Adhesion from tethered ligand-receptor bonds with microsecond lifetimes.

According to classical thermodynamics, biological ligand-receptor bonds should have a median lifetime of about 2 ms, and nearly half should have lifetimes of nanoseconds to microseconds. As a result, it is clear that many "weak" bonds are indispensable for cellular adhesion, signaling, and other critical events. However, the forces required to rupture such weak bonds and the adhesion they provide between surfaces are largely unknown because of their propensity to dissociate rapidly from a measuring probe. To measure such weak bond forces quantitatively, we followed nature's example of adhering surfaces with many weak ligand-receptor bonds. Analogously to how multiplicity promotes stronger adhesion between cellular membranes, multiple bonds created significant adhesion between model cellular surfaces. Specifically, we used an automated surface forces apparatus to measure the adhesion between complementary surfaces bearing dense populations of streptavidin receptors and flexible PEG tethers that each anchored a weakly binding ligand (HABA, or 2-(4-hydroxyphenylazo) benzoic acid). We show that this short-lived bond (< 100 mu s) leads to low forces of dissociation and only a small fraction being simultaneously bound. These results are significant because the HABA-streptavidin bond energy (similar to 10.5k(B)T) is similar to the average found in nature (14.7k(B)T)The measurements exemplify how a single ligand-receptor bond may fall apart and rejoin many times before completing a cellular function yet can still exhibit strength in numbers.