Journal of the American Chemical Society, Vol.132, No.2, 773-776, 2010
Gated Molecular Recognition and Dynamic Discrimination of Guests
Some highly efficient enzymes, e.g., acetylcholinesterase, use gating as a tool for controlling the rate by which substrates access their active site to direct product formation. Mastering gated molecular encapsulation could therefore be important for manipulating reactivity in artificial environments, albeit quantitative relationships that describe these processes are unknown. In this work, we examined the interdependence between the thermodynamics (Delta G degrees) and the kinetics (Delta G(in)(double dagger) and Delta G(out)(double dagger)) of encapsulation as mediated by gated molecular basket 1. For a series of isosteric guests (2-6, 106-107 angstrom(3)) entering/exiting 1, we found a linear correlation between the host-guest affinities (Delta G degrees) and the free energies of the activation (Delta G(in)(double dagger) and Delta G(out)(double dagger)), which was fit to the following equation: Delta G(double dagger) = rho Delta G degrees+ delta. Markedly, the kinetics for the entrapment of smaller guest 7 (93 angstrom(3)) and bigger guest 8 (121 angstrom(3)) did not follow the free energy trends observed for 2-6. Thus, it appears that the kinetics of the gated encapsulation mediated by 1 is a function of the encapsulation's favorability (Delta G degrees) and the guest's profile. When the size/shape of guests is kept constant, a linear dependence between the encapsulation potential (Delta G degrees) and the rate of guests' entering/departing basket (Delta G(in/out)(double dagger)) holds. However, when the potential (Delta G degrees) is fixed, the basket discriminates guests on the basis of their size/shape via dynamic modulation of the binding site's access.