Mechanically induced silyl ester cleavage under acidic conditions investigated by AFM-based single-molecule force spectroscopy in the force-ramp mode

AFM-based dynamic single-molecule force spectroscopy was used to
stretch carboxymethylated amylose (CMA) polymers, which have been
covalently tethered between a silanized glass substrate and a
silanized AFM tip via acid-catalyzed ester condensation at pH 2.0.
Rupture forces were measured as a function of temperature and force
loading rate in the force-ramp mode. The data exhibit significant
statistical scattering, which is fitted with a maximum likelihood
estimation (MLE) algorithm. Bond rupture is described with a Morse
potential based Arrhenius kinetics model. The fit yields a bond
dissociation energy De = 35 kJ mol−1 and an Arrhenius pre-factor A
= 6.6 × 104 s−1. The bond dissociation energy is consistent with
previous experiments under identical conditions, where the
force-clamp mode was employed. However, the bi-exponential decay
kinetics, which the force-clamp results unambiguously revealed, are
not evident in the force-ramp data. While it is possible to fit the
force-ramp data with a bi-exponential model, the fit parameters
differ from the force-clamp experiments. Overall, single-molecule
force spectroscopy in the force-ramp mode yields data whose
information content is more limited than force-clamp data. It may,
however, still be necessary and advantageous to perform force-ramp
experiments. The number of successful events is often higher in the
force-ramp mode, and competing reaction pathways may make
force-clamp experiments impossible.