How Do We Know when Single-Molecule Force Spectroscopy Really Tests Single Bonds?

Unique stick-slip crack dynamics of double-network hydrogels under pure-shear loading

In this work, we have found that a prenotched double-network (DN) hydrogel, when subjected to tensile loading in a pure-shear geometry, exhibits intriguing stick-slip crack dynamics. These dynamics synchronize with the oscillation of the damage (yielding) zone at the crack tip. Through manipulation of the loading rate and the predamage level of the brittle network in DN gels, we have clarified that this phenomenon stems from the significant amount of energy dissipation required to form the damage zone at the crack tip, as well as a kinetic contrast between the rapid crack extension through the yielding zone (slip process) and the slow formation of a new yielding zone controlled by the external loading rate (stick process).

FimH-mannose noncovalent bonds survive minutes to hours under force

The adhesin FimH is expressed by commensal Escherichia coli and is implicated in urinary tract infections, where it mediates adhesion to mannosylated glycoproteins on urinary and intestinal epithelial cells in the presence of a high-shear fluid environment. The FimH-mannose bond exhibits catch behavior in which bond lifetime increases with force, because tensile force induces a transition in FimH from a compact native to an elongated activated conformation with a higher affinity to mannose. However, the lifetime of the activated state of FimH has not been measured under force. Here we apply multiplexed magnetic tweezers to apply a preload force to activate FimH bonds with yeast mannan, then we measure the lifetime of these activated bonds under a wide range of forces above and below the preload force. A higher fraction of FimH-mannan bonds were activated above than below a critical preload force, confirming the FimH catch bond behavior. Once activated, FimH detached from mannose with multi-state kinetics, suggesting the existence of two bound states with a 20-fold difference in dissociation rates. The average lifetime of activated FimH-mannose bonds was 1000 to 10,000 s at forces of 30-70 pN. Structural explanations of the two bound states and the high force resistance provide insights into structural mechanisms for long-lived, force-resistant biomolecular interactions.

Use of DNA forceps to measure receptor-ligand dissociation equilibrium constants in a single-molecule competition assay

The ability of biophysicists to decipher the behavior of individual biomolecules has steadily improved over the past thirty years. However, it still remains unclear how an ensemble of data acquired at the single-molecule level compares with the data acquired on an ensemble of the same molecules. We here propose an assay to tackle this question in the context of dissociation equilibrium constant measurements. A sensor is built by engrafting a receptor and a ligand onto a flexible dsDNA scaffold and mounting this assembly on magnetic tweezers. This way, looking at the position of the magnetic bead enables one to determine in real-time if the two molecular partners are associated or not. Next, to quantify the affinity of the scrutinized single-receptor for a given competitor, various amounts of the latter molecule are introduced in solution and the equilibrium response of the sensor is monitored throughout the titration protocol. Proofs of concept are established for the binding of three rapamycin analogs to the FKBP12 cis-trans prolyl isomerase. For each of these drugs the mean affinity constant obtained on a ten of individual receptors agrees with the one previously determined in a bulk assay. Furthermore, experimental contingencies are sufficient to explain the dispersion observed over the single-molecule values.

Tension-tuned receptors for synthetic mechanotransduction and intercellular force detection

Cells interpret mechanical stimuli from their environments and neighbors, but the ability to engineer customized mechanosensing capabilities has remained a synthetic and mechanobiology challenge. Here we introduce tension-tuned synthetic Notch (SynNotch) receptors to convert extracellular and intercellular forces into specifiable gene expression changes. By elevating the tension requirements of SynNotch activation, in combination with structure-guided mutagenesis, we designed a set of receptors with mechanical sensitivities spanning the physiologically relevant picoNewton range. Cells expressing these receptors can distinguish between varying tensile forces and respond by enacting customizable transcriptional programs. We applied these tools to design a decision-making circuit, through which fibroblasts differentiate into myoblasts upon stimulation with distinct tension magnitudes. We also characterize cell-generated forces transmitted between cells during Notch signaling. Overall, this work provides insight into how mechanically induced changes in protein structure can be used to transduce physical forces into biochemical signals. The system should facilitate the further programming and dissection of force-related phenomena in biological systems.

Combining DNA scaffolds and acoustic force spectroscopy to characterize individual protein bonds

Single-molecule data are of great significance in biology, chemistry, and medicine. However, new experimental tools to characterize, in a multiplexed manner, protein bond rupture under force are still needed. Acoustic force spectroscopy is an emerging manipulation technique which generates acoustic waves to apply force in parallel on multiple microbeads tethered to a surface. We here exploit this configuration in combination with the recently developed modular junctured-DNA scaffold that has been designed to study protein-protein interactions at the single-molecule level. By applying repetitive constant force steps on the FKBP12-rapamycin-FRB complex, we measure its unbinding kinetics under force at the single-bond level. Special efforts are made in analyzing the data to identify potential pitfalls. We propose a calibration method allowing in situ force determination during the course of the unbinding measurement. We compare our results with well-established techniques, such as magnetic tweezers, to ensure their accuracy. We also apply our strategy to study the force-dependent rupture of a single-domain antibody with its antigen. Overall, we get a good agreement with the published parameters that have been obtained at zero force and population level. Thus, our technique offers single-molecule precision for multiplexed measurements of interactions of biotechnological and medical interest.

Biochemical Interactions through Microscopic Techniques: Structural and Molecular Characterization

Many researchers and scientists have contributed significantly to provide structural and molecular characterizations of biochemical interactions using microscopic techniques in the recent decade, as these biochemical interactions play a crucial role in the production of diverse biomaterials and the organization of biological systems. The properties, activities, and functionalities of the biomaterials and biological systems need to be identified and modified for different purposes in both the material and life sciences. The present study aimed to review the advantages and disadvantages of three main branches of microscopy techniques (optical microscopy, electron microscopy, and scanning probe microscopy) developed for the characterization of these interactions. First, we explain the basic concepts of microscopy and then the breadth of their applicability to different fields of research. This work could be useful for future research works on biochemical self-assembly, biochemical aggregation and localization, biological functionalities, cell viability, live-cell imaging, material stability, and membrane permeability, among others. This understanding is of high importance in rapid, inexpensive, and accurate analysis of biochemical interactions.

Modeling multiple duplex DNA attachments in a force-extension experiment

Optical tweezers-based DNA stretching often relies on tethering a single end-activated DNA molecule between optically manipulated end-binding beads. Measurement success can depend on DNA concentration. At lower DNA concentrations tethering is less common, and many trials may be required to observe a single-molecule stretch. At higher DNA concentrations tethering is more common; however, the resulting force-extensions observed are more complex and may vary from measurement to measurement. Typically these more complex results are attributed to the formation of multiple tethers between the beads; however, to date there does not appear to have been a critical examination of this hypothesis or the potential usefulness of such data. Here we examine stretches at a higher DNA concentration and use analysis and simulation to show how the more complex force-extensions observed can be understood in terms of multiple DNA attachments.

Is There a Need for a More Precise Description of Biomolecule Interactions to Understand Cell Function?

An important goal of biological research is to explain and hopefully predict cell behavior from the molecular properties of cellular components. Accordingly, much work was done to build extensive “omic” datasets and develop theoretical methods, including computer simulation and network analysis to process as quantitatively as possible the parameters contained in these resources. Furthermore, substantial effort was made to standardize data presentation and make experimental results accessible to data scientists. However, the power and complexity of current experimental and theoretical tools make it more and more difficult to assess the capacity of gathered parameters to support optimal progress in our understanding of cell function. The purpose of this review is to focus on biomolecule interactions, the interactome, as a specific and important example, and examine the limitations of the explanatory and predictive power of parameters that are considered as suitable descriptors of molecular interactions. Recent experimental studies on important cell functions, such as adhesion and processing of environmental cues for decision-making, support the suggestion that it should be rewarding to complement standard binding properties such as affinity and kinetic constants, or even force dependence, with less frequently used parameters such as conformational flexibility or size of binding molecules.

Atomic force spectroscopy with magainin 1 functionalized tips and biomimetic supported lipid membranes

Antimicrobial peptides are molecules synthesized by living organisms as the first line of defense against bacteria, fungi, parasites, or viruses. Since their biological activity is based on destabilization of the microbial membranes, a study of direct interaction forces between antimicrobial peptides and biomimetic membranes is very important for understanding the molecular mechanisms of their action. Herein, we use atomic force spectroscopy to probe the interaction between atomic force microscopy (AFM) tips functionalized with magainin 1 and supported lipid bilayers (SLBs) mimicking electrically uncharged membranes of normal eukaryotic cells and negatively charged membranes of bacterial cells. The investigations performed on negatively charged SLBs showed that the magainin 1 functionalized AFM tips are quickly adsorbed into the SLBs when they approach, while they adhere strongly to the lipid membrane when retracted. On contrary, same investigations performed on neutral SLBs showed mechanical resistance of the lipid membrane to the tip breakthrough and negligible adhesion force at detachment.

Monitoring the binding and insertion of a single transmembrane protein by an insertase

Cells employ highly conserved families of insertases and translocases to insert and fold proteins into membranes. How insertases insert and fold membrane proteins is not fully known. To investigate how the bacterial insertase YidC facilitates this process, we here combine single-molecule force spectroscopy and fluorescence spectroscopy approaches, and molecular dynamics simulations. We observe that within 2 ms, the cytoplasmic α-helical hairpin of YidC binds the polypeptide of the membrane protein Pf3 at high conformational variability and kinetic stability. Within 52 ms, YidC strengthens its binding to the substrate and uses the cytoplasmic α-helical hairpin domain and hydrophilic groove to transfer Pf3 to the membrane-inserted, folded state. In this inserted state, Pf3 exposes low conformational variability such as typical for transmembrane α-helical proteins. The presence of YidC homologues in all domains of life gives our mechanistic insight into insertase-mediated membrane protein binding and insertion general relevance for membrane protein biogenesis.

An asymmetric mechanical code ciphers curvature-dependent proprioceptor activity

A repetitive gait cycle is an archetypical component within the behavioral repertoire of many animals including humans. It originates from mechanical feedback within proprioceptors to adjust the motor program during locomotion and thus leads to a periodic orbit in a low-dimensional space. Here, we investigate the mechanics, molecules, and neurons responsible for proprioception in Caenorhabditis elegans to gain insight into how mechanosensation shapes the orbital trajectory to a well-defined limit cycle. We used genome editing, force spectroscopy, and multiscale modeling and found that alternating tension and compression with the spectrin network of a single proprioceptor encodes body posture and informs TRP-4/NOMPC and TWK-16/TREK2 homologs of mechanosensitive ion channels during locomotion. In contrast to a widely accepted model of proprioceptive “stretch” reception, we found that proprioceptors activated locally under compressive stresses in-vivo and in-vitro and propose that this property leads to compartmentalized activity within long axons delimited by curvature-dependent mechanical stresses.

Details of Single-Molecule Force Spectroscopy Data Decoded by a Network-Based Automatic Clustering Algorithm

Atomic force microscopy-single-molecule force spectroscopy (AFM-SMFS) is a powerful methodology to probe intermolecular and intramolecular interactions in biological systems because of its operability in physiological conditions, facile and rapid sample preparation, versatile molecular manipulation, and combined functionality with high-resolution imaging. Since a huge number of AFM-SMFS force-distance curves are collected to avoid human bias and errors and to save time, numerous algorithms have been developed to analyze the AFM-SMFS curves. Nevertheless, there is still a need to develop new algorithms for the analysis of AFM-SMFS data since the current algorithms cannot specify an unbinding force to a corresponding/each binding site due to the lack of networking functionality to model the relationship between the unbinding forces. To address this challenge, herein, we develop an unsupervised method, i.e., a network-based automatic clustering algorithm (NASA), to decode the details of specific molecules, e.g., the unbinding force of each binding site, given the input of AFM-SMFS curves. Using the interaction of heparan sulfate (HS)-antithrombin (AT) on different endothelial cell surfaces as a model system, we demonstrate that NASA is able to automatically detect the peak and calculate the unbinding force. More importantly, NASA successfully identifies three unbinding force clusters, which could belong to three different binding sites, for both Ext1^f/f and Ndst1^f/f cell lines. NASA has great potential to be applied either readily or slightly modified to other AFM-based SMFS measurements that result in "saw-tooth"-shaped force-distance curves showing jumps related to the force unbinding, such as antibody-antigen interaction and DNA-protein interaction.

Probing Ligand-Receptor Interaction in Living Cells Using Force Measurements With Optical Tweezers

This work probes the binding kinetics of COOH-terminus of Clostridium perfringens enterotoxin (c-CPE) and claudin expressing MCF-7 cells using force spectroscopy with optical tweezers. c-CPE is of high biomedical interest due to its ability to specifically bind to claudin with high affinity as well as reversibly disrupt tight junctions whilst maintaining cell viability. We observed single-step rupture events between silica particles functionalized with c-CPE and MCF-7 cells. Extensive calibration of the optical tweezers' trap stiffness and displacement of the particle from trap center extracted a probable bond rupture force of ≈ 18 pN. The probability of rupture events with c-CPE functionalized silica particles increased by 50% compared to unfunctionalized particles. Additionally, rupture events were not observed when probing cells not expressing claudin with c-CPE coated particles. Overall, this work demonstrates that optical tweezers are invaluable tools to probe ligand-receptor interactions and their potential to study dynamic molecular events in drug-binding scenarios.

Next Generation Methods for Single-Molecule Force Spectroscopy on Polyproteins and Receptor-Ligand Complexes

Single-molecule force spectroscopy with the atomic force microscope provides molecular level insights into protein function, allowing researchers to reconstruct energy landscapes and understand functional mechanisms in biology. With steadily advancing methods, this technique has greatly accelerated our understanding of force transduction, mechanical deformation, and mechanostability within single- and multi-domain polyproteins, and receptor-ligand complexes. In this focused review, we summarize the state of the art in terms of methodology and highlight recent methodological improvements for AFM-SMFS experiments, including developments in surface chemistry, considerations for protein engineering, as well as theory and algorithms for data analysis. We hope that by condensing and disseminating these methods, they can assist the community in improving data yield, reliability, and throughput and thereby enhance the information that researchers can extract from such experiments. These leading edge methods for AFM-SMFS will serve as a groundwork for researchers cognizant of its current limitations who seek to improve the technique in the future for in-depth studies of molecular biomechanics.

Environment-dependent single-chain mechanics of synthetic polymers and biomacromolecules by atomic force microscopy-based single-molecule force spectroscopy and the implications for advanced polymer materials

"The Tao begets the One. One begets all things of the world." This quote from Tao Te Ching is still inspiring for scientists in chemistry and materials science: The "One" can refer to a single molecule. A macroscopic material is composed of numerous molecules. Although the relationship between the properties of the single molecule and macroscopic material is not well understood yet, it is expected that a deeper understanding of the single-chain mechanics of macromolecules will certainly facilitate the development of materials science. Atomic force microscopy-based single-molecule force spectroscopy (AFM-SMFS) has been exploited extensively as a powerful tool to study the single-chain behaviors of macromolecules. In this review, we summarize the recent advances in the emerging field of environment-dependent single-chain mechanics of synthetic polymers and biomacromolecules by means of AFM-SMFS. First, the single-chain inherent elasticities of several typical linear macromolecules are introduced, which are also confirmed by one of three polymer models with theoretical elasticities of the corresponding macromolecules obtained from quantum mechanical (QM) calculations. Then, the effects of the external environments on the single-chain mechanics of synthetic polymers and biomacromolecules are reviewed. Finally, the impacts of single-chain mechanics of macromolecules on the development of polymer science especially polymer materials are illustrated.

Temperature-Induced Catch-Slip to Slip Bond Transit in Plasmodium falciparum-Infected Erythrocytes

Plasmodium falciparum malaria-infected red blood cells (IRBCs), or erythrocytes, avoid splenic clearance by adhering to host endothelium. Upregulation of endothelial receptors intercellular adhesion molecule-1 (ICAM-1) and cluster of differentiation 36 (CD36) are associated with severe disease pathology. Most in vitro studies of IRBCs interacting with these molecules were conducted at room temperature. However, as IRBCs are exposed to temperature variations between 37°C (body temperature) and 41°C (febrile temperature) in the host, it is important to understand IRBC-receptor interactions at these physiologically relevant temperatures. Here, we probe IRBC interactions against ICAM-1 and CD36 at 37 and 41°C. Single bond force-clamp spectroscopy is used to determine the bond dissociation rates and hence, unravel the nature of the IRBC-receptor interaction. The association rates are also extracted from a multiple bond flow assay using a cellular stochastic model. Surprisingly, IRBC-ICAM-1 bond transits from a catch-slip bond at 37°C toward a slip bond at 41°C. Moreover, binding affinities of both IRBC-ICAM-1 and IRBC-CD36 decrease as the temperature rises from 37 to 41°C. This study highlights the significance of examining receptor-ligand interactions at physiologically relevant temperatures and reveals biophysical insight into the temperature dependence of P. falciparum malaria cytoadherent bonds.

Atomic force microscopy-based single-molecule force spectroscopy detects DNA base mismatches

Atomic force microscopy-based single-molecule-force spectroscopy is limited by low throughput. We introduce addressable DNA origami to study multiple target molecules. Six target DNAs that differed by only a single base-pair mismatch were clearly differentiated a rupture force of only 4 pN.

A modular DNA scaffold to study protein-protein interactions at single-molecule resolution

The residence time of a drug on its target has been suggested as a more pertinent metric of therapeutic efficacy than the traditionally used affinity constant. Here, we introduce junctured-DNA tweezers as a generic platform that enables real-time observation, at the single-molecule level, of biomolecular interactions. This tool corresponds to a double-strand DNA scaffold that can be nanomanipulated and on which proteins of interest can be engrafted thanks to widely used genetic tagging strategies. Thus, junctured-DNA tweezers allow a straightforward and robust access to single-molecule force spectroscopy in drug discovery, and more generally in biophysics. Proof-of-principle experiments are provided for the rapamycin-mediated association between FKBP12 and FRB, a system relevant in both medicine and chemical biology. Individual interactions were monitored under a range of applied forces and temperatures, yielding after analysis the characteristic features of the energy profile along the dissociation landscape.

Removal of a Conserved Disulfide Bond Does Not Compromise Mechanical Stability of a VHH Antibody Complex

Single-domain VHH antibodies are promising reagents for medical therapy. A conserved disulfide bond within the VHH framework region is known to be critical for thermal stability, however, no prior studies have investigated its influence on the stability of VHH antibody-antigen complexes under mechanical load. Here, we used single-molecule force spectroscopy to test the influence of a VHH domain's conserved disulfide bond on the mechanical strength of the interaction with its antigen mCherry. We found that although removal of the disulfide bond through cysteine-to-alanine mutagenesis significantly lowered VHH domain denaturation temperature, it had no significant impact on the mechanical strength of the VHH:mCherry interaction with complex rupture occurring at ∼60 pN at 103-104 pN/sec regardless of disulfide bond state. These results demonstrate that mechanostable binding interactions can be built on molecular scaffolds that may be thermodynamically compromised at equilibrium.

TCR-pMHC kinetics under force in a cell-free system show no intrinsic catch bond, but a minimal encounter duration before binding

The T cell receptor (TCR)-peptide-MHC (pMHC) interaction is the only antigen-specific interaction during T lymphocyte activation. Recent work suggests that formation of catch bonds is characteristic of activating TCR-pMHC interactions. However, whether this binding behavior is an intrinsic feature of the molecular bond, or a consequence of more complex multimolecular or cellular responses, remains unclear. We used a laminar flow chamber to measure, first, 2D TCR-pMHC dissociation kinetics of peptides of various activating potency in a cell-free system in the force range (6 to 15 pN) previously associated with catch-slip transitions and, second, 2D TCR-pMHC association kinetics, for which the method is well suited. We did not observe catch bonds in dissociation, and the off-rate measured in the 6- to 15-pN range correlated well with activation potency, suggesting that formation of catch bonds is not an intrinsic feature of the TCR-pMHC interaction. The association kinetics were better explained by a model with a minimal encounter duration rather than a standard on-rate constant, suggesting that membrane fluidity and dynamics may strongly influence bond formation.

Nanobody-CD16 Catch Bond Reveals NK Cell Mechanosensitivity

Antibodies are key tools in biomedical research and medicine. Their binding properties are classically measured in solution and characterized by an affinity. However, in physiological conditions, antibodies can bridge an immune effector cell and an antigen-presenting cell, implying that mechanical forces may apply to the bonds. For example, in antibody-dependent cell cytotoxicity-a major mode of action of therapeutic monoclonal antibodies-the Fab domains bind the antigens on the target cell, whereas the Fc domain binds to the activating receptor CD16 (also known as FcgRIII) of an immune effector cell, in a quasi-bidimensional environment (2D). Therefore, there is a strong need to investigate antigen/antibody binding under force (2D) to better understand and predict antibody activity in vivo. We used two anti-CD16 nanobodies targeting two different epitopes and laminar flow chamber assay to measure the association and dissociation of single bonds formed between microsphere-bound CD16 antigens and surface-bound anti-CD16 nanobodies (or single-domain antibodies), simulating 2D encounters. The two nanobodies exhibit similar 2D association kinetics, characterized by a strong dependence on the molecular encounter duration. However, their 2D dissociation kinetics strongly differ as a function of applied force: one exhibits a slip bond behavior in which off rate increases with force, and the other exhibits a catch-bond behavior in which off rate decreases with force. This is the first time, to our knowledge, that catch-bond behavior was reported for antigen-antibody bond. Quantification of natural killer cells spreading on surfaces coated with the nanobodies provides a comparison between 2D and three-dimensional adhesion in a cellular context, supporting the hypothesis of natural killer cell mechanosensitivity. Our results may also have strong implications for the design of efficient bispecific antibodies for therapeutic applications.

Heterogeneous and rate-dependent streptavidin-biotin unbinding revealed by high-speed force spectroscopy and atomistic simulations

Receptor-ligand interactions are essential for biological function and their binding strength is commonly explained in terms of static lock-and-key models based on molecular complementarity. However, detailed information on the full unbinding pathway is often lacking due, in part, to the static nature of atomic structures and ensemble averaging inherent to bulk biophysics approaches. Here we combine molecular dynamics and high-speed force spectroscopy on the streptavidin-biotin complex to determine the binding strength and unbinding pathways over the widest dynamic range. Experiment and simulation show excellent agreement at overlapping velocities and provided evidence of the unbinding mechanisms. During unbinding, biotin crosses multiple energy barriers and visits various intermediate states far from the binding pocket, while streptavidin undergoes transient induced fits, all varying with loading rate. This multistate process slows down the transition to the unbound state and favors rebinding, thus explaining the long lifetime of the complex. We provide an atomistic, dynamic picture of the unbinding process, replacing a simple two-state picture with one that involves many routes to the lock and rate-dependent induced-fit motions for intermediates, which might be relevant for other receptor-ligand bonds.

Force Spectroscopy Shows Dynamic Binding of Influenza Hemagglutinin and Neuraminidase to Sialic Acid

The influenza A virus infects target cells through multivalent interactions of its major spike proteins, hemagglutinin (HA) and neuraminidase (NA), with the cellular receptor sialic acid (SA). HA is known to mediate the attachment of the virion to the cell, whereas NA enables the release of newly formed virions by cleaving SA from the cell. Because both proteins target the same receptor but have antagonistic functions, virus infection depends on a properly tuned balance of the kinetics of HA and NA activities for viral entry to and release from the host cell. Here, dynamic single-molecule force spectroscopy, based on scanning force microscopy, was employed to determine these bond-specific kinetics, characterized by the off rate k_off, rupture length x_β and on rate k_on, as well as the related free-energy barrier ΔG and the dissociation constant K_D. Measurements were conducted using surface-immobilized HA and NA of the influenza A virus strain A/California/04/2009 and a novel, to our knowledge, synthetic SA-displaying receptor for functionalization of the force probe. Single-molecule force spectroscopy at force loading rates between 100 and 50,000 pN/s revealed most probable rupture forces of the protein-SA bond in the range of 10-100 pN. Using an extension of the widely applied Bell-Evans formalism by Friddle, De Yoreo, and co-workers, it is shown that HA features a smaller x_β, a larger k_off and a smaller ΔG than NA. Measurements of the binding probability at increasing contact time between the scanning force microscopy force probe and the surface allow an estimation of K_D, which is found to be three times as large for HA than for NA. This suggests a stronger interaction for NA-SA than for HA-SA. The biological implications in regard to virus binding to the host cell and the release of new virions from the host cell are discussed.

Force Spectroscopy and Beyond: Innovations and Opportunities

Life operates at the intersection of chemistry and mechanics. Over the years, we have made remarkable progress in understanding life from a biochemical perspective and the mechanics of life at the single-molecule scale. Yet the full integration of physical and mechanical models into mainstream biology has been impeded by technical and conceptual barriers, including limitations in our ability to 1) easily measure and apply mechanical forces to biological systems, 2) scale these measurements from single-molecule characterization to more complex biomolecular systems, and 3) model and interpret biophysical data in a coherent way across length scales that span single molecules to cells to multicellular organisms. In this manuscript, through a look at historical and recent developments in force spectroscopy techniques and a discussion of a few exemplary open problems in cellular biomechanics, we aim to identify research opportunities that will help us reach our goal of a more complete and integrated understanding of the role of force and mechanics in biological systems.

Modified Pluronic F127 Surface for Bioconjugation and Blocking Nonspecific Adsorption of Microspheres and Biomacromolecules

Many experiments and applications require the chemical coupling of target molecules to surfaces, during which the elimination of nonspecific interactions presents a difficult challenge. We report on a technologically accessible surface passivation and chemical conjugation method based on an NHS end-labeled F127 Pluronic block copolymer (F127-NHS). To quantify interactions between the F127-NHS surface and magnetic microspheres, we developed a simple assay: the microsphere adhesion by gravity, inversion, then counting, or "MAGIC" assay. To improve blocking of microspheres while maintaining the ability to chemically couple additional molecules, we implemented a pH-dependent two-step chemical modification process for amine microspheres. This process achieves an extremely high level of blocking nonspecific interactions (less than 2% nonspecific adhesion) for a variety of microsphere surface charges and chemical functionalities. We also demonstrate the ability to specifically tether magnetic microspheres to an F127-NHS surface, using single DNA molecules. Using the DNA microspheres, we establish the applicability of the surface for force spectroscopy (stable with an applied load >30 pN) via the massively parallel technique of centrifuge force microscopy. Finally, we demonstrate that the surface can be used in fluorescence studies with a fluorogenic peptide cleavage assay, with high levels of blocking achieved for both the fluorogenic peptide and trypsin. These results suggest applications including, but not limited to, single-molecule force spectroscopy and fluorescence, biosensors, medical implants, and anti-biofouling, which could make use of the F127-NHS surface.