The monolayer technique as a tool to study the energetics of protein-protein interactions

In this paper, we explore the possibility of using the monolayer technique and hydrophobic homopolypeptides to study the energetics of protein stability. We have studied the stabilization of the bilayer state of poly-L-alanine in its alpha-helical conformation at the air-water interface by measuring compression and expansion surface pressure (II)-residual area (A) isotherms at 22 +/- 2 degrees C. The Gibbs free energy of stabilization per alanyl residue transferred from the water exposed state in the monolayer to the inside of the bilayer was calculated from the surface area of the hysteresis loops obtained during compression-expansion cycles performed during the monolayer to bilayer transition. Using atomic solvation parameters and the water accessible surface area per atom group for an alanyl residue in a standard alpha-helix, we have dissected the free energy of stabilization per alanyl residue into the change of solvation free energy (delta Gs) upon transfer from the water surface to the inside of the bilayer state, and the free energy associated to the formation of hydrophobic van der Waals interactions (delta GvdW) in the bilayer. We estimate a value of 25 +/- 4 cal/(mol A2) for the hydrophobic interaction, as defined by the sum of delta Gs and delta GvdW per unit of hydrophobic (aliphatic) accessible surface area in an alanyl residue.