Proteins unfold by applying an external force, although the microscopic mechanism is still not well understood. In this work, we use steered molecular dynamics to probe fundamental aspects of the stretching transition of alpha-helices, in particular how proline kinks and side chain dynamics would influence their ability to resist the applied force. We find that proline residues effectively 'cut' a helix in half when introduced on stable homopolymers, whereas their effect is smaller when present in helices that are more easily deformed. Our findings provide insight into the factors that may regulate the mechanical stretching of realistic protein domains. (C) 2004 Elsevier B.V. All rights reserved.