Mechanism of Reversible Peptide–Bilayer Attachment: Combined Simulation and Experimental Single-Molecule Study

The binding of peptides and proteins to lipid membrane surfaces is of fundamental importance for many membrane-mediated cellular processes. Using closely matched molecular dynamics simulations and atomic force microscopy experiments, we study the force-induced desorption of single peptide chains from phospholipid bilayers to gain microscopic insight into the mechanism of reversible attachment. This approach allows quantification of desorption forces and decomposition of peptide–membrane interactions into energetic and entropic contributions. In both simulations and experiments, the desorption forces of peptides with charged and polar side chains are much smaller than those for hydrophobic peptides. The adsorption of charged/polar peptides to the membrane surface is disfavored by the energetic components, requires breaking of hydrogen bonds involving the peptides, and is favored only slightly by entropy. By contrast, the stronger adsorption of hydrophobic peptides is favored both by energy and by entropy and the desorption forces increase with increasing side-chain hydrophobicity. Interestingly, the calculated net adsorption free energies per residue correlate with experimental results of single residues, indicating that side-chain free energy contributions are largely additive. This observation can help in the design of peptides with tailored adsorption properties and in the estimation of membrane binding properties of peripheral membrane proteins

Peptide Desorption Kinetics from Single Molecule Force Spectroscopy Studies

We use a combined experimental/theoretical approach to determine the intrinsic monomeric desorption rate <i>k</i>₀ 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 <i>L</i>, Kuhn length <i>a</i>, adsorption free energy λ, and intrinsic monomeric desorption rate <i>k</i>₀. 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 <i>k</i>₀ ≈ 10⁵ Hz points to significant cooperativity in the desorption process of single polymers

Effect of Molecular Architecture on Single Polymer Adhesion

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

Thermoswitchable Nanoparticles Based on Elastin-like Polypeptides

The design of biocompatible particles with defined size on the nanometer scale has proven to be a challenging task in current biomedical research. Here we present an approach toward temperature-responsive nanoparticles by covalently cross-linking micelles based on trimeric constructs of elastin-like polypeptides. These trimers can be triggered to assemble into micelles by heating the solution above a specific transition temperature (<i>T</i>_t) which was shown in previous studies. Here we show that the disassembly of the micelles below the <i>T</i>_t can be prevented by the incorporation of covalent cross-links in the core of the micelles. This facilitates a temperature-triggered swelling and collapsing by around 35% in diameter, as determined by dynamic light scattering. Size distribution was confirmed by fluorescence correlation spectroscopy, atomic force microscopy, and transmission electron microscopy. We show switchable nanoparticles with reversible volume changes in the temperature region between 30 and 40 °C, making these particles promising candidates for switchable drug delivery carriers