In the absence of ligand, the normalized force versus distance curves obtained with the surface force apparatus were reversible and exhibited intersurface repulsion due to steric interactions between the protein and bare lipid monolayers ( Figure S1 ). The thickness of the unbound protein D T was determined from the distance at which the repulsive force exceeded the measurement standard deviation of (0.03 mN/m. Thicknesses of 31.5 ( 0.5, 27.6 ( 0.5, and 22.4 ( 0.5 nm were thus obtained for the 7-, 6-, and 5-repeat DC-SIGNR monolayers, respectively.