Force dependence of E-selectin/sialyl Lewisx single bond dissociation

When a range of force loading rates, r_f, is applied to single receptor-ligand bonds, a plot of the peak in the observed force distribution vs ln r_f yields the location of transition states in the bonding potential as well as estimates of the reverse reaction rate under zero load (Merkel et al., Nature, 397:50-3, 1999). This analysis was applied to the molecules that mediate rolling adhesion of leukocytes in blood vessels by adsorbing E-selectin onto 3 μm-diam. glass fibers and coupling the selectin ligand, sialyl Lewis ^x (sLe^x) to latex microspheres. After gentle apposition of sphere and fiber by micromanipulation, the microsphere was retracted using a protocol that combines hydrodynamic and Hookean forces to apply a constant r_f. The [sLe^x] is titrated so adhesion occurs in only ~25% of tests, implying >80% single bonds (simulations show that a few multiple bonds have little effect on parameter estimation). The peak in the force distribution was determined from video microscopy for a variety of r_f. The parameters agree with those predicted to produce rolling in Monte Carlo simulations of adhesion in flow