1. Departments of Physics and Pathology, University of British Columbia, Vancouver, British Columbia , Canada V6T 1Z1
2. Physikdepartment der Technischen Universitt Mnchen, 85748 Garching, Germany
3. Physico-Chemie de l'Institut Curie , 75231 Paris, France
4. Biomedical Engineering, Boston University , Boston, Massachusetts 02215, USA

Correspondence to: E. Evans^1,4 Correspondence and request for materials should be addressed to E.E. (e-mail: Email: evans@physics.ubc.ca).

Abstract

Atomic force microscopy (AFM)¹, ² has been used to measure the strength of bonds between biological receptor molecules and their ligands^{3, 4, 5, 6}. But for weak noncovalent bonds, a dynamic spectrum of bond strengths is predicted as the loading rate is altered, with the measured strength being governed by the prominent barriers traversed in the energy landscape along the force-driven bond-dissociation pathway⁷. In other words, the pioneering early AFM measurements represent only a single point in a continuous spectrum of bond strengths, because theory predicts that these will depend on the rate at which the load is applied. Here we report the strength spectra for the bonds between streptavidin (oravidin) and biotin⁸—the prototype of receptor–ligand interactions used in earlier AFM studies^{3, 4, 5}, and which have been modelled by molecular dynamics⁹, ¹⁰. We have probed bond formation over six orders of magnitude in loading rate, and find that the bond survival time diminished from about 1 min to 0.001 s with increasing loading rate over this range. The bond strength, meanwhile, increased from about 5 pN to 170 pN. Thus, although they are among the strongest noncovalent linkages in biology (affinity of 10¹³ to 10¹⁵  M^-1)⁸, ¹¹, these bonds in fact appear strong or weak depending on how fast they are loaded. We are also able to relate the activation barriers derived from our strength spectra to the shape of the energy landscape derived from simulations of the biotin–avidin complex.