Molecular Level Forces Research Articles

Recent developments in DNA-based mechanical nanodevices.

Cellular processes and functions can be regulated by mechanical forces. Nanodevices that can measure and manipulate these forces are critical tools in chemical and cellular biology. Synthetic DNA oligonucleotides have been used to develop a wide range of powerful nanodevices due to their programmable nature and precise and predictable self-assembly. In recent years, various types of DNA-based mechanical nanodevices have been engineered for studying molecular-level forces. With the help of these nanodevices, our understanding of cellular responses to physical forces has been significantly advanced. In this article, we have reviewed some recent developments in DNA-based mechanical sensors and regulators for application in the characterization of cellular biomechanics and the manipulation of cellular morphology, motion and other functions. The design principles discussed in this article can be further used to inspire other types of powerful DNA-based mechanical nanodevices.

Double-stranded DNA force sensors to study the molecular level forces required to activate signaling pathways

Single-molecule force measurement techniques provide unique opportunities to investigate simple molecular interactions and complicated force-induced cell responses. This has revolutionized our understanding of how mechanical cues influence cell behaviors. Recent advances in DNA technologies have led to the development of tension sensors for high-throughput screening of pN scale forces with single-molecule precision. Thus, the emerging DNA-based mechano-sensors are replacing conventional single-molecule force spectroscopes. This review provides an overview of double-stranded DNA-based mechano-sensors, such as the tension gauge tether (TGT) and its derivatives, and their biophysical applications. First, we review the theoretical background of the force-induced rupture of DNA duplexes and force sensing mechanisms using DNA structures. Next, we survey the use of double-stranded DNA-based force sensors to study the role of mechanical forces, mostly receptor tensions, in signaling pathways.

Real-time synchrotron-based X-ray computed microtomography during in situ emulsification

Molecular-level forces experienced by a surfactant during emulsification can have a significant impact on emulsion structure and effective properties. Understanding the interplay of molecular-level forces and effective phase properties during in situ emulsification is a fundamental question relevant to various subsurface engineering applications. Herein, we use dynamic synchrotron-based X-ray microtomography to capture flow dynamics during an oil emulsification process, whereby brine salinity influences the predominate molecular-level forces. We measure oil recovery, phase viscosities, phase morphologies, contact angles and water relative permeability to elucidate the underlying oil recovery mechanisms. Optimum salinity formed a stable emulsion phase with ultra-low interfacial tension, viscosity high enough to reduce the mobility of the injected solution and a reduced adhesive force that resulted in less contact between oil and solid grains. These mechanisms are attributed to favorable flow dynamics that lead to improved oil recovery that can be tuned by brine salinity.

Molecular interactions and forces of adhesion between single human neural stem cells and gelatin methacrylate hydrogels of varying stiffness

Single Cell Force Spectroscopy was applied to measure the single cell de-adhesion between human neural stem cells (hNSC) and gelatin methacrylate (GelMA) hydrogel with varying modulus in the range equivalent to brain tissue. The cell de-adhesion force and energy were predominately generated via unbinding of complexes formed between RGD groups of the GelMA and cell surface integrin receptors and the de-adhesion force/energy were found to increase with decreasing modulus of the GelMA hydrogel. For the softer GelMA hydrogels (160 Pa and 450 Pa) it was proposed that a lower degree of cross-linking enables a greater number of polymer chains to bind and freely extend to increase the force and energy of the hNSC-GelMA de-adhesion. In this case, the multiple polymer chains are believed to act together in parallel like ‘molecular tensors’ to generate tensile forces on the bound receptors until the cell detaches. Counterintuitively for softer substrates, this type of interaction gave rise to higher force loading rates, including the appearance of high and low dynamic force regimes in de-adhesion rupture force versus loading rate analysis. For the stiffer GelMA hydrogel (900 Pa) it was observed that the extension and elastic restoring forces of the polymer chains contributed less to the cell de-adhesion. Due to the apparent lower extent of freely interacting chains on the stiffer GelMA hydrogel the intrinsic RGD groups are presumed to be “more fixed” to the substrate. Hence, the cell de-adhesion is suggested to be mainly governed by the discrete unbinding of integrin-RGD complexes as opposed to elastic restoring forces of polymer chains, leading to smaller piconewton rupture forces and only a single lower dynamic force regime. Intriguingly, when integrin antibodies were introduced for binding integrin α5β1, β1- and αv-subunits it was revealed that the cell modifies the de-adhesion force depending on the substrate stiffness. The antibody binding supressed the de-adhesion on the softer GelMA hydrogel while on the stiffer GelMA hydrogel caused an opposing reinforcement in the de-adhesion. Statement of SignificanceConceptual models on cell mechanosensing have provided molecular-level insight to rationalize the effects of substrate stiffness. However most experimental studies evaluate the cell adhesion by analysing the bulk material properties. As such there is a discrepancy in the scale between the bulk properties versus the nano- and micro-scale cell interactions. Furthermore there is a paucity of experimental studies on directly measuring the molecular-level forces of cell-material interactions.Here we apply Single Cell Force Spectroscopy to directly measure the adhesion forces between human neural stem cells and gelatin-methacrylate hydrogel. We elucidate the mechanisms by which single cells bind and physically interact with hydrogels of varying stiffness. The study highlights the use of single cell analysis tools to probe molecular-level interactions at the cell-material interface which is of importance in designing material cues for regulating cell function.

Revealing the Dependence of Molecular-Level Force Transfer and Distribution on Polymer Cross-Link Density via Mechanophores.

The correlation between polymer architecture and molecular-level forces has long been a challenging research subject. Herein, spiropyran, a mechanophore that exhibits fluorescence change under force, was incorporated as a cross-linker between PMMA backbone segments. Using an in situ opto-mechanical setup to probe the molecular-level forces, the mechano-response of SP-linked PMMA as a function of the cross-link density was monitored during deformation. The dependence of the molecular-level force on cross-link density was quantitatively examined and revealed. First, a higher cross-link density shifted the fluorescence onset, that is, the onset of the spiropyran-to-merocyanine transition, to lower strains, eventually shifting the onset long before yield, without requiring sufficient chain mobility, owing to the higher efficiency of the force transfer. Under the same energy, the increase in cross-link density allowed for faster force transfer, but only to a certain level. Finally, the overall amount of spiropyran-to-merocyanine conversion linearly decreased with increasing cross-link density.

Mesoscopic Impurities Expose a Nucleation-Limited Regime of Crystal Growth.

Nanoscale self-assembly is naturally subject to impediments at the nanoscale. The recently developed ability to follow processes at the molecular level forces us to resolve older, coarse-grained concepts in terms of their molecular mechanisms. In this Letter, we highlight one such example. We present evidence based on experimental and simulation data that one of the cornerstones of crystal growth theory, the Cabrera-Vermilyea model of step advancement in the presence of impurities, is based on incomplete physics. We demonstrate that the piercing of an impurity fence by elementary steps is not solely determined by the Gibbs-Thomson effect, as assumed by Cabrera-Vermilyea. Our data show that for conditions leading up to growth cessation, step retardation is dominated by the formation of critically sized fluctuations. The growth recovery of steps is counter to what is typically assumed, not instantaneous. Our observations on mesoscopic impurities for lysozyme expose a nucleation-dominated regime of growth that has not been hitherto considered, where the system alternates between zero and near-pure velocity. The time spent by the system in arrest is the nucleation induction time required for the step to amass a supercritical fluctuation that pierces the impurity fence.

Nanofiber Optic Mechanical Transducers for Measuring Molecular Level Displacements and Forces

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Liquid film drag out in the presence of molecular forces

From a practical as well as a conceptual point of view, one of the most interesting problems of physicochemical hydrodynamics is the drag out of a liquid film by a moving solid out of a pool of liquid. The basic problem, sometimes denoted the Landau-Levich problem [L. Landau and B. Levich, “Dragging of a liquid by a moving plate,” Acta Physicochim. USSR 17, 42–54 (1942)], involves an interesting blend of capillary and viscous forces plus a matching of the static solution for capillary rise with a numerical solution of the film evolution equation, neglecting gravity, on the downstream region of the flow field. The original solution describes experimental data for a wide range of Capillary numbers but fails to match results for large and very small Capillary numbers. Molecular level forces are introduced to create an augmented version of the film evolution equation to show the effect of van der Waals forces at the lower range of Capillary numbers. A closed form solution for static capillary rise, including molecular forces, was matched with a numerical solution of the augmented film evolution equation in the dynamic meniscus region. Molecular forces do not sensibly modify the static capillary rise region, since film thicknesses are larger than the range of influence of van der Waals forces, but are determinant in shaping the downstream dynamic meniscus of the very thin liquid films. As expected, a quantitatively different level of disjoining pressure for different values of molecular constants remains in the very thin liquid film far downstream. Computational results for a wide range of Capillary numbers and Hamaker constants show a clear transition towards a region where the film thickness becomes independent of the coating speed.

AFM Determination of the Elasticity of Model Biomembranes

The utility of AFM (Atomic Force Microscopy) to study surface morphologies and molecular level forces is well known. It is also possible to apply well-defined small forces on soft surfaces using the AFM. The latter is being used here to determine the elasticity (Young's modulus) of model membrane lipid bilayers. The mechanical properties of biological membranes are of interest in understanding various biologically related processes like cell hemolysis, viral fusion and pore formation, etc. and are closely related to the cell stability. The method involves determination of the force curves by compressing the lipid bilayer by indenting the AFM tip on the bilayer surface, and the resulting elastic response of the bilayer to the applied force (load) is analysed using theoretical models for the tip geometry. We will discuss the sample preparation technique and the results of our experiments on coexisting fluid-phase domains in typical ‘lipid raft’ mixtures, as well as the effects of varying the level of unsaturation of the lipid carbon chains.

Liquid glass electrodes for nanofluidics

Nanofluidic devices exploit molecular-level forces and phenomena to increase their density, speed and accuracy1. However, fabrication is challenging because dissimilar materials need to be integrated in three dimensions with nanoscale precision. Here we report a three-dimensional nanoscale liquid glass electrode (NLGE) made from monolithic substrates without conductive materials by femtosecond laser nanomachining. The electrode consists of a nanochannel terminating at a nanoscale glass tip that becomes a conductor in the presence of high electric fields through dielectric breakdown, and returns to an insulator when this field is removed. This reversibility relies on control of nanoampere breakdown currents and extremely fast heat dissipation at nanoscale volumes. We use the NLGE to fabricate a nano-injector that includes an electrokinetic pump, 4 µm across with 0.6 µm channels, and capable of well-controlled flow rates below 1 fL/s. The electrode can be easily integrated into other nanodevices and fluidic systems, including actuators and sensors.

Chemical force microscopy: probing chemical origin of interfacial forces and adhesion

Intermolecular interactions between distinct chemical functionalities define a multitude of adhesion events in chemistry, biology and materials science. Modern techniques for measuring molecular level forces have allowed direct quantitative characterization of these interactions. In particular, chemical force microscopy (CFM), which uses the probe tip of a force microscope covalently modified with specific organic functional groups, provides a flexible approach for studying interactions between specific chemical functionalities. In this review, we survey the progress in CFM in recent years as it applies to adhesion of soft materials. We show how new developments in the experimental and theoretical approaches continue to build a realistic and detailed picture of adhesion interaction in condensed phases. We specifically highlight the importance of the kinetics of the unbinding processes and solvation effects in determining the strength of intermolecular interactions. We also describe some recent new directions in CFM, such as high-throughput adhesion measurements and mapping of full intermolecular potentials.

Measurement and interpretation of molecular-level forces of interaction between the siderophore azotobactin and mineral surfaces

The forces of interaction were measured between the siderophore azotobactin and the minerals goethite (α-FeOOH) and diaspore (α-AlOOH) in aqueous solution using force microscopy. Azotobactin, a pyoverdin-type siderophore, was covalently linked to a hydrazide terminated atomic force microscope tip using a standard active ester protein coupling technique. Upon contact with each mineral surface, the adhesion force between azotobactin and goethite was two to three times the value observed for the isostructural Al-equivalent diaspore. The affinity for the solid iron oxide surface reflected in the force measurements correlates with the specificity of azotobactin for aqueous ferric iron. Further, the adhesion force between azotobactin and goethite significantly decreases (4 nN to 2 nN) when small amounts of soluble iron (0.1 μM FeCl 3 · 6H 2O) are added to the system at pH 3.5 suggesting a significant specific interaction between the chelating reactive center of azotobactin and the mineral surface. Changes in the force signature with pH and ionic strength were fairly predictable when considering mineral solubility, the charge character of the mineral surfaces, the molecular structure of azotobactin, and the intervening solution. For example, azotobactin–goethite adhesion values were consistently smaller at pH 3.5 relative to the forces at pH 7. At the lower pH, the large number of protons and the increase in the mineral solubility provides additional electron acceptors (e.g., H + and Fe 3+(aq)) that are free to compete for the basic oxygen chelating sites in the azotobactin structure. It is believed that this competition disrupts siderophore affinity for the surface resulting in decreased adhesion values.

Tuning dimensionality in low-dimensional electronic materials

We demonstrate how dimensionality can be tuned in complex low-dimensional electronic materials via small perturbations in competing molecular forces. The delicate balance between molecular level forces on observed dimensionality in materials is illustrated by the 1-d to 3-d structural reorganization following deuteration of the ancillary ligands in the halogen-bridged transition-metal charge-transfer complex [Pt(en)2I2][Pt(en)2]I2 (PtI) where en denotes ethylenediamine. Specifically, the impact of competing forces on dimensionality is clearly demonstrated by the temperature-dependent phase transitions in perdeuterated PtI (D-PtI), where small changes in inter- and intra-sheet interactions drive mesoscopic structural changes.

Models of Rebound and Capture for Oblique Microparticle Impacts

ABSTRACT Simple algebraic, rigid body impact models using coefficients of restitution and coefficient of friction have been used extensively in the study of the impact of microparticles with surfaces. Recent work by the authors has shown that rigid body impact theory can be extended to include an adhesion coefficient to model oblique impacts of microparticles with surfaces in the presence of molecular level forces. In this paper, the model is fully described and exploited for engineering applications. The behavior of coefficients is investigated both analytically and experimentally as initial impact velocity and angel of incidence vary. As the initial velocity of microparticles grows smaller and smaller, the significance of adhesion forces increases, eventually reaching a point where no rebound occurs and the particle is captured. Equations for this region of the growth of the influence of adhesion, designed for empirically fitting, are presented. They are capable of representing the behavior of the mater...

Molecular electronic properties of a series of 4-quinolinecarbinolamines define antimalarial activity profile.

A detailed computational study on a series of 4-quinolinecarbinolamine antimalarials was performed using the semiempirical Austin model 1 (AM1) quantum chemical method to correlate the electronic features with antimalarial activity and to illuminate more completely the fundamental molecular level forces that affect the function and utility of the compounds. Ab initio (3-21G level) calculations were performed on mefloquine, the lead compound in this series, to check the reliability of the AM1 method. Electron density in specific regions of the molecules appears to play the pivotal role toward activity. A large laterally extended negative potential in the frontal portion of the nitrogen atom of the quinoline ring and the absence of negative potential over the molecular plane are crucial for the potent antimalarials. These electrostatic features are likely to be the modulator of hydrophobicity or lipophilicity of the compounds and, hence, determine their activities. The magnitude of the positive potential located by the hydroxyl hydrogen atom also correlates with potent antimalarial activity. Two negative potential regions occur near the hydroxyl oxygen and piperidyl nitrogen atoms. The two negative potential regions and the positive potential located by the hydroxyl hydrogen atom are consistent with intermolecular hydrogen bonding with the cellular effectors. The present modeling study should aid in efficient designing of this class of antimalarial agents.

A Semiempirical Computational Investigation of the Antipsoriatic Drug Anthralin

A series of computational studies was carried out, by using the highly successful Austin Model 1 (AM1) semiempirical method, to illuminate more completely the fundamental, molecular-level forces that affect the function and utility of the antipsoriatic drug anthralin. First, examination of the keto–enol tautomeric equilibrium by AM1 showed that the keto tautomer of the drug is 9.5 kcal/mol more stable than the enol form. Strong electrostatic forces involving the hydrogens on the hydroxy groups (a partial charge of +0.25 in both forms) and the keto oxygen (a partial charge of −0.40) apparently overcome the effects of the increased aromatic stabilization present in the enol form. Second, AM1 was applied to the degradation products of anthralin, 1,8-dihydroxy-9, 10-anthra-quinone, 1,8,1′,8′-tetrahydroxy-10,10′-dianthrone, and a further oxidized form of the dimer. These molecules have been implicated in some of the unpleasant side effects of anthralin. Third, AM1 was used to predict the preferred site of ionization of anthralin.

On the inertial impaction of small particles at the entrance of a pore including hydrodynamic and molecular wall interaction effects

Existing analyses for the inertial impaction at low Reynolds number of solid particulates and aerosol droplets at the entrance to pores and theories for the collection efficiency of nuclepore filters have heretofore neglected the hydrodynamic interaction of the particles with the entrance geometry of the pore and the effect of molecular level electrostatic and electrodynamic forces on the collection efficiency of the pores. In the present paper a new theoretical framework is proposed in which both these effects are considered and the behaviour of an isolated pore is considered in detail. Approximate expressions are derived for the hydrodynamic interaction tensor describing the spatially varying non-isotropic resistance of the particle as it approaches the pore entrance, and a simple effective interaction model is developed for describing capture by molecular level forces. This combined theory provides a rational explanation for the capture of particles by the wall of the pore when the viscous draining of the fluid layer between the particle and the wall is considered. The theory also predicts a heretofore unrecognized new phenomenon in which for finite particle inertia, large-angle incoming particle trajectories can cross low-angle trajectories and lead to multiple solutions for the range of angles in which the particles will be captured by the wall.

Hydrodynamic and mechanical aspects of endothelial permeability

The passive transport of water through the endothelial cell layer junctions is considered from the standpoint of hydrodynamic theories based on ultrastructural information. The local geometry of tight junctions based on molecular level forces and elastic membrane properties has been modeled and leads to estimates of the hydraulic resistance of the clefts. It is shown that the large resistance measured experimentally can be accounted for in this model. The transport of large macromolecules via vesicles which diffuse across the endothelial cell has been developed, but recent experimental data do not appear to support this mechanism as a primary pathway. Fused vesicles forming an open channel appear to be rare. Leaky junctions, such as around dying endothelial cells or produced by cytoskeletal changes within the cells, may be important in control of endothelial permeability. Another kind of model is a fiber matrix model of the endocapillary layer, extending into the intercellular clefts which can also account for the molecular seiving properties of the endothelial cell layer but may produce a large resistance to water flux.

Theory for the formation of intercellular junctions based on intramembranous particle patterns observed in the freeze fracture technique

A heterogeneous continuum theory of biological membrane interactions is presented which takes account of the spatial inhomogeneity of intramembranous particle patterns observed in ultrastructural studies of junctional complexes using the freeze-cleaved technique. The theory attempts to explain (i) how electrostatic and electrodynamic forces between particles of different biochemical composition in the same and opposing membranes might give rise to the specialized particle configurations characteristic of tight and gap junctions, (ii) how the spatial non-uniformity of the membrane proteins quantitatively modifies the local long range molecular level force field between adjacent membrane bilayers and (iii) how membrane elastic stresses and the modified molecular level forces combine to determine the equilibrium configurations of the various junctional complexes. The mathematical problem is highly non-linear since the molecular forces are a rapidly varying function of the local membrane spacing which is a priori unknown. The simplified dimensionless boundary value problem to determine the junction geometry has been reduced to a fourth order quasi one-dimensional equation with split end point conditions. The equation is singular in the vicinity of the junction complex and special solution techniques had to be employed. The numerical results are in reasonable quantitative agreement with electron microscopic observations of tight, gap and venous junctions.