Effect of cyclodextrins on the behaviour of insulin at the air–water interface: A Langmuir monolayer study

• β-Cyclodextrins affect the behaviour of bovine insulin Langmuir monolayers.
• β-Cyclodextrins were added to the insulin spreading solutions or to the subphases.
• The effect of β-cyclodextrins depends on their location.

Herein we report the effect of β-cyclodextrins on the behaviour of bovine insulin Langmuir monolayers spread at the air–water interface. The insulin–β-cyclodextrin interaction was studied using two different approaches: first, the macrocycles were dissolved in the aqueous subphase, and second, they were included in the insulin spreading solutions. The experiments were performed under various subphase conditions of pH and salt content. The behaviour of the insulin monolayers in the presence of β-cyclodextrins was investigated by measuring the isotherms of the surface pressure (π) as a function of the area per molecule and the surface compressional modulus–π curves. Brewster angle microscopy was applied to confirm the formation of insulin aggregates at the air–water interface. Our results indicate that the characteristics and behaviour of the insulin monolayers are influenced by β-cyclodextrins and their effect on the monolayers depends on the location of the macrocycles.