We develop a physical model to describe the kinetic behavior in cell-adhesion molecules. Unbinding of non-covalent biological bonds is decomposed into entropic and energetic controlled debonding. Such a treatment on debonding processes is a space decomposition of bond breaking events. Entropy controlled dissociation under thermal fluctuation is non-directional in a 3-dimensional space, and its energy barrier to escape may be not influenced by a tensile force but the microstates which can lead to dissociation are changed by the tensile force; An applied force effectively lowers the energy barrier to escape along the force direction. Such energetic effect will accelerate dissociate mainly along directions parallel to the loading direction. The lifetime of the biological bond, due to the superimposition of two concurrent off-rates, may grow with increasing tensile force to moderate amount and decrease with further increasing load, as debonding events dominated by entropy transit to those controlled by an applied force. We hypothesize that a catch-to-slip bond transition is a generic feature in biological bonds. The model also predicts that catch bonds in compliant molecular structure have longer lifetimes and may be activated at lower forces [1].
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