Enantioselective Sensing of Amino Acids Using a Structurally Distinct Ni-Based H-MOF/MOF: Insights from Spectroscopy and Molecular Simulations

Abstract

This research uses single-crystal X-ray diffraction (SC-XRD) to analyze the structures of hydrogen-bonded metal–organic frameworks (H-MOF(1)), {[Ni(dTMP)(4,4′-azpy)(H₂O)₅]•(H₂O)}_n, and MOF(2), {[Ni(dTMP)(bpe)(H₂O)₃]•5H₂O}_n, synthesized by a slow evaporation method using organic ligands 2′-deoxythymidine-5′-monophosphate (dTMP), 4,4′-azopyridine (4,4′-azpy), and 1,2-bis(4-pyridyl)ethene (bpe). The chiral recognition behavior of these MOFs toward d- and l-amino acids was studied via absorption, circular dichroism (CD), fluorescence spectroscopy, and time-resolved measurements. H-MOF(1) shows fluorescence enhancement with d- and l-asparagine, driven by hydrogen bonding and structural rigidity. d-Asparagine has a higher quantum yield (Φ = 18.4%) and lower detection limit (LoD = 0.315 μM) than l-asparagine (Φ = 17.3%, LoD = 0.415 μM), indicating moderate enantioselectivity. The CD signals for l- and d-asparagine on interaction with H-MOF(1) are distinctly different but diverge from those observed for the complex itself, so H-MOF(1) serves as a dual probe for separating asparagine, not for chiral separation. MOF(2) exhibits fluorescence quenching with l-phenylalanine, attributed to both static and dynamic mechanisms, evidenced by Stern–Volmer quenching plots and a decrease in lifetime from 4.4 to 1.58 ns and a drop in quantum yield (Φ = 20.1% for l-phenylalanine, 11.1% for d-phenylalanine), with LoD values of 0.292 and 2.992 μM. CD spectra reveal a negative band between 300 and 400 nm for the MOF(2): l-phenylalanine complex, suggesting strong π–π stacking and a chiral environment. These results point to MOF(2)’s high enantioselectivity toward l-phenylalanine and its dual role as a fluorescent and CD-active chiral probe. Molecular dynamics simulations reveal that π–π interactions, along with hydrogen bonding and electrostatics, play a crucial role in chiral recognition.