Thomas Utzig

Deciphering the scaling of single-molecule interactions using Jarzynski’s equality

Unravelling the complexity of the macroscopic world relies on understanding the scaling of single-molecule interactions towards integral macroscopic interactions. Here, we demonstrate the scaling of single acid-amine interactions through a synergistic experimental approach combining macroscopic surface forces apparatus experiments and single-molecule force spectroscopy. This experimental framework is ideal for testing the well-renowned Jarzynskis equality, which relates work performed under non-equilibrium conditions with equilibrium free energy. Macroscopic equilibrium measurements scale linearly with the number density of interfacial bonds, providing acid-amine interaction energies of 10.9 ± 0.2 kT. Irrespective of how far from equilibrium single-molecule experiments are performed, the Jarzynskis free energy converges to 11 ± 1 kT. Our results validate the applicability of Jarzynskis equality to unravel the scaling of non-equilibrium single-molecule experiments to scenarios where large numbers of molecules interacts simultaneously in equilibrium. The developed scaling strategy predicts large-scale properties such as adhesion or cell-cell interactions on the basis of single-molecule measurements.

Direct and quantitative AFM measurements of the concentration and temperature dependence of the hydrophobic force law at nanoscopic contacts

By virtue of its importance for self-organization of biological matter the hydrophobic force law and the range of hydrophobic interactions (HI) have been debated extensively over the last 40 years. Here, we directly measure and quantify the hydrophobic force-distance law over large temperature and concentration ranges. In particular, we study the HI between molecularly smooth hydrophobic self-assembled monolayers, and similarly modified gold-coated AFM tips (radii∼8-50 nm). We present quantitative and direct evidence that the hydrophobic force is both long-ranged and exponential down to distances of about 1-2 nm. Therefore, we introduce a self-consistent radius-normalization for atomic force microscopy data. This approach allows quantitative data fitting of AFM-based experimental data to the recently proposed Hydra-model. With a statistical significance of r(2)⩾0.96 our fitting and data directly reveal an exponential HI decay length of 7.2±1.2 Å that is independent of the salt concentration up to 750 mM. As such, electrostatic screening does not have a significant influence on the HI in electrolyte concentrations ranging from 1 mM to 750 mM. In 1 M solutions the observed instability during approach shifts to longer distances, indicating ion correlation/adsorption effects at high salt concentrations. With increasing temperature the magnitude of HI decreases monotonically, while the range increases slightly. We compare our results to the large body of available literature, and shed new light into range and magnitude of hydrophobic interactions at very close distances and over wide temperature and concentration regimes.

Resolving Non-Specific and Specific Adhesive Interactions of Catechols at Solid/Liquid Interfaces at the Molecular Scale.

Abstract The adhesive system of mussels evolved into a powerful and adaptive system with affinity to a wide range of surfaces. It is widely known that thereby 3,4‐dihydroxyphenylalanine (Dopa) plays a central role. However underlying binding energies remain unknown at the single molecular scale. Here, we use single‐molecule force spectroscopy to estimate binding energies of single catechols with a large range of opposing chemical functionalities. Our data demonstrate significant interactions of Dopa with all functionalities, yet most interactions fall within the medium–strong range of 10–20 k B T. Only bidentate binding to TiO2 surfaces exhibits a higher binding energy of 29 k B T. Our data also demonstrate at the single‐molecule level that oxidized Dopa and amines exhibit interaction energies in the range of covalent bonds, confirming the important role of Dopa for cross‐linking in the bulk mussel adhesive. We anticipate that our approach and data will further advance the understanding of biologic and technologic adhesives.

How specific halide adsorption varies hydrophobic interactions

Hydrophobic interactions (HI) are driven by the water structure around hydrophobes in aqueous electrolytes. How water structures at hydrophobic interfaces and how this influences the HI was subject to numerous studies. However, the effect of specific ion adsorption on HI and hydrophobic interfaces remains largely unexplored or controversial. Here, the authors utilized atomic force microscopy force spectroscopy at well-defined nanoscopic hydrophobic interfaces to experimentally address how specific ion adsorption of halide ions as well as NH4 (+), Cs(+), and Na(+) cations alters interaction forces across hydrophobic interfaces. Our data demonstrate that iodide adsorption at hydrophobic interfaces profoundly varies the hydrophobic interaction potential. A long-range and strong hydration repulsion at distances D > 3 nm, is followed by an instability which could be explained by a subsequent rapid ejection of adsorbed iodides from approaching hydrophobic interfaces. In addition, the authors find only a weakly pronounced influence of bromide, and as expected no influence of chloride. Also, all tested cations do not have any significant influence on HI. Complementary, x-ray photoelectron spectroscopy and quartz-crystal-microbalance with dissipation monitoring showed a clear adsorption of large halide ions (Br(-)/I(-)) onto hydrophobic self-assembled monolayers (SAMs). Interestingly, iodide can even lead to a full disintegration of SAMs due to specific and strong interactions of iodide with gold. Our data suggest that hydrophobic surfaces are not intrinsically charged negatively by hydroxide adsorption, as it was generally believed. Hydrophobic surfaces rather interact strongly with negatively charged large halide ions, leading to a surface charging and significant variation of interaction forces.

Adhesive barnacle peptides exhibit a steric-driven design rule to enhance adhesion between asymmetric surfaces

Barnacles exhibit superior underwater adhesion simply through sequencing of the 21 proteinogenic amino acids, without post processing or using special amino acids. Here, we measure and discuss the molecular interaction of two distinct and recurring short peptide sequences (Bp1 and Bp2) inspired from the surface binding 19kDa protein from the barnacle attachment interface. Using self-assembled monolayer (SAMs) of known physical and chemical properties on molecularly smooth gold substrates in 5mM NaCl at pH 7.3, (1) the adsorption mechanisms of the barnacle inspired peptides are explored using quartz crystal microbalance, and (2) adhesion mediating properties are measured using the surface force apparatus. The hydrophobic Bp1 peptide with a cysteine residue adsorbs irreversibly onto Au surfaces due to thiol bond formation, while on hydrophobic CH3 SAM surface, the interactions are hydrophobic in nature. Interestingly, Bp2 that contains both hydrophobic and protonated amine units exhibits asymmetric bridging with an exceptionally high adhesion energy up to 100mJ/m2 between mica and both gold and CH3 SAM. Surprisingly on hydrophilic surfaces such as COOH- or OH-SAMs both peptides fail to show any interactions, implying the necessity of surface charge to promote bridging. Our results provide insights into the molecular aspects of manipulating and utilizing barnacle-mediated peptides to promote or inhibit underwater adhesion.

Targeted Tuning of Interactive Forces by Engineering of Molecular Bonds in Series and Parallel Using Peptide-Based Adhesives.

Polymer-mediated adhesion plays a major role for both technical glues and biological processes like self-assembly or biorecognition. In contrast to engineering systems, adhesive strength in biological systems is precisely tuned via well-adjusted arrangement of individual bonds. How adhesion may be engineered by arrangement of individual bonds is however not yet well-understood. Here we show how the number of bonds in series and parallel can significantly influence adhesion forces using specifically designed surface-bridging peptides. We directly measure how adhesion forces between -COOH and -NH2 functionalized surfaces across aqueous media vary as a function of the number of bonds in parallel. We also introduce surface bridging peptide sequences that are similarly end-functionalized with amines and carboxylic acid. Compared to single molecular junctions, adhesive strength mediated by these surface bridging peptides decreases by a factor of 2 for adhesive junctions that consist of two acid/base bonds in series. Furthermore, adhesive strength varies with the density of bonds in parallel. For dense systems, we observe that the formation of a bridging peptide monolayer is sterically hindered and therefore adhesion is further reduced significantly by 20%. Our results unravel how the arrangement of individual bonds in an adhesive junction allows for a wide tuning of adhesive strength on the basis of utilizing just one single specific bond. As such, for peptide adhesives it is essential to consider bonds in parallel in a wide range of applications where both high adhesion and triggered release of adhesive bonds is essential.

Tether-length dependence of bias in equilibrium free-energy estimates for surface-to-molecule unbinding experiments

The capabilities of atomic force microscopes and optical tweezers to probe unfolding or surface-to-molecule bond rupture at a single-molecular level are widely appreciated. These measurements are typically carried out unidirectionally under nonequilibrium conditions. Jarzynski’s equality has proven useful to relate the work obtained along these nonequilibrium trajectories to the underlying free energy of the unfolding or unbinding process. Here, we quantify biases that arise from the molecular design of the bond rupture experiment for probing surface-to-molecule bonds. In particular, we probe the well-studied amine/gold bond as a function of the linker’s length which is used to anchor the specific amine functionality during a single molecule unbinding experiment. With increasing linker length, we observe a significant increase in the average work spent on polymer stretching and a strongly biased estimated interaction free energy. Our data demonstrate that free energy estimates converge well for linker lengths below 20 nm, where the bias is <10–15%. With longer linkers severe methodical limits of the method are reached, and convergence within a reasonable number of realizations of the bond rupture is not feasible. Our results also provide new insights into stability and work dissipation mechanisms at adhesive interfaces at the single-molecular level, and offer important design and analysis aspects for single-molecular surface-to-molecule experiments.

Phänomen der Adhäsion entschlüsselt

Stabile industrielle Klebund Beschichtungstechnik erfordert ein delikates Zusammenspiel einer Vielzahl physikalischer und chemischer Prozesse. Auch wenn Formulierungstechnik heute auf einem großen Erfahrungsschatz beruht, werden die zugrundeliegenden molekularen Wechselwirkungen bisher kaum verstanden. Durch den rasenden Fortschritt in den nanoskalierten und molekularen Grundlagenwissenschaften ist es nun erstmals möglich, ein Konzept zu formulieren, welches mittelfristig auch ein molekulares Design von Klebstoffen und Haftung ermöglicht.

Phänomen der Adhäsion entschlüsselt: Klebstoffentwicklung aus dem molekularen Baukasten

Stabile industrielle Klebund Beschichtungstechnik erfordert ein delikates Zusammenspiel einer Vielzahl physikalischer und chemischer Prozesse. Auch wenn Formulierungstechnik heute auf einem großen Erfahrungsschatz beruht, werden die zugrundeliegenden molekularen Wechselwirkungen bisher kaum verstanden. Durch den rasenden Fortschritt in den nanoskalierten und molekularen Grundlagenwissenschaften ist es nun erstmals möglich, ein Konzept zu formulieren, welches mittelfristig auch ein molekulares Design von Klebstoffen und Haftung ermöglicht.

Klebstoffentwicklung aus dem molekularen Baukasten - Phänomen der Adhäsion entschlüsselt

Stabile industrielle Klebund Beschichtungstechnik erfordert ein delikates Zusammenspiel einer Vielzahl physikalischer und chemischer Prozesse. Auch wenn Formulierungstechnik heute auf einem großen Erfahrungsschatz beruht, werden die zugrundeliegenden molekularen Wechselwirkungen bisher kaum verstanden. Durch den rasenden Fortschritt in den nanoskalierten und molekularen Grundlagenwissenschaften ist es nun erstmals möglich, ein Konzept zu formulieren, welches mittelfristig auch ein molekulares Design von Klebstoffen und Haftung ermöglicht.