C3-symmetric ferrichrome-mimicking Fe3+-complexes containing the 1-hydroxypyrimidinone Fe3+-binding moieties based on α-cyclodextrin: helicities in solvent environments

Abstract

Two homologous C₃-symmetric ferrichrome mimics, La and Lb, equipped with three 1-hydroxypyrimidinone terminals for receiving an Fe³⁺ ion have been designed and synthesized; the side chains of La and Lb, which include dimethylene and pentamethylene spacers, respectively, are anchored to the narrower rim of the α-cyclodextrin framework through an amide linkage. The 1∶1 Fe³⁺-complexes of La and Lb provide strong circular dichroic exciton coupling of negative and positive signs at 480 nm, showing Δ- and Λ-helices, respectively, in polar aprotic solvents such as dimethylformamide and dimethyl sulfoxide. The magnitude of the signals increases with the increasing hydrogen-bond accepting ability of the solvent. It is inferred, therefore, that hydrogen-bonding interactions of the linker amide N–H groups with the solvent play a primary role in inducing a helicity within aprotic solvents. The mechanisms of the helicity inductions are discussed.

Meanwhile, the signals for La are still negative, but weak in alcohols and water. However, the signal sign for Lb having the relatively flexible spacers depends on the nature of alcohols, and no CD signal was observed in water.

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