PACKING OF HYDROPHOBIC ALPHA-HELICES - A STUDY AT THE AIR-WATER-INTERFACE

In order to mimic the packing of alpha-helices in alpha-helical proteins, two hydrophobic homopolypeptides, poly-L-alanine (PLA) and poly-L-leucine (PLL), in their alpha-helical conformation were spread at the air/water interface and studied by the monolayer technique. The surface pressure and surface potential isotherms of both polypeptides were recorded at 22 +/- 2-degrees-C. In addition to the arrest point already identified in the literature as the starting point for the monolayer/bilayer transition, the surface pressure isotherms also showed an inflection point at 12.8 angstrom2/residue for PLA and 17.0 angstrom2/residue for PLL. Starting from these residual areas, a surface potential pseudoplateau was encountered for both polypeptides. From calculations of the internal and external radii of the PLA and PLL alpha-helices, we have rationalized the organization of the hydrophobic helices at the inflection point. It is demonstrated that, at these residual areas, the hydrophobic alpha-helices are organized such that their side chains are interdigitated. From the residual areas at the inflection point, we calculated the interhelical distance to be 8.5 and 11.0 angstrom for PLA and PLL, respectively. These values agree with the interhelical distances found in a variety of alpha-helical proteins as well as those obtained from theoretical studies. On the basis of these results, and from what is known in the literature on the packing of the alpha-helices in such proteins, we suggest that the hydrophobic alpha-helices of PLA and PLL are organized as coiled-coil structures at the air/water interface.