The electrical properties of the cellular membrane are important for ion transport across cells and electrophysiology. Plasma membranes also resist bending and stretching, and mechanical properties of the membrane influence cell shape and forces in membrane tethers pulled from cells. There exists a coupling between the electrical and mechanical properties of the membrane. Previous work has shown that applied voltages can induce forces and movements in the lipid bilayer. We present a theory that computes membrane bending deformations and forces as the applied voltage is changed. We discover that electromechanical coupling in lipid bilayers depends on the voltage-dependent adsorption of ions into the region occupied by the phospholipid head groups. A simple model of counter-ion absorption is investigated. We show that electromechanical coupling can be measured using membrane tethers and we use our model to predict the membrane tether tension as a function of applied voltage. We also discuss how electromechanical coupling in membranes may influence transmembrane protein function.
Monday, April 19, 2010
Voltage-induced bending and electromechanical coupling in lipid bilayers
Ben Harland, William E. Brownell, Alexander A. Spector, and Sean X. Sun
The electrical properties of the cellular membrane are important for ion transport across cells and electrophysiology. Plasma membranes also resist bending and stretching, and mechanical properties of the membrane influence cell shape and forces in membrane tethers pulled from cells. There exists a coupling between the electrical and mechanical properties of the membrane. Previous work has shown that applied voltages can induce forces and movements in the lipid bilayer. We present a theory that computes membrane bending deformations and forces as the applied voltage is changed. We discover that electromechanical coupling in lipid bilayers depends on the voltage-dependent adsorption of ions into the region occupied by the phospholipid head groups. A simple model of counter-ion absorption is investigated. We show that electromechanical coupling can be measured using membrane tethers and we use our model to predict the membrane tether tension as a function of applied voltage. We also discuss how electromechanical coupling in membranes may influence transmembrane protein function.
The electrical properties of the cellular membrane are important for ion transport across cells and electrophysiology. Plasma membranes also resist bending and stretching, and mechanical properties of the membrane influence cell shape and forces in membrane tethers pulled from cells. There exists a coupling between the electrical and mechanical properties of the membrane. Previous work has shown that applied voltages can induce forces and movements in the lipid bilayer. We present a theory that computes membrane bending deformations and forces as the applied voltage is changed. We discover that electromechanical coupling in lipid bilayers depends on the voltage-dependent adsorption of ions into the region occupied by the phospholipid head groups. A simple model of counter-ion absorption is investigated. We show that electromechanical coupling can be measured using membrane tethers and we use our model to predict the membrane tether tension as a function of applied voltage. We also discuss how electromechanical coupling in membranes may influence transmembrane protein function.
Hydrodynamic synchronization of colloidal oscillators
Jurij Kotar, Marco Leoni, Bruno Bassetti, Marco Cosentino Lagomarsino, and Pietro Cicuta
Two colloidal spheres are maintained in oscillation by switching the position of an optical trap when a sphere reaches a limit position, leading to oscillations that are bounded in amplitude but free in phase and period. The interaction between the oscillators is only through the hydrodynamic flow induced by their motion. We prove that in the absence of stochastic noise the antiphase dynamical state is stable, and we show how the period depends on coupling strength. Both features are observed experimentally. As the natural frequencies of the oscillators are made progressively different, the coordination is quickly lost. These results help one to understand the origin of hydrodynamic synchronization and how the dynamics can be tuned. Cilia and flagella are biological systems coupled hydrodynamically, exhibiting dramatic collective motions. We propose that weakly correlated phase fluctuations, with one of the oscillators typically precessing the other, are characteristic of hydrodynamically coupled systems in the presence of thermal noise.
Two colloidal spheres are maintained in oscillation by switching the position of an optical trap when a sphere reaches a limit position, leading to oscillations that are bounded in amplitude but free in phase and period. The interaction between the oscillators is only through the hydrodynamic flow induced by their motion. We prove that in the absence of stochastic noise the antiphase dynamical state is stable, and we show how the period depends on coupling strength. Both features are observed experimentally. As the natural frequencies of the oscillators are made progressively different, the coordination is quickly lost. These results help one to understand the origin of hydrodynamic synchronization and how the dynamics can be tuned. Cilia and flagella are biological systems coupled hydrodynamically, exhibiting dramatic collective motions. We propose that weakly correlated phase fluctuations, with one of the oscillators typically precessing the other, are characteristic of hydrodynamically coupled systems in the presence of thermal noise.
Contribution of the myosin VI tail domain to processive stepping and intramolecular tension sensing
Alexander R. Dunna, Peiying Chuana, Zev Bryantb, and James A. Spudich
Myosin VI is proposed to act as both a molecular transporter and as an anchor in vivo. A portion of the molecule C-terminal to the canonical lever arm, termed the medial tail (MT), has been proposed to act as either a lever arm extension or as a dimerization motif. We describe constructs in which the MT is interrupted by a glycine-rich molecular swivel. Disruption of the MT results in decreased processive run lengths measured using single-molecule fluorescence microscopy and a decreased step size under applied load as measured in an optical trap. We used single-molecule gold nanoparticle tracking and optical trapping to examine the mechanism of coordination between the heads of dimeric myosin VI. We detect two rate-limiting kinetic processes at low ATP concentrations. Our data can be explained by a model in which intramolecular tension greatly increases the affinity of the lead head for ADP, likely by slowing ADP release from the lead head. This mechanism likely increases both the motor's processivity and its ability to act as an anchor under physiological conditions.
Myosin VI is proposed to act as both a molecular transporter and as an anchor in vivo. A portion of the molecule C-terminal to the canonical lever arm, termed the medial tail (MT), has been proposed to act as either a lever arm extension or as a dimerization motif. We describe constructs in which the MT is interrupted by a glycine-rich molecular swivel. Disruption of the MT results in decreased processive run lengths measured using single-molecule fluorescence microscopy and a decreased step size under applied load as measured in an optical trap. We used single-molecule gold nanoparticle tracking and optical trapping to examine the mechanism of coordination between the heads of dimeric myosin VI. We detect two rate-limiting kinetic processes at low ATP concentrations. Our data can be explained by a model in which intramolecular tension greatly increases the affinity of the lead head for ADP, likely by slowing ADP release from the lead head. This mechanism likely increases both the motor's processivity and its ability to act as an anchor under physiological conditions.
Dark focal spot shaping of hyperbolic-cosine-Gaussian beam
Xiumin Gao, Qiufang Zhan, Jinsong Li, Song Hu, Jian Wang, and Songlin Zhuang
Dark focal spot shaping is investigated by vector diffraction theory in the focal region of a hyperbolic-cosine beam that contains one on-axis spiral optical vortex. Results show that a dark focal shape can be altered considerably by the decentered parameters in cosh parts of the beam and topological charge of the vortex. Many novel and interesting dark focal shapes may appear, including rhombic, quadrangular, cross-shaped, and foursquare dark foci. Some dark focal spot chains can also occur. In addition, the numerical aperture of the focusing system can also affect dark focal shapes remarkably, which may lead to dark focal spots that disappear or change focal shape due to the depolarization effect for a high numerical aperture. All of the above dark focal shapes can be used in an optical manipulation system to construct alterable optical traps.
Dark focal spot shaping is investigated by vector diffraction theory in the focal region of a hyperbolic-cosine beam that contains one on-axis spiral optical vortex. Results show that a dark focal shape can be altered considerably by the decentered parameters in cosh parts of the beam and topological charge of the vortex. Many novel and interesting dark focal shapes may appear, including rhombic, quadrangular, cross-shaped, and foursquare dark foci. Some dark focal spot chains can also occur. In addition, the numerical aperture of the focusing system can also affect dark focal shapes remarkably, which may lead to dark focal spots that disappear or change focal shape due to the depolarization effect for a high numerical aperture. All of the above dark focal shapes can be used in an optical manipulation system to construct alterable optical traps.
On the Positioning Problem of a Microscopic Particle Trapped in Optical Tweezers
Carlos Aguilar-Ibañez and Luis I. Rosas-Soriano
We solve the positioning problem of a spherical microparticle trapped by Optical Tweezers, under the assumption that the drag viscous force is presented. To do it, we develop two control strategies for the manipulation of this kind of optical trap. The first control strategy is developed assuming that the damping coefficient is known, while in the second strategy this parameter value is only partially known, which in practice it is more realistic due to the difficulty to estimate it. Both strategies are based on the traditional Lyapunov method in conjunction with the use of a saturation function. The stability analysis of both strategies was carried out by using the standard Lyapunov stability theory. Finally, numerical simulations validate the effectiveness of both control approaches in reducing the random position fluctuations produced by the inherent thermal noise.
DOI
We solve the positioning problem of a spherical microparticle trapped by Optical Tweezers, under the assumption that the drag viscous force is presented. To do it, we develop two control strategies for the manipulation of this kind of optical trap. The first control strategy is developed assuming that the damping coefficient is known, while in the second strategy this parameter value is only partially known, which in practice it is more realistic due to the difficulty to estimate it. Both strategies are based on the traditional Lyapunov method in conjunction with the use of a saturation function. The stability analysis of both strategies was carried out by using the standard Lyapunov stability theory. Finally, numerical simulations validate the effectiveness of both control approaches in reducing the random position fluctuations produced by the inherent thermal noise.
DOI
Thursday, April 8, 2010
Raman tweezers and their application to the study of singly trapped eukaryotic cells
Richard D. Snook, Timothy J. Harvey, Elsa Correia Faria and Peter Gardner
In this review the recent emergence of Raman tweezers as an analytical technique for single eukaryotic cell analysis is described. The Raman tweezer technique combines Raman spectroscopy as a diagnostic tool with optical tweezers by which means single cells can be trapped and manipulated in a laser beam using a high numerical aperture imaging microscope. Necessary instrumental requirements to facilitate Raman tweezer experiments are discussed together with practical considerations such as the potential for photodamage of cells subjected to trapping and Raman excitation. Specific applications of Raman tweezers to the analysis of cancer cells, erythrocytes and lymphocytes, micro-organisms and sub-cellular components e.g. chromosomes and mitochondria are then discussed followed by a summary of the future potential of the technique for single cell analysis.
DOI
In this review the recent emergence of Raman tweezers as an analytical technique for single eukaryotic cell analysis is described. The Raman tweezer technique combines Raman spectroscopy as a diagnostic tool with optical tweezers by which means single cells can be trapped and manipulated in a laser beam using a high numerical aperture imaging microscope. Necessary instrumental requirements to facilitate Raman tweezer experiments are discussed together with practical considerations such as the potential for photodamage of cells subjected to trapping and Raman excitation. Specific applications of Raman tweezers to the analysis of cancer cells, erythrocytes and lymphocytes, micro-organisms and sub-cellular components e.g. chromosomes and mitochondria are then discussed followed by a summary of the future potential of the technique for single cell analysis.
DOI
Factors Affecting Daughter Cells' Arrangement during the Early Bacterial Divisions
Pin-Tzu Su, Pei-Wen Yen, Shao-Hung Wang, Chi-Hung Lin, Arthur Chiou, Wan-Jr Syu
On agar plates, daughter cells of Escherichia coli mutually slide and align side-by-side in parallel during the first round of binary fission. This phenomenon has been previously attributed to an elastic material that restricts apparently separated bacteria from being in string. We hypothesize that the interaction between bacteria and the underneath substratum may affect the arrangement of the daughter bacteria. To test this hypothesis, bacterial division on hyaluronic acid (HA) gel, as an alternative substratum, was examined. Consistent with our proposition, the HA gel differs from agar by suppressing the typical side-by-side alignments to a rare population. Examination of bacterial surface molecules that may contribute to the daughter cells' arrangement yielded an observation that, with disrupted lpp, the E. coli daughter cells increasingly formed non-typical patterns, i.e. neither sliding side-by-side in parallel nor forming elongated strings. Therefore, our results suggest strongly that the early cell patterning is affected by multiple interaction factors. With oscillatory optical tweezers, we further demonstrated that the interaction force decreased in bacteria without Lpp, a result substantiating our notion that the side-by-side sliding phenomenon directly reflects the strength of in-situ interaction between bacteria and substratum.
On agar plates, daughter cells of Escherichia coli mutually slide and align side-by-side in parallel during the first round of binary fission. This phenomenon has been previously attributed to an elastic material that restricts apparently separated bacteria from being in string. We hypothesize that the interaction between bacteria and the underneath substratum may affect the arrangement of the daughter bacteria. To test this hypothesis, bacterial division on hyaluronic acid (HA) gel, as an alternative substratum, was examined. Consistent with our proposition, the HA gel differs from agar by suppressing the typical side-by-side alignments to a rare population. Examination of bacterial surface molecules that may contribute to the daughter cells' arrangement yielded an observation that, with disrupted lpp, the E. coli daughter cells increasingly formed non-typical patterns, i.e. neither sliding side-by-side in parallel nor forming elongated strings. Therefore, our results suggest strongly that the early cell patterning is affected by multiple interaction factors. With oscillatory optical tweezers, we further demonstrated that the interaction force decreased in bacteria without Lpp, a result substantiating our notion that the side-by-side sliding phenomenon directly reflects the strength of in-situ interaction between bacteria and substratum.
Wednesday, April 7, 2010
Optical sorting using an array of optical vortices with fractional topological charge
Cheng-Shan Guo, Ya-Nan Yu and Zhengping Hong
An array of optical vortices with fractional topological charge is generated using a phase-only Talbot array illuminator and used to sort microparticles. Our theoretical analysis and experimental results reveal that when a particle passes through a fractional vortex array, it will be driven by two forces, intensity-gradient force and phase-gradient force, and the cooperation of these two forces can improve its ability in optical sorting because of the special intensity and phase distribution of the fractional optical vortex array. Larger angle separation could be obtained with moderate laser power.
An array of optical vortices with fractional topological charge is generated using a phase-only Talbot array illuminator and used to sort microparticles. Our theoretical analysis and experimental results reveal that when a particle passes through a fractional vortex array, it will be driven by two forces, intensity-gradient force and phase-gradient force, and the cooperation of these two forces can improve its ability in optical sorting because of the special intensity and phase distribution of the fractional optical vortex array. Larger angle separation could be obtained with moderate laser power.
Thursday, April 1, 2010
Force Generation in Lamellipodia Is a Probabilistic Process with Fast Growth and Retraction Events
Rajesh Shahapure, Francesco Difato, Alessandro Laio, Giacomo Bisson, Erika Ercolini, Ladan Amin, Enrico Ferrari and Vincent Torre
Polymerization of actin filaments is the primary source of motility in lamellipodia and it is controlled by a variety of regulatory proteins. The underlying molecular mechanisms are only partially understood and a precise determination of dynamical properties of force generation is necessary. Using optical tweezers, we have measured with millisecond (ms) temporal resolution and picoNewton (pN) sensitivity the force-velocity (Fv) relationship and the power dissipated by lamellipodia of dorsal root ganglia neurons. When force and velocity are averaged over 3–5 s, the Fv relationships can be flat. On a finer timescale, random occurrence of fast growth and subsecond retractions become predominant. The maximal power dissipated by lamellipodia over a silica bead with a diameter of 1 μm is 10−16 W. Our results clarify the dynamical properties of force generation: i), force generation is a probabilistic process; ii), underlying biological events have a bandwidth up to at least 10 Hz; and iii), fast growth of lamellipodia leading edge alternates with local retractions.
Optical Trapping of Amino Acids in Aqueous Solutions
Yasuyuki Tsuboi, Tatsuya Shoji and Noboru Kitamura
In this paper, we demonstrated that the photon forces that are generated by a near-infrared (1064 nm) focused laser beam can manipulate (optically trap) small amino acid molecules in aqueous solutions. Under observation with an optical microscope, we observed the gradual growth of a particle-like assembly of objects at the focal point during laser irradiation into an aqueous arginine solution. The observation was a molecular assembly of arginine by means of confocal Raman microspectroscopy. Such molecular assemblies were also observed for other amino acids (glycine, proline, serine, and alanine), showing that the optical manipulation technique can be extensively applied to the micromanipulation of amino acids. From experimental observations and numeral calculations, we consider that the origin of the present manipulation phenomenon is ascribed to optical trapping, not of individual molecules, but of molecular clusters of amino acids.
In this paper, we demonstrated that the photon forces that are generated by a near-infrared (1064 nm) focused laser beam can manipulate (optically trap) small amino acid molecules in aqueous solutions. Under observation with an optical microscope, we observed the gradual growth of a particle-like assembly of objects at the focal point during laser irradiation into an aqueous arginine solution. The observation was a molecular assembly of arginine by means of confocal Raman microspectroscopy. Such molecular assemblies were also observed for other amino acids (glycine, proline, serine, and alanine), showing that the optical manipulation technique can be extensively applied to the micromanipulation of amino acids. From experimental observations and numeral calculations, we consider that the origin of the present manipulation phenomenon is ascribed to optical trapping, not of individual molecules, but of molecular clusters of amino acids.
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