Aletta Adriana Verhoeff, Francois Lavergne, Denis Bartolo, Dirk G. A. L. Aarts and Roel P. A. Dullens
We achieve active control of interfacial phenomena by optically trapping the interface using the gradient forces of a strongly focussed laser beam parallel to the interface. We illustrate our technique in a phase separated colloid-polymer mixture by distorting the interface in a very controlled way. The static structure of the manipulated interface as well as its dynamic relaxation behaviour are analysed. Both the statics and dynamics can be related to the capillary wave height-height correlations using the fluctuation dissipation theorem up to surprisingly large deformations of the interface. To underline the novelty and potential of our approach we also show multiple interface distortions and the controlled snap-off of liquid droplets.
DOI
Saturday, March 7, 2015
Characterizing the interaction between DNA and GelRed fluorescent stain
F. A. P. Crisafuli, E. B. Ramos, M. S. Rocha
We have performed single-molecule stretching and dynamic light-scattering (DLS) experiments to characterize the interaction between the DNA molecule and the fluorescent stain GelRed. The results from single-molecule stretching show that the persistence length of DNA–GelRed complexes increases as the ligand concentration increases up to a critical concentration, then decreases for higher concentrations. The contour length of the complexes, on the other hand, increases monotonically as a function of GelRed concentration, suggesting that intercalation is the main binding mechanism. To characterize the physical chemistry of the interaction, we used the McGhee–von Hippel binding isotherm to extract physicochemical data for the interaction from the contour length data. Such analysis enabled us to conclude that the GelRed stain is, in fact, a bis-intercalator. In addition, DLS experiments were performed to study the changes of the effective size of the DNA–GelRed complexes, measured as the hydrodynamic radius, as a function of ligand concentration. We observed qualitative agreement between the results obtained from the two techniques by comparing the behavior of the hydrodynamics radius and the radius of gyration, because the latter quantity can be expressed as a function of mechanical properties determined from the stretching experiments.
DOI
We have performed single-molecule stretching and dynamic light-scattering (DLS) experiments to characterize the interaction between the DNA molecule and the fluorescent stain GelRed. The results from single-molecule stretching show that the persistence length of DNA–GelRed complexes increases as the ligand concentration increases up to a critical concentration, then decreases for higher concentrations. The contour length of the complexes, on the other hand, increases monotonically as a function of GelRed concentration, suggesting that intercalation is the main binding mechanism. To characterize the physical chemistry of the interaction, we used the McGhee–von Hippel binding isotherm to extract physicochemical data for the interaction from the contour length data. Such analysis enabled us to conclude that the GelRed stain is, in fact, a bis-intercalator. In addition, DLS experiments were performed to study the changes of the effective size of the DNA–GelRed complexes, measured as the hydrodynamic radius, as a function of ligand concentration. We observed qualitative agreement between the results obtained from the two techniques by comparing the behavior of the hydrodynamics radius and the radius of gyration, because the latter quantity can be expressed as a function of mechanical properties determined from the stretching experiments.
DOI
Condensation transition and forced unravelling of DNA-histone H1 toroids: a multi-state free energy landscape
A H Mack, D J Schlingman, R D Salinas, L Regan and S G J Mochrie
DNA is known to condense with multivalent cations and positively charged proteins. However, the properties and energetics of DNA superstructures, such as chromatin, are poorly understood. As a model system, we investigate histone H1 condensation of DNA with tethered particle motion and force-extension measurements. We show that after the addition of H1 to DNA, a concentration dependent lag time is followed by the DNA spontaneously condensing. The trigger for this condensation phase transition can be modeled as sufficient H1s having bound to the DNA, providing insight into the 30 nm fiber condensation upon H1 binding. Furthermore, optical tweezers force-extension measurements of histone H1 condensed DNA reveals a sequence of state transitions corresponding to the unwinding of superhelical turns. We determine the complete, experimental, multi-state free energy landscape for the complex using Crooks fluctuation theorem. The measured force-versus-extension and free energy landscape are compared to predictions from a simple, theoretical model. This work encourages the theoretical description of DNA/protein structure and energetics and their role in chromatin and other, more complex, systems.
DOI
DNA is known to condense with multivalent cations and positively charged proteins. However, the properties and energetics of DNA superstructures, such as chromatin, are poorly understood. As a model system, we investigate histone H1 condensation of DNA with tethered particle motion and force-extension measurements. We show that after the addition of H1 to DNA, a concentration dependent lag time is followed by the DNA spontaneously condensing. The trigger for this condensation phase transition can be modeled as sufficient H1s having bound to the DNA, providing insight into the 30 nm fiber condensation upon H1 binding. Furthermore, optical tweezers force-extension measurements of histone H1 condensed DNA reveals a sequence of state transitions corresponding to the unwinding of superhelical turns. We determine the complete, experimental, multi-state free energy landscape for the complex using Crooks fluctuation theorem. The measured force-versus-extension and free energy landscape are compared to predictions from a simple, theoretical model. This work encourages the theoretical description of DNA/protein structure and energetics and their role in chromatin and other, more complex, systems.
DOI
Dynamics of Membrane Tethers Reveal Novel Aspects of Cytoskeleton-Membrane Interactions in Axons
Anagha Datar, Thomas Bornschlögl, Patricia Bassereau, Jacques Prost, Pramod A. Pullarkat
Mechanical properties of cell membranes are known to be significantly influenced by the underlying cortical cytoskeleton. The technique of pulling membrane tethers from cells is one of the most effective ways of studying the membrane mechanics and the membrane-cortex interaction. In this article, we show that axon membranes make an interesting system to explore as they exhibit both free membrane-like behavior where the tether-membrane junction is movable on the surface of the axons (unlike many other cell membranes) as well as cell-like behavior where there are transient and spontaneous eruptions in the tether force that vanish when F-actin is depolymerized. We analyze the passive and spontaneous responses of axonal membrane tethers and propose theoretical models to explain the observed behavior.
DOI
Mechanical properties of cell membranes are known to be significantly influenced by the underlying cortical cytoskeleton. The technique of pulling membrane tethers from cells is one of the most effective ways of studying the membrane mechanics and the membrane-cortex interaction. In this article, we show that axon membranes make an interesting system to explore as they exhibit both free membrane-like behavior where the tether-membrane junction is movable on the surface of the axons (unlike many other cell membranes) as well as cell-like behavior where there are transient and spontaneous eruptions in the tether force that vanish when F-actin is depolymerized. We analyze the passive and spontaneous responses of axonal membrane tethers and propose theoretical models to explain the observed behavior.
DOI
Thursday, March 5, 2015
Local Heat Activation of Single Myosins Based on Optical Trapping of Gold Nanoparticles
Mitsuhiro Iwaki, Atsuko H. Iwane, Keigo Ikezaki, and Toshio Yanagida
Myosin is a mechano-enzyme that hydrolyzes ATP in order to move unidirectionally along actin filaments. Here we show by single molecule imaging that myosin V motion can be activated by local heat. We constructed a dark-field microscopy that included optical tweezers to monitor 80 nm gold nanoparticles (GNP) bound to single myosin V molecules with nanometer and submillisecond accuracy. We observed 34 nm processive steps along actin filaments like those seen when using 200 nm polystyrene beads (PB) but dwell times (ATPase activity) that were 4.5 times faster. Further, by using DNA nanotechnology (DNA origami) and myosin V as a nanometric thermometer, the temperature gradient surrounding optically trapped GNP could be estimated with nanometer accuracy. We propose our single molecule measurement system should advance quantitative analysis of the thermal control of biological and artificial systems like nanoscale thermal ratchet motors.
DOI
Myosin is a mechano-enzyme that hydrolyzes ATP in order to move unidirectionally along actin filaments. Here we show by single molecule imaging that myosin V motion can be activated by local heat. We constructed a dark-field microscopy that included optical tweezers to monitor 80 nm gold nanoparticles (GNP) bound to single myosin V molecules with nanometer and submillisecond accuracy. We observed 34 nm processive steps along actin filaments like those seen when using 200 nm polystyrene beads (PB) but dwell times (ATPase activity) that were 4.5 times faster. Further, by using DNA nanotechnology (DNA origami) and myosin V as a nanometric thermometer, the temperature gradient surrounding optically trapped GNP could be estimated with nanometer accuracy. We propose our single molecule measurement system should advance quantitative analysis of the thermal control of biological and artificial systems like nanoscale thermal ratchet motors.
DOI
Plasmonically Enhanced Chiral Optical Fields and Forces in Achiral Split Ring Resonators
M. H. Alizadeh and Björn M. Reinhard
The two enantiomers (mirror images) of a biomolecule can show drastically different behaviors, requiring the development of sensitive approaches for their identification and separation. Plasmonic nanostructures have shown promise for enhancing the sensitivities of chiral spectroscopies, but the generation of chiral near fields with a specific handedness in the spatial domain surrounding the plasmonic structures remains a challenge. Here we demonstrate that achiral bianisotropic structures, which couple the electric and magnetic fields, can achieve high enhancements of optical chirality in an extended spatial region. Magneto-electric coupling in such structures facilitates electrically excited magnetic resonances in the near IR and optical regimes, which in turn can result in highly enhanced optical chirality in an extended region of space. We apply this concept to achiral double split ring resonators (DSRRs) and demonstrate their potential in generating enhanced chiral fields and forces. Also, the behavior of optical chirality density gradient and chirality flux in such structures is examined, and it is shown that plasmonically generated chiral forces may pave the way to a new class of chiral biosensors
DOI
The two enantiomers (mirror images) of a biomolecule can show drastically different behaviors, requiring the development of sensitive approaches for their identification and separation. Plasmonic nanostructures have shown promise for enhancing the sensitivities of chiral spectroscopies, but the generation of chiral near fields with a specific handedness in the spatial domain surrounding the plasmonic structures remains a challenge. Here we demonstrate that achiral bianisotropic structures, which couple the electric and magnetic fields, can achieve high enhancements of optical chirality in an extended spatial region. Magneto-electric coupling in such structures facilitates electrically excited magnetic resonances in the near IR and optical regimes, which in turn can result in highly enhanced optical chirality in an extended region of space. We apply this concept to achiral double split ring resonators (DSRRs) and demonstrate their potential in generating enhanced chiral fields and forces. Also, the behavior of optical chirality density gradient and chirality flux in such structures is examined, and it is shown that plasmonically generated chiral forces may pave the way to a new class of chiral biosensors
DOI
Maxwell stress on a small dielectric sphere in a dielectric
Vitaly V. Datsyuk and Oleg R. Pavlyniuk
Electrically induced normal pressure and tangential stress at the surface of a small dielectric sphere (or cavity) in a dielectric are calculated using the Minkowski, Einstein-Laub, Abraham, and Lorentz forms of the Maxwell stress tensor. Only the Lorentz tensor is in agreement with the following observations: (1) A spherical cavity in a dielectric transforms into a sharp-edge plate perpendicular to the electric field; (2) a liquid drop placed in a medium with a slightly lower refractive index is stretched along the electric field; and (3) there is a torque on a small birefringent sphere. These phenomena cannot be explained by the conventional theory using the Minkowski stress tensor. For example, the Minkowski stress tensor predicts lateral compression of a spherical cavity in a dielectric.
DOI
Electrically induced normal pressure and tangential stress at the surface of a small dielectric sphere (or cavity) in a dielectric are calculated using the Minkowski, Einstein-Laub, Abraham, and Lorentz forms of the Maxwell stress tensor. Only the Lorentz tensor is in agreement with the following observations: (1) A spherical cavity in a dielectric transforms into a sharp-edge plate perpendicular to the electric field; (2) a liquid drop placed in a medium with a slightly lower refractive index is stretched along the electric field; and (3) there is a torque on a small birefringent sphere. These phenomena cannot be explained by the conventional theory using the Minkowski stress tensor. For example, the Minkowski stress tensor predicts lateral compression of a spherical cavity in a dielectric.
DOI
Precision Assembly of Complex Cellular Microenvironments using Holographic Optical Tweezers
Glen R. Kirkham, Emily Britchford, Thomas Upton, James Ware, Graham M. Gibson, Yannick Devaud, Martin Ehrbar, Miles Padgett, Stephanie Allen, Lee D. Buttery & Kevin Shakesheff
The accurate study of cellular microenvironments is limited by the lack of technologies that can manipulate cells in 3D at a sufficiently small length scale. The ability to build and manipulate multicellular microscopic structures will facilitate a more detailed understanding of cellular function in fields such as developmental and stem cell biology. We present a holographic optical tweezers based technology to accurately generate bespoke cellular micro-architectures. Using embryonic stem cells, 3D structures of varying geometries were created and stabilized using hydrogels and cell-cell adhesion methods. Control of chemical microenvironments was achieved by the temporal release of specific factors from polymer microparticles positioned within these constructs. Complex co-culture micro-environmental analogues were also generated to reproduce structures found within adult stem cell niches. The application of holographic optical tweezers-based micromanipulation will enable novel insights into biological microenvironments by allowing researchers to form complex architectures with sub-micron precision of cells, matrices and molecules.
DOI
The accurate study of cellular microenvironments is limited by the lack of technologies that can manipulate cells in 3D at a sufficiently small length scale. The ability to build and manipulate multicellular microscopic structures will facilitate a more detailed understanding of cellular function in fields such as developmental and stem cell biology. We present a holographic optical tweezers based technology to accurately generate bespoke cellular micro-architectures. Using embryonic stem cells, 3D structures of varying geometries were created and stabilized using hydrogels and cell-cell adhesion methods. Control of chemical microenvironments was achieved by the temporal release of specific factors from polymer microparticles positioned within these constructs. Complex co-culture micro-environmental analogues were also generated to reproduce structures found within adult stem cell niches. The application of holographic optical tweezers-based micromanipulation will enable novel insights into biological microenvironments by allowing researchers to form complex architectures with sub-micron precision of cells, matrices and molecules.
DOI
Wednesday, March 4, 2015
Force-induced remodelling of proteins and their complexes
Yun Chen, Sheena E Radford, David J Brockwell
Force can drive conformational changes in proteins, as well as modulate their stability and the affinity of their complexes, allowing a mechanical input to be converted into a biochemical output. These properties have been utilised by nature and force is now recognised to be widely used at the cellular level. The effects of force on the biophysical properties of biological systems can be large and varied. As these effects are only apparent in the presence of force, studies on the same proteins using traditional ensemble biophysical methods can yield apparently conflicting results. Where appropriate, therefore, force measurements should be integrated with other experimental approaches to understand the physiological context of the system under study.
DOI
Force can drive conformational changes in proteins, as well as modulate their stability and the affinity of their complexes, allowing a mechanical input to be converted into a biochemical output. These properties have been utilised by nature and force is now recognised to be widely used at the cellular level. The effects of force on the biophysical properties of biological systems can be large and varied. As these effects are only apparent in the presence of force, studies on the same proteins using traditional ensemble biophysical methods can yield apparently conflicting results. Where appropriate, therefore, force measurements should be integrated with other experimental approaches to understand the physiological context of the system under study.
DOI
Precise revolution control in three-dimensional off-axis trapping with single Laguerre-Gaussian beam
Tomoko Otsu, Taro Ando, Yu Takiguchi, Yoshiyuki Ohtake, Haruyoshi Toyoda and Hiroyasu Itoh
We report the precise study of orbital angular momentum transfer from Laguerre-Gaussian (LG) beams to submicrometer-sized dielectric spheres. Stable three-dimensional off-axis trapping of the spheres was achieved and confirmed by the constant behavior of the sphere’s revolution radius against the trapping LG-beam power. By measuring the revolutions of the sphere that was trapped and revolved in mid-water, we confirmed excellent linearity between the revolution rate and the trapping light free from the hydrodynamic coupling with walls of a sample chamber. This result suggests a torque source of precise controllability, which will contribute to the study of biological molecules, nonequilibrium statistical physics, and micromachines.
DOI
We report the precise study of orbital angular momentum transfer from Laguerre-Gaussian (LG) beams to submicrometer-sized dielectric spheres. Stable three-dimensional off-axis trapping of the spheres was achieved and confirmed by the constant behavior of the sphere’s revolution radius against the trapping LG-beam power. By measuring the revolutions of the sphere that was trapped and revolved in mid-water, we confirmed excellent linearity between the revolution rate and the trapping light free from the hydrodynamic coupling with walls of a sample chamber. This result suggests a torque source of precise controllability, which will contribute to the study of biological molecules, nonequilibrium statistical physics, and micromachines.
DOI
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