Copper(ii) and zinc(ii) complexes with C-glycoside-pendant dipicolylamine (DPA)-amino acid conjugates

Abstract

A series of dipicolylamine (DPA)-functionalized amino acid ligands with a pendant C-glycosyl amide has been synthesized and complexed with Cu(II) and Zn(II). Amino acid derivatives equipped with a DPA unit were conjugated with C-glycosylamine prepared from acetobromoglucose in 4 steps. These ligands bind to metal ions as a N₃ or N₃O chelate utilizing three DPA nitrogens and carbamoyl oxygen atoms. In the crystal structures of copper(II) and zinc(II) complexes of AcL1 (2-(N′,N′-bis(2‴-pyridylmethyl)amino)-N-(2′-(2″,3″,4″,6″-tetra-O-acetyl-β-D-glucopyranosyl)ethyl)acetamide, carbamoyl oxygen binds to the metal centre, however, NMR analysis reveals that AcL1 acts as a tridentate ligand in solution. The CD spectrum of the copper(II) complex with glycine-based AcL1 at the d–d transition region indicates that the chirality at the sugar moiety does not induce an asymmetric metal coordination environment. In contrast, the chirality at the amino acid moiety for alanine and phenylalanine derivatives, AcL2 and AcL3, respectively, induces characteristic Cotton effect in the 500–800 nm region depending on the nature of the α-carbon substituent of the amino acid moiety.