Ester Bond: Chemically Labile Yet Mechanically Stable

Due to the dynamic nature of ester linkages, ester-bond-containing materials are well known for their outstanding degradability and stimuli responsiveness. However, whether ester hydrolysis is affected by mechanical forces remains unclear. Here, we develop a single-molecule assay to quantitatively study the force-dependent ester hydrolysis using an engineered circular permutant protein with a caged ester bond as a model. Our single-molecule force spectroscopy results show that the ester hydrolysis rate is surprisingly insensitive to forces, with a ∼7 s dissociation rate that remains almost unchanged in the force range of 80 to 200 pN. Quantum calculations reveal that the ester hydrolysis involves an intermediate state formed by either HO- or OH-bonded tetrahedral intermediates. The measured ester-hydrolysis kinetics at the single-molecule level may primarily correspond to the rupture of these intermediate states. However, the rate-limiting step appears to be the formation of the tetrahedral intermediates, which cannot be quantitatively characterized in our experiments. Nonetheless, based on the quantum calculations, this step is also insensitive to applied forces. Altogether, our study suggests that the ester bond is chemically labile yet mechanically stable, serving as the basis for the design of responsive materials using ester bonds as mechanically inert units.