investigating-single-molecule-adhesion-by-atomic-force-spectroscopy

Investigating Single Molecule Adhesion by Atomic Force Spectroscopy

Atomic force spectroscopy is an ideal tool to study molecules at surfaces and interfaces. An experimental protocol to couple a large variety of single ...

Technische Universität München

Atomic force microscopy (AFM) based single polymer force spectroscopy allows to detect the interaction (energy) between single polymers and interfaces in aqueous environment. We use this method to delineate the effect of ions, pH, co-solutes and temperature on the adhesion of biopolymers onto solid substrates.

Measuring the interaction between ions, biopolymers and interfaces--one polymer at a time.

We use a combined experimental/theoretical approach to determine the intrinsic monomeric desorption rate k0 of polytyrosine and polylysine homopeptides from flat surfaces. To this end, single polypeptide molecules are covalently attached to an AFM cantilever tip and desorbed from hydrophobic self-assembled monolayers in two complementary experimental protocols. In the constant-pulling-velocity protocol, the cantilever is moved at finite velocity away from the surface and the distance at which the constant plateau force regime ends and the polymer detaches is recorded. In the waiting-time protocol, the cantilever is held at a fixed distance above the surface and the time until the polymer detaches is recorded. The desorption plateau force is varied between 10 and 90 pN, by systematically changing the aqueous solvent quality via the addition of ethanol or salt. A simultaneous fit of the experimental data from both protocols with simple two-state kinetic polymer theory allows to unambiguously disentangle and determine the model parameters corresponding to polymer contour length L, Kuhn length a, adsorption free energy λ, and intrinsic monomeric desorption rate k0. Crucial to our analysis is that a statistically significant number of single-polymer desorption experiments are done with one and the same single polymer molecule for different solvent qualities. The surprisingly low value of about k0 ≈ 10(5) Hz points to significant cooperativity in the desorption process of single polymers.

Peptide desorption kinetics from single molecule force spectroscopy studies.

Several applications require strong noncovalent adhesion of polymers to substrates. Graft and branched polymers have proven superior to linear polymers, but the molecular mechanism is still unclear. Here, this question is addressed on the single molecule level with an atomic force microscopy (AFM) based method. It is determined how the presence of side chains and their molecular architecture influence the adhesion and the mobility of polymers on solid substrates. Surprisingly, the adhesion of mobile polymers cannot significantly be improved by side chains or their architecture. Only for immobile polymers a significantly higher maximum rupture force for graft, bottle-brush, and branched polymers compared to linear chains is measured. Our results suggest that a combination of polymer architecture and strong molecular bonds is necessary to increase the polymer-surface contact area. An increased contact area together with intrachain cohesion (e.g., by entanglements) leads to improved polymer adhesion. These findings may prove useful for the design of stable polymer coatings.

Effect of molecular architecture on single polymer adhesion.

A set of new catecholic monolayer coatings was developed to improve the antifouling performance of TiO2 surfaces. To solve the problem of the weak charge-transfer interaction between a single catechol anchor and TiO2, multiple catechol groups were combined with hyperbranched polyglycerol (hPG) which is a distinct dendritic scaffold that exposes its multivalent anchor groups on the surface. Thus, multivalent catecholic hPGs can be easily prepared for surface modification. The immobilization of the compounds was monitored by quartz crystal microbalance with dissipation monitoring. Surface properties of the coatings were analyzed by water contact angle, X-ray photoelectron spectroscopy, and atomic force microscopy. The antifouling ability and stability were investigated by protein adsorption and cell adhesion. By increasing the number of catechol groups on the hPG scaffold, the stability and surface coverage could be significantly enhanced. Moreover, the inner-layer crosslinking of the coatings by grafting and initiating vinyl groups clearly improved their long-term stability. As a result, hPG with a catecholic functional degree of 10% (hPG-Cat10) and hPG with both catecholic and vinylic functional degree of 5% (hPG-Cat5-V5) were identified as the best catecholic hPGs to prepare bioinert and stable monolayer coatings on TiO2.

Article	Year
Measuring the interaction between ions, biopolymers and interfaces--one polymer at a time. Faraday discussions\| PubMed ID: 23795508	2013
Peptide desorption kinetics from single molecule force spectroscopy studies. Journal of the American Chemical Society\| PubMed ID: 24328353	2014
Effect of molecular architecture on single polymer adhesion. Langmuir : the ACS journal of surfaces and colloids\| PubMed ID: 24679005	2014
Multivalent anchored and crosslinked hyperbranched polyglycerol monolayers as antifouling coating for titanium oxide surfaces. Colloids and surfaces. B, Biointerfaces\| PubMed ID: 25189471	2014

Stefanie Krysiak

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