Host-guest interactions of chiral Schiff-base macrocyclic Zn(II) complexes with α-amino acids

Abstract

Two chiral Schiff-base macrocyclic Zn(II) complexes (M_R,R and M_S,S) were synthesized from the condensation reaction of 3,3’-methylene-bis(2-hydroxy-5-methylbenzaldehyde) and 1R,2R- or 1S,2S-cyclohexanediamine in the presence of Zn(NO₃)₂. Their structures were characterized using ¹H/¹³C NMR, ESI-MS, and elemental analysis, and the crystal structure of M_R,R was further determined via single-crystal X-ray diffraction. In the solution of MeOH/H₂O (9:1), the UV-Vis spectra were utilized for the host-guest interaction studies between M_R,R or M_S,S and a series of α-amino acid enantiomers. The results demonstrated that both M_R,R and M_S,S exhibited quantifiable binding interactions with D-/L-Arginine (Arg), D-/L-Lysine (Lys), and D-/L-Glutamic acid (Glu), and the binding ratio was consistently 1:1. Moreover, the density functional theory (DFT) calculation indicated that the binding affinity as well as the observed enantioselectivity of M_R,R or M_S,S toward these amino acid enantiomers was mainly attributed to the hydrogen-bonding interactions between the amino acid enantiomers with the bridging water molecules located in the binuclear Zn(II) complexes.