  We analyze the response of molecular adhesion complexes to increasing pulling
forces (dynamic force spectroscopy) when dissociation can occur along either
one of two alternative trajectories in the underlying multidimensional energy
landscape. A great diversity of behaviors (e.g. non-monotonicity) is found for
the unbinding force and time as a function of the rate at which the pulling
force is increased. We highlight an intrinsic difficulty in unambiguously
determining the features of the energy landscape from single-molecule pulling
experiments. We also suggest a class of ``harpoon'' stickers that bind easily
but resist strong pulling efficiently.
