Stiffness variations during the conformational change of a single alpha -helix. polylysine peptide molecule were measured in a liquid environment using atomic force microscopy (AFM) with magnetic cantilever modulation. At the initial stage of the stretching process the stiffness decreased due to the breaking of hydrogen bonds and then increased due to the stretching of the helix backbone. These changes were reversible on reversal of the stretching motion. Below pK, the stiffness did not show increase on reversal, indicating that the reforming of hydrogen bonds did not take place. Conformational changes in the molecule were examined via these changes in stiffness.