The recognition mechanisms and dissociation pathways of the avidin-bio
tin complex and of actin monomers in actin filaments were investigated
. The unbinding forces of discrete complexes of avidin or streptavidin
with biotin analogs are proportional to the enthalpy change of the co
mplex formation but independent of changes in the free energy. This re
sult indicates that the unbinding process is adiabatic and that entrop
ic changes occur after unbinding. On the basis of the measured forces
and binding energies, an effective rupture length of 9.5 +/- 1 angstro
ms was calculated for all biotin-avidin pairs and approximately 1 to 3
angstroms for the actin monomer-monomer interaction. A model for the
correlation among binding forces, intermolecular potential, and molecu
lar function is proposed.