THE MONOLAYER TECHNIQUE AS A TOOL TO STUDY THE ENERGETICS OF PROTEIN-PROTEIN INTERACTIONS

It this paper, we explore the possibility of using the monolayer techn ique and hydrophobic homopolypeptides to study the energetics of prote in stability. We have studied the stabilization of the bilayer state o f poly-L-alanine in its alpha-helical conformation at the air-water in terface by measuring compression and expansion surface pressure (II)-r esidual area (A) isotherms at 22 +/- 2 degrees C. The Gibbs free energ y of stabilization per alanyl residue transferred from the water expos ed state in the monolayer to the inside of the bilayer was calculated from the surface area of the hysteresis loops obtained during compress ion-expansion cycles performed during the monolayer to bilayer transit ion. Using atomic solvation parameters and the water accessible surfac e 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 G(s)) upon transfer f rom the water surface to the inside of the bilayer state, and the free energy associated to the formation of hydrophobic van der Waals inter actions (Delta G(vdW)) in the bilayer. We estimate a value of 25 +/- 4 cal/(mol Angstrom(2)) for the hydrophobic interaction, as defined by the sum of Delta G(s) and Delta G(vdW) per unit of hydrophobic (alipha tic) accessible surface area in an alanyl residue. (C) 1998 Elsevier S cience B.V.