The complexation mode of metal ions with Langmuir monolayers of nitrogen-containing flavonoid glycoside-based surfactants derived from rutin

The molecular recognition activities of flavonoid derivatives toward other guest molecules have been the focus of interest recently. This study reports the metal-complexation behavior of nitrogen-containing rutin (RuCn) in the organized Langmuir films. Three model compounds with different lipid chain lengths (n = 8, 12 and 18) were designed and synthesized. The monolayer structures and metal-binding activities of RuCn were studied by the monolayer technology, FT-IR spectroscope and thermodynamic calculations (experimentally and theoretically). Results showed that hard Lewis acid-type metal cations, such as Na(I), K(I), Ca(II) and Fe(III), and soft metal cations, such as Cu(II), exhibited clearly different binding activities to RuCn monolayers. The sequence of the effects of metal ions on the monolayer properties and thermodynamic stability varies in the order: univalent ions < pure water < bivalent ions ≈ tervalent ions. Importantly, the typical binding mode (including the binding site and coordination number) of Cu(II) and Fe(III) with RuCn at the interface was clarified. All evidences implied that two primary molecular configurations in the form of O–M–O and O–M–N linkage (M represents Cu or Fe) could account for the unusual structure–property relationship of RuCn monolayers at the air–electrolyte interfaces. These results provided a theoretical basis for the practical applications of such metal complexes as antimicrobials or anticancer drugs in the molecular level.