Beta-cyclodextrin host-guest complexes probed under thermodynamic equilibrium: thermodynamics and AFM force spectroscopy.

The rupture forces of individual host-guest complexes between beta-cyclodextrin (beta-CD) heptathioether monolayers on Au(111) and several surface-confined guests were measured in aqueous medium by single molecule force spectroscopy using an atomic force microscope. Anilyl, toluidyl, tert-butylphenyl, and adamantylthiols (0.2-1%) were immobilized in mixed monolayers with 2-mercaptoethanol on gold-coated AFM tips. For all guests and for all surface coverages, the force-displacement curves measured between the functionalized tips and monolayers of beta-CD exhibited single, as well as multiple, pull-off events. The histograms of the pull-off forces showed several maxima at equidistant forces, with force quanta characteristic for each guest of 39 +/- 15, 45 +/- 15, 89 +/- 15, and 102 +/- 15 pN, respectively. These force quanta were independent of the loading rate, indicating that, because of the fast complexation/decomplexation kinetics, the rupture forces were probed under thermodynamic equilibrium. The force values followed the same trend as the free binding energy Delta G degrees measured for model guest compounds in solution or on beta-CD monolayers, as determined by microcalorimetry and surface plasmon resonance measurements, respectively. A descriptive model was developed to correlate quantitatively the pull-off force values with the Delta G degrees of the complexes, based on the evaluation of the energy potential landscape of tip-surface interaction.