alpha- and 3(10)-helix interconversion: A quantum-chemical study on polyalanine systems in the gas phase and in aqueous solvent

Helices are among the predominant secondary structures in globular proteins . About 90% of the residues in them are found to be in the alpha -helical c onformation, and another 10% in the 3(10) conformation. There is a standing controversy between experimental and some theoretical results, and controv ersy among theoretical results concerning the predominance of each conforma tion, in particular, helices, We address this controversy by ab initio Hart ree-Fock and density functional theory studies of helices with different le ngths in a vacuum and in the aqueous phase. Our results show that (1) in a vacuum, all oligo(Ala) helices of 4-10 residues adopt the 3(10) - conformat ion; (2) in aqueous solution, the 6-10 residue peptides adopt the alpha -he lical conformation; (3) there might be two intermediates between these heli cal conformers allowing for their interconversion. The relevance of these r esults to the structure and folding of proteins is discussed.