Preparation and Formation Process of Zn(II)-Coordinated Nanovesicles

Mixing a glycylglycine lipid and zinc acetate has been reported to form novel supramolecular Zn(II)-coordinated nanovesicles in ethanol. In this study, we investigate in detail the formation of nanovesicles by using three lipids at different temperatures and discuss their formation process. The original lipids show extremely low solubilities and appear as plate structures in ethanol. Within a small window of lipid solubility, the formation of lipid-Zn(II) complexes occurs mainly on the solid surfaces of plate structures. Controlling of the lipid solubility by temperature affects the kinetics of complex formation and the subsequent transformation of the complexes into nanovesicles and nanotubes. An improved method of two-step control of temperature is developed for preparing all the three kinds of nanovesicles. We provide new insights into the formation process of nanovesicles based on several control experiments. A tetrahedral lipid-cobalt(II) complex similarly produces nanovesicles, whereas an octahedral complex gives sheet structures. Mixing of zinc acetate with a β-alanyl-β-alanine lipid can only give sheet structures, which lack a polyglycine II hydrogen-bond network and induce no morphological changes. We conclude that the formation of the lipid-Zn(II) complexes on solid plate structures, tetrahedral geometry, and polyglycine II hydrogen-bond network in the complexes shall work cooperatively for the formation of Zn(II)-coordinated nanovesicles.