Monolayers of three amphiphilic esters of rutin at the air-water interface

The π-A isotherms for monolayers of three amphiphilic rutin esters, rutin-4”’-O-stearate (RS), rutin-4”’-O-laurate (RL) and rutin-4”’-O-caproate (RC), were measured at the air-water interface and an air-aqueous aluminum sulfate solution interface. Although RS and RL showed scant water solubility, they formed monolayers with the liquid-expanded phase when spread on water. An extraordinary value of the limiting molecular area was observed for the RC monolayer, which may be due to the larger water solubility of RC. A model was proposed for the molecular conformations of RS and RL monolayers at the surface pressure at collapse (πcoll). In this model, two saccharide groups were under the water and perpendicular to the surface, the aglucone portion was spread out on the surface and the alkyl chain was orientationally disordered in air. When spread on an aqueous aluminum sulfate solution, the formation of a complex between the aluminum ions and the rutin ester resulted in higher values of acoll and lower values of πcoll for RS and RL monolayers compared with their values on water, and RC could spread as liquid-expanded phase monolayer. The results described in this paper provide valuable information for investigating the interaction of flavonoids and biomembranes and the anti-oxidant mechanism of flavonoids.