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Abstract
Forced detachment of a single polymer chain, strongly adsorbed on a solid substrate, is investigated by two complementary methods: a coarse-grained analytical dynamical model, based on the Onsager stochastic equation, and Molecular Dynamics (MD) simulations with a Langevin thermostat. The suggested approach makes it possible to go beyond the limitations of the conventional Bell-Evans model. We observe a series of characteristic force spikes when the pulling force is measured against the cantilever displacement during detachment at constant velocity vc (displacement control mode) and find that the average magnitude of this force increases as vc increases. The probability distributions of the pulling force and the end-monomer distance from the surface at the moment of the final detachment are investigated for different adsorption energies ε and pulling velocities vc. Our extensive MD simulations validate and support the main theoretical findings. Moreover, the simulations reveal a novel behavior: for a strong-friction and massive cantilever the force spike pattern is smeared out at large vc. As a challenging task for experimental bio-polymer sequencing in future we suggest the fabrication of a stiff, super-light, nanometer-sized AFM probe.
Highlights
In recent years single-molecule pulling techniques based on the use of laser optical tweezers (LOTs) or an atomic force microscope (AFM) have gained prominence as a versatile tool in the studies of non-covalent bonds and self-associating bio-molecular systems.[1,2,3,4,5,6,7,8,9]
We suggested a theory of the force-induced polymer desorption in the isotensional[27,28] and isometric[29] equilibrium ensembles supported by extensive Monte Carlo (MC) simulations
We need to de ne the adsorption–desorption potential pro le Fads(n). This plays the role of the potential of mean force (PMF) which depends on n
Summary
The method of dynamic force spectroscopy (DFS) is used to probe the force–extension relationship, rupture force distribution, and the force vs. loading rate dependence for singlemolecule bonds or for more complicated multiply bonded attachments. As a result of the master equation solution, the authors obtained a probability distribution of detachment heights (i.e., distances between the cantilever tip and the substrate) as well as an average detachment height as a function of the pulling velocity. Irrespective of all these efforts, a detailed theoretical interpretation of the dynamic force spectroscopy experiments is still missing. The corresponding free-energy-based stochastic equations (known as Onsager equations26) are derived and solved numerically This solution makes it possible to provide force–displacement diagrams and the ensuing dependence on the cantilever displacement velocity vc and the detachment force probability distribution function (PDF).
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