A series of molecular dynamics calculations have been performed mimicking the mechanical extension of alpha-helical polyalanine molecule in water on the AFM. Force profile has been evaluated as a function of extension. In an initial stage of the extension, similar to5%, the force was found to follow the Hooke's law up to about 140 pN. At the extension of about 10%, however, the restoring force diminishes, indicating the rupture of alpha-helix. Then, of alpha-helix region and beta-strand region separate from each other in one molecule. Further extension causes stepwise breakages of hydrogen bonds to give the wholly beta-stranded polyalanine. (C) 2002 Elsevier Science B.V. All rights reserved.