Intermolecular Interactions of Polymer Molecules Determined by Single-Molecule Force Spectroscopy

Abstract

Force spectroscopy on the single-molecule level allows the investigation of intramolecular as well as interdomain and intermolecular interactions of both synthetic and natural polymers. In these experiments, a single molecule is usually stretched between two strong attachment points. The force then increases with increasing extension. Under certain conditions, however, force-extension curves result in force plateaus; an increase in extension is observed under constant force. These plateaus do not depend on pulling speed and therefore indicate transitions at equilibrium. Here we present single-molecule data where carboxymethycellulose (CMC) molecules are pulled out of a polymer film into a poor solvent. The resulting force-extension curves show not only one but up to seven force plateaus. The step height from one force plateau to the next is nonuniform but follows a characteristic spacing. This is the first time that nonuniform force plateaus are reported for polymer molecules. A simple model for the polymerpolymer and polymer-solvent interactions explains the measured data. In this model the new solventpolymer interface gain during the pull is minimized by the formation of a tightly packed polymer bundle. By introducing a parameter for the product of effective radius of the polymer multiplied by surface tension, as well as a second parameter describing the interaction between polymer strands in the bundle, one finds excellent agreement between predicted and measured plateau heights. This model, therefore, provides a basis to investigate the interactions of polymer chains and the influence of solvent in well-defined geometries using single-molecule force spectroscopy.

Signature of hydrophobic hydration in a single polymer

“…It has been predicted theoretically and shown in simulations that forced hydration of a collapsed hydrophobic polymer produces a constant force-distance profile. (29-31) Force-extension curves of single hydrophobic polymers such as polystyrene (PS), poly(4-tert-butylstyrene) (PtBS), and poly(4-vinylbiphenyl) (PVBP) show characteristic constant force plateaus corresponding to single-chain hydration (10,(32)(33)(34), followed by entropic elastic stretching before the chain detaches from the AFM cantilever or the Si substrate (Fig. 2B).…”

Experimental and Theoretical Investigations on the Catalytic Hydrosilylation of Carbon Dioxide with Ruthenium Nitrile Complexes

“…Finally, we speculate that for many systems in aqueous environment (such as those investigated here) the adsorbed amount of polymer as well as the adhesion of coatings is only marginally determined by the adsorption strength of a single polymer but much more by cohesion effects. Previous SMFS studies showed that the adsorption strength of polymers is roughly proportional to the number of chains that are desorbed in parallel 52. Hence, the cohesion of a couple of polymers, which have to be desorbed in parallel, will already increase the adhesion strength significantly.…”

Correlation between Desorption Force Measured by Atomic Force Microscopy and Adsorption Free Energy Measured by Surface Plasmon Resonance Spectroscopy for Peptide−Surface Interactions

“…A solution to the first difficulty is provided by linking the interacting molecules to the probe tip of an AFM tip or the substrate’s surface with long, flexible, hydrophilic molecular tethers that can provide a defined tip–sample distance to spatially isolate the nonspecific probe–sample interactions from the peptide–surface interaction. 14–19 In addition, if the tether length is longer than the peptides that are tethered, the tether will remain flexible during the adsorption process when the probe tip is brought closer than the extended length of the tether to the surface, thus allowing the peptide to adsorb in a manner that is minimally influenced by the tether 20–21. While the use of a tether provides a means of effectively separating the influence of the probe tip from the peptide-surface interactions, a method of dealing with the uncertainty in the number of tethered peptides per unit area of the probe tip remains to be found.…”