Designing processive catalytic systems. Threading polymers through a flexible macrocycle ring.

Abstract

The translocation of polymers through pores is widely observed in nature and studying their mechanism may help understand the fundamental features of these processes. We describe here the mechanism of threading of a series of polymers through a flexible macrocyclic ring. Detailed kinetic studies show that the translocation speed is slower than the translocation speed through previously described more rigid macrocycles, most likely as a result of the wrapping of the macrocycle around the polymer chain. Temperature-dependent studies reveal that the threading rate increases on decreasing the temperature, resulting in a negative activation enthalpy of threading. The latter is related to the opening of the cavity of the macrocycle at lower temperatures, which facilitates binding. The translocation process along the polymer chain, on the other hand, is enthalpically unfavorable, which can be ascribed to the release of the tight binding of the macrocycle to the chain upon translocation. The combined kinetic and thermodynamic data are analyzed with our previously proposed consecutive-hopping model of threading. Our findings provide valuable insight into the translocation mechanism of macrocycles on polymers, which is of interest for the development of processive catalysts, i.e., catalysts that thread onto polymers and move along it while performing a catalytic action.