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Same system-different results: the importance of protein-introduction protocols in Langmuir-monolayer studies of lipid-protein interactions.

For studies of protein-lipid interactions, thin films at the air-water surface are often employed as model systems for cell membranes. A convenient manner in which to study these interactions is the Langmuir technique, which allows for formation of monolayer phospholipid films together with a choice of where and how to introduce proteins, according to the desired response variable. Here, a distinction has been made between different interaction protocols and it is also commented upon to what extent introduction of protein to a solution prior to spreading of a lipid film affects the results. This paper describes commonly used methods when working with Langmuir monolayers as membrane mimics and compares the results of four different experimental protocols: formation of a lipid film on top of a protein-containing subphase, injection of protein under an existing, semicompressed phospholipid film (surface pressure 5 mN/m), and deposition of a protein solution on top of a lipid film contained at either surface pressure 0 mN/m or at surface pressure 5 mN/m. Results obtained from Langmuir isotherms and Brewster angle microscope clearly differentiate between these methods and give insight into under which conditions and at which interfaces the protein interactions are predominant (protein-air or protein-lipid).