Noncovalent induced circular dichroism sensors based on a chiral metal–organic framework: chiral induction synthesis, quantitative enantioselective sensing and noncovalent sensing mechanism

Abstract

As an important class of chiral optical sensors, chiral induced circular dichroism (ICD) sensors have received increasing attention. Herein, a series of monometallic and bimetallic centered metal–organic frameworks (MOFs) with the same crystal structure, namely [Cu(L)(2,2′-bipy)]·H₂O (LNNU-2), [Cd(L)(2,2′-bipy)(H₂O)] (LNNU-3) and [Zn_xCu_y(L)(2,2′-bipy)]·H₂O (x : y = 9.6 : 0.4–1.0 : 9.0) (H₂L = HOOCC₆H₄CH₂PO(OH)(OC₂H₅), 2,2′-bipy = 2,2′-bipyridine), have been successfully synthesized for studying the non-covalent ICD sensing mechanism. LNNU-2 shows excellent ICD sensing performance due to the strong metal coordination interaction between LNNU-2 and chiral tryptophan (Trp). In contrast, there is no ICD effect between LNNU-3 and chiral Trp due to the absence of metal coordination interaction. The results indicate that the metal coordination interaction plays a decisive role in generating and amplifying ICD signals in this sensing system. Compared to the previously reported LNNU-1 ([Zn(L)(2,2′-bipy)]·H₂O) sensor, LNNU-2 is not only capable of quantifying the enantiomeric composition of chiral Trp samples at a lower concentration but also enables highly selective and accurate determination of L-Trp concentration in water, mixed natural amino acids and simulated blood plasma components by the circular dichroism (CD) titration method. LNNU-2 also allows the accurate determination of the absolute configuration and enantiomeric excess (ee) values of two chiral aromatic amino alcohols in pure water. Additionally, the effects of different chiral amino acids as chiral inducers on the chiral induction synthesis of enantioenriched LNNU-2 were investigated. As a result, the enantioenriched (P)-LNNU-2 and (M)-LNNU-2 were successfully obtained using L-alanine (L-Ala) and D-alanine (D-Ala) as chiral inducers, respectively.