Stabilization of α-helices by dipole-dipole interactions within α-helices

Including solvation effects (in the Poisson-Boltzmann continuum solvent approximation) we report ab initio quantum mechanical calculations (KF/6-31G**) on the conformational energies for adding alanine to the amino or carboxyl terminus of a polyalanine alpha-helix as a function of helix length N. We find that extending the length of an alpha-helix increasingly favors the alpha-helix conformation for adding an additional residue, even in hydrophobic environment. Thus, alpha-helix formation is a cooperative process. Using charges from the QM calculations, we find that the electrostatic energy dominates the QM results, showing that this increasing preference for alpha-helix formation results from dipole-dipole interaction within the alpha-helix. These results provide quantitative preferences and insight into the conformational preferences and kinetics of protein folding.