We investigated the role of receptor binding affinity in surface adhesion. A sensitive technique was developed to measure the surface energy of receptor-mediated adhesion. The experimental system involved a functionalized elastic agarose bead resting on a functionalized glass coverslip. Attractive intersurface forces pulled the two surfaces together, deforming the bead to produce an enlarged contact area. The Johnson-Kendall-Roberts (JKR) model was used to relate the surface energy of the interaction to the elasticity of the bead and the area of contact. The surface energies for different combinations of modified surfaces in solution were obtained from reflection interference contrast microscopy (RICM) measurements of the contact area formed by the bead and the coverslip. Studies with surfaces functionalized with ligand-receptor pairs showed that the relationship between surface energy and the association constant of the ligand binding has two regimes. At low binding affinity, surface energy increased linearly with the association constant, while surface energy increased logarithmically with the association constant in the high affinity regime.
The atomic force microscope (AFM) is capable of measuring the interaction between tip and sample with high sensitivity and unparalleled spatial resolution. The chemical functionalization of the AFM tips has expanded the versatility of the AFM to experiments where specific molecular interactions are measured. We present here measurements of the interaction between complementary strands of DNA. A necessary prerequisite for the quantitative analysis of the interaction force is knowledge of the spring constant of the cantilevers. We report a method that allows for the in situ measurement of the absolute value of the spring constant of cantilevers based on spectral analysis of the thermal excitations of the cantilever.
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