Single-molecule force-clamp experiments reveal kinetics of mechanically activated silyl ester hydrolysis

We have investigated the strength of silyl ester bonds formed between carboxymethylated amylose (CMA) molecules and silane-functionalized silicon oxide surfaces using AFM-based single-molecule force spectroscopy in the force-clamp mode. Single tethered CMA molecules were picked up, and bond lifetimes were determined at constant clamp forces of 0.8, 1.0, and 1.2 nN at seven temperatures between 295 and 320 K at pH 2.0. The results reveal biexponential rupture kinetics. To obtain the reaction rate constants for each force and temperature individually, the results were analyzed with a biexponential kinetic model using the maximum likelihood estimation (MLE) method. The force-independent kinetic and structural parameters of the underlying bond rupture mechanisms were extracted by fitting the entire data set with a parallel MLE fit procedure using the Zhurkov/Bell model and, alternatively, an Arrhenius kinetics model combined with a Morse potential as an analytic representation of the binding potential. With activation energies between 37 and 40 kJ mol(-1), and with Arrhenius prefactors between 5 × 10(4) and 2 × 10(6) s(-1), the results point to the hydrolysis of the silyl ester bond.