Folding of α-helices into bundles in long polyalanines

► A breakup of α-helix into shorter pieces that organize into bundles is predicted. ► Several bundles differing in the number of helical fragments coexist in polyalanines. ► The simulation data concur with the ion mobility experiments on Ala-rich peptides. ► Average length of helices in a bundle well matches the data for membrane proteins. ► The ability of helix-coil theories to account for the helix bundles is scrutinized.