed of DNA-like right-handed double-helix structure. HMOF-1 shows high solvent and thermal stability and is also stable
in neutral, weak acidic and weak basic aqueous solution. Emulsified
HMOF-1 shows strong inherent circular dichroism (CD)
signal in aqueous solution, which can show regular intensity change
by induction of amino acids. On the basis of the measuring of CD signal
intensity, a chemosensor for unmodified amino acids is fabricated,
which differ from reported those in which CD signal is amplified by
a complicated chemical reaction of originally CD-silent molecule with
probed amino acids. This chemosensor can be used for rapid, convenient
and sensitive detection of micro amount of amino acids. Most remarkably,
3 × 10–8 mol of l-aspartic acid and
4 × 10–8 mol of d-aspartic acid in
aqueous solution can completely quench CD signal of emulsified HMOF-1 in H2O. It is found that the difference of recognition
ability between d- and l-proline is the largest
in all probed amino acids. The LOD (limit of detection) of the proposed
sensor for the determination of aspartic acid is 13.31 ppm. The recognition
efficiency η = [A0−AA0] ×
100% for l-aspartic acid
is as high as 92.1%. The interacting mechanism of DNA-like HMOF-1
with probed amino acids is similar to that of groove binding of targeting
drug with DNA.