Monday, November 30, 2009

Mode-selective thermal radiation from a microparticle

Hitoshi Odashima, Maki Tachikawa, and Kei Takehiro

We experimentally demonstrate that thermal radiation from a micron-sized dielectric particle depends sensitively on its size and shape through the cavity quantum-electrodynamic effect. Our laser trapping technique levitated a high-temperature microsphere of Al2O3 and enabled emission spectroscopy of the single particle. As the particle becomes smaller, a blackbody like spectrum turns into a spectrum dominated by multiple peaks resonant with whispering gallery modes of the spherical resonator. The observed sharp frequency selectivity is applicable to spectral control of thermal radiation.

Spontaneous symmetry breaking and circulation by optically bound microparticle chains in Gaussian beam traps

J. M. Taylor and G. D. Love

It has been known for some time that simple “optically bound” chains of dielectric microparticles can form in a counter propagating Gaussian beam optical trap. Here we report experimental observations of more complex trapped states, which do not reflect the underlying symmetry of the optical beam trap they are confined in. We discuss both stationary off-axis trapping and dynamic motion. We confirm the results using a rigorous Mie scattering model and also give a physical explanation for these static and dynamic off-axis trapped states.

Design and realization of a high-stability optical tweezer

Slawomir Drobczynski, Pascal Hébraud, Jean Pierre Munch, and Sébastien Harlepp

We present a method in which we stabilize mechanically an optical tweezer setup over a period ranging up to 30 min. A feedback loop is used to correct the mechanical and thermal drifts. The position of a fixed object on the sample surface is measured with a CCD device and its fluctuations analyzed and used to maintain its position fixed with threepiezoelectric devices. With this setup, we obtain a spatial stability of 1.5 nm in the radial direction and 5 nm in the axial direction. This method opens the route for real-time measurements of kinetics of macromolecules association, at a single molecule level, on very long time scales.

Thursday, November 26, 2009

Binding Kinetics of Bisintercalator Triostin A with Optical Tweezers Force Mechanics

Christoph Kleimann, Andy Sischka, Andre Spiering, Katja Tönsing, Norbert Sewald, Ulf Diederichsen and Dario Anselmetti

The binding kinetics of the intercalative binding of Triostin A to λ-DNA was investigated by measuring the force extension response of the DNA-ligand complexes with an optical tweezers system. These force response curves, containing the information about different binding properties, were analyzed based on a recent method (put forth by another research group) for monointercalators that was extended to bisintercalators. Our binding analysis reveals an exponential dependence of the association constant on the applied external force as well as a decreasing binding site size. In general, our results are in agreement with those for the monointercalator ethidium. However, to explain the high-force binding site size, a new model for bisintercalation of Triostin A at high forces is proposed.

Tug-of-war between dissimilar teams of microtubule motors regulates transport and fission of endosomes

Virupakshi Soppina, Arpan Kumar Rai, Avin Jayesh Ramaiya,Pradeep Barak and Roop Mallik

Intracellular transport is interspersed with frequent reversals in direction due to the presence of opposing kinesin and dynein motors on organelles that are carried as cargo. The cause and the mechanism of reversals are unknown, but are a key to understanding how cargos are delivered in a regulated manner to specific cellular locations. Unlike established single-motor biophysical assays, this problem requires understanding of the cooperative behavior of multiple interacting motors. Here we present measurements inside live Dictyosteliumcells, in a cell extract and with purified motors to quantify such an ensemble function of motors. We show through precise motion analysis that reversals during endosome motion are caused by a tug-of-war between kinesin and dynein. Further, we use a combination of optical trap-based force measurements and Monte Carlo simulations to make the surprising discovery that endosome transport uses many (approximately four to eight) weak and detachment-prone dyneins in a tug-of-war against a single strong and tenacious kinesin. We elucidate how this clever choice of dissimilar motors and motor teams achieves net transport together with endosome fission, both of which are important in controlling the balance of endocytic sorting. To the best of our knowledge, this is a unique demonstration that dynein and kinesin function differently at the molecular level inside cells and of how this difference is used in a specific cellular process, namely endosome biogenesis. Our work may provide a platform to understand intracellular transport of a variety of organelles in terms of measurable quantities.

Increasing trap stiffness with position clamping in holographic optical tweezers

Daryl Preece, Richard Bowman, Anna Linnenberger, Graham Gibson, Steven Serati, and Miles Padgett

We present a holographic optical tweezers system capable of position clamping multiple particles. Moving an optical trap in response to the trapped object’s motion is a powerful technique for optical control and force measurement. We have now realised this experimentally using a Boulder Nonlinear Systems Spatial Light Modulator (SLM) with a refresh rate of 203Hz. We obtain a reduction of 44% in the variance of the bead’s position, corresponding to an increase in effective trap stiffness of 77%. This reduction relies on the generation of holograms at high speed. We present software capable of calculating holograms in under 1ms using a graphics processor unit.

Micro Manipulation with Optical Responsive Cholesteric and Compensated Nematic Liquid Crystal

Abu Kausar; Hiroto Nagano; Soichiro Okada; Tomonari Ogata; Seiji Kurihara

We have developed photoresponsive liquid crystalline films (cholesteric liquid crystal and compensated nematic liquid crystal) having azobenzene compound to manipulate micro particles on the surface by light irradiation. When few solid particles (polystyrene bead) were placed on the surface of the film, movement of the solid particles was observed upon irradiation of UV (365 nm) or visible light (>420 nm). Only rotational motion of the particles was observed on the cholesteric liquid crystal film. Both translation and rotation was observed on the compensated nematic liquid crystal film. So the mode of motion (either rotation or both translation and rotation) was changed by changing the film from cholesteric film to compensated liquid crystal film. The rotational direction of the solid was controlled by using chiral azobenzene compound with right or left handed twisting ability.

Tuesday, November 24, 2009

Forces between Blank Surfaces As Measured by the Colloidal Probe Technique and by Optical Tweezers − A Comparison

Mahdy M. Elmahdy, Astrid Drechsler, Christof Gutsche, Alla Synytska, Petra Uhlmann, Friedrich Kremer and Manfred Stamm

The well-established atomic force microscopy (AFM)-based colloidal probe technique (CPT) and optical tweezers (OT) are combined to measure the interaction forces between blank SiO2 surfaces in aqueous ionic solutions (CaCl2) of varying concentration at pH 7. Spherical colloids (SiO2, diameter 4.63 ± 0.05 μm) taken out of the same batch are used by both methods. In the case of CPT, a single colloid is glued to a cantilever, and the interaction forces with a plain SiO2 surface are determined in dependence on the concentration of the surrounding medium. For the OT studies, two colloids (one fixed to a micropipet by capillary action, the other held with the optical trap) are approached to each other in nanometer steps, and the resulting forces are measured for the same media as in the CPT experiment. Both techniques fit well to each other and enable one to cover interaction energies ranging from 10−5 to 1 mN/m. The experimental data are well described by the Derjaguin−Landau−Verwey−Overbeek (DLVO) theory revealing that the effective surface charge density changes slightly with concentration.

Determination of cell elasticity through hybrid ray optics and continuum mechanics modeling of cell deformation in the optical stretcher

Andrew E. Ekpenyong, Carolyn L. Posey, Joy L. Chaput, Anya K. Burkart, Meg M. Marquardt, Timothy J. Smith, and Michael G. Nichols

The optical stretcher is a dual-beam trap capable of stretching individual cells. Previous studies have used either ray-or wave-optical models to compute the optical pressure on the surface of a spherical cell. We have extended the ray-optics model to account for focusing by the spherical interface and the effects of multiple internal reflections. Simulation results for red-blood cells (RBCs) show that internal reflections can lead to significant perturbation of the deformation, leading to a systematic error in the determination of cellular elasticity. Calibration studies show excellent agreement between the predicted and measured escape force, and RBC stiffness measurements are consistent with literature values. Measurements of the elasticity of murine osteogenic cells reveal that these cells are approximately 5.4 times stiffer than RBCs.

Microfluidic sorting system based on optical force switching

S.-K. Hoi, C. Udalagama, C.-H. Sow, F. Watt and A. A. Bettiol

We report a versatile, and automatic method for sorting cells and particles in a three dimensional polydimethylsiloxane (PDMS) structure consisting of two cross-microchannels. As microspheres or yeast cells are fed continuously into a lower channel, a line shaped focused laser beam is applied (perpendicular to the direction of flow) at the crossing junction of the two channels. The scattering force of the laser beam was employed to push microparticles matching specific criteria upwards from one channel to another. The force depends on the intrinsic properties of the particles such as their refractive index and size, as well as the laser power and the fluid flow speed. The combination of these parameters gives a tunable selection criterion for the effective and efficient sorting of the particles. The introduction of the cylindrical lens into the optical train allows for simultaneous manipulation of multiple particles which has significantly increased the efficiency and throughput of the sorting. A high aspect ratio microchannel (A.R.=1.6) was found to enhance the sorting performance of the device. By careful control of the microparticle flow rate, near 100% sorting efficiency was achieved.

Forces of Interaction between Poly(2-vinylpyridine) Brushes As Measured by Optical Tweezers

Mahdy M. Elmahdy, Alla Synytska, Astrid Drechsler, Christof Gutsche, Petra Uhlmann, Manfred Stamm and Friedrich Kremer

Forces of interaction within single pairs of poly(2-vinylpyridine) (P2VP) grafted colloids have been measured by optical tweezers (OT) with an extraordinary resolution of ±0.5 pN. Parameters to be varied are the concentration and type of salt (KCl, CaCl2, and LaCl3) of the surrounding medium as well as its pH. The observed force−distance relation is quantitativelydescribed by the Jusufi model [Colloid Polym. Sci. 2004, 282, 910−917] for spherical polyelectrolyte brushes which takes into account the entropic effect of the counterions and enables one to estimate the ionic concentration inside the brush. The transition from an osmotic to the salted brush regime is analyzed in detail. For the scaling of the brush height a power law is found having an exponent of 0.24 ± 0.01, which ranges between the values expected for spherical and planar brushes. At pH 4 a strong transition from a brush to a pancake conformation takes place.

Light-Induced Nonlinear Rotations of Nematic Liquid Crystal Droplets Trapped in Laser Tweezers

Etienne Brasselet, Tadas Balciunas; Naoki Murazawa; Saulius Juodkazis; Hiroaki Misawa

We report on optically induced rotations of nematic liquid crystal droplets. We show experimentally that bipolar nematic liquid crystal droplets trapped in laser tweezers having circular polarization can exhibit nonlinear rotational motion, unlike optically trapped solid birefringent micro-plates. Such nonlinear rotations are retrieved by analyzing the polarization dynamics analysis of the light beam after it has passed through the droplet. The occurrence of complex dynamics is only found for large enough trapping power and droplet diameter. The analogy with optically induced nonlinear rotations in nematic liquid crystal films is briefly discussed.

Thursday, November 19, 2009

Precise balancing of viscous and radiation forces on a particle in liquid-filled photonic bandgap fiber

T. G. Euser, M. K. Garbos, J. S. Y. Chen, and P. St.J. Russell

A great challenge in microfluidics is the precise control of laser radiation forces acting on single particles or cells, while allowing monitoring of their optical and chemical properties. We show that, in the liquid-filled hollow core of a single-mode photonic crystal fiber, a micrometer-sized particle can be held stably against a fluidic counterflow using radiation pressure and can be moved to and fro (over tens of centimeters) by ramping the laser power up and down. Accurate studies of the microfluidic drag forces become possible, because the particle is trapped in the center of the single guided optical mode, resulting in highly reproducible radiation forces. The counterflowing liquid can be loaded with sequences of chemicals in precisely controlled concentrations and doses, making possible studies of single particles, vesicles, or cells.

Tuesday, November 17, 2009

Trapping double negative particles in the ray optics regime using optical tweezers with focused beams

Leonardo A. Ambrosio and H. E. Hernández-Figueroa

The capabilities of optical tweezers to trap DNG (double negative) spherical particles, with both negative permittivity and permeability, are explored in detail by analyzing some interesting theoretical features not seeing in conventional DPS (double positive) particles possessing positive refractive index. The ray optics regime is adopted and, although this regime is quite simple and limited, its validity is already known and tested for DPS particles such as biological cells and molecules trapped by highly focused beams. Simulation results confirm that even for ray optics, DNG particles present unusual and interesting trapping characteristics.

Highly birefringent vaterite microspheres: production, characterization and applications for optical micromanipulation

Simon J. Parkin, Robert Vogel, Martin Persson, Maren Funk, Vincent L. Loke, Timo A. Nieminen, Norman R. Heckenberg, and Halina Rubinsztein-Dunlop

This paper reports on a simple synthesis and characterization of highly birefringent vaterite microspheres, which are composed of 20–30 nm sized nanocrystalls. Scanning electron microscopy shows a quite disordered assembly of nanocrystals within the microspheres. However, using optical tweezers, the effective birefringence of the microspheres was measured to be Δn = 0.06, which compares to Δn = 0.1 of vaterite single crystals. This suggests a very high orientation of the nanocrystals within the microspheres. A hyperbolic model of the direction of the optical axis throughout the vaterite spherulite best fits the experimental data. Results from polarized light microscopy further confirm the hyperbolic model.

Monday, November 16, 2009

Forces in nematic liquid crystals: from nanoscale interfacial forces to long-range forces in nematic colloids

Igor Muscaroneviccaron

In this paper we give an overview of experiments that provided an insight into the nature of forces between surfaces and objects in a nematic liquid crystal. These forces, also called 'structural forces', are the consequence of the long-range orientational order and orientational elasticity of nematic liquid crystals. Owing to their fundamental as well as technological importance, forces between objects in liquid crystals have been a subject of growing interest during the last decade. Experimental observations and studies of structural forces are described from nanoscale interfacial forces, measured by an atomic force microscope, to the micro-scale forces between colloidal particles in nematics, studied by laser tweezers and optical video microscopy.

An optical-manipulation technique for cells in physiological flows

Hu Zhang, Neng H. Chen, Alicia El Haj and Kuo-Kang Liu

We have developed a technique to manipulate human red blood cells (RBCs) in hydrodynamic flows. This method applies optical tweezers to trap and move microbead-attached RBCs in a liquid medium at various speeds, while it significantly minimizes laser heating and photon-induced stress for normal operation with laser-trapped cells. Computational fluid dynamics is applied to simulate flow-induced shear stress over the cell membrane and to correlate quantitatively the forces with the cell deformations. RBCs can be manipulated under physiological conditions by this approach, which may open an avenue to design principles for the next generation of cell sorting and delivery.

High-resolution, long-term characterization of bacterial motility using optical tweezers

Taejin L Min, Patrick J Mears, Lon M Chubiz, Christopher V Rao, Ido Golding & Yann R Chemla

We present a single-cell motility assay, which allows the quantification of bacterial swimming in a well-controlled environment, for durations of up to an hour and with a temporal resolution greater than the flagellar rotation rates of 100 Hz. The assay is based on an instrument combining optical tweezers, light and fluorescence microscopy, and a microfluidic chamber. Using this device we characterized the long-term statistics of the run-tumble time series in individual Escherichia coli cells. We also quantified higher-order features of bacterial swimming, such as changes in velocity and reversals of swimming direction.

Parallel trapping of multiwalled carbon nanotubes with optoelectronic tweezers

Peter J. Pauzauskie, Arash Jamshidi, Justin K. Valley, Joe H. Satcher, Jr., and Ming C. Wu

Here we report the use of optoelectronic tweezers and dynamic virtual electrodes to address multiwalled carbon nanotubes (MWCNTs) with trap stiffness values of approximately 50 fN/µm. Both high-speed translation (>200 µm/s) of individual-MWCNTsand two-dimensional trapping of MWCNT ensembles are achieved using 100,000 times less optical power density than single beam laser tweezers. Modulating the virtual electrode's intensity enables tuning of the MWCNT ensemble's number density by an order of magnitude on the time scale of seconds promising a broad range of applications in MWCNT science and technology.

Amplitude and frequency spectra of thermal fluctuations of a translocating RNA molecule

Henk Vocks, Debabrata Panja and Gerard T Barkema

Using a combination of theory and computer simulations, we study the translocation of an RNA molecule, pulled through a solid-state nanopore by an optical tweezer, as a method for determining its secondary structure. The resolution with which the elements of the secondary structure can be determined is limited by thermal fluctuations. We present a detailed study of these thermal fluctuations, including the frequency spectrum, and show that these rule out single-nucleotide resolution under the experimental conditions which we simulated. Two possible ways to improve this resolution are strongly stretching the RNA with a back-pulling voltage across the membrane, and stiffening the translocated part of the RNA by biochemical means.

Friday, November 13, 2009

Multiple traps created with an inclined dual-fiber system

Yuxiang Liu and Miao Yu

Multiple optical traps allow one to manipulate multiple particles simultaneously, to characterize interactions in colloidal systems, and to assemble particles into complex structures. Most of the current multiple optical traps are realized with microscope objective-based optical tweezers, which are bulky in size. In this article, we created multiple optical traps with an inclined dual-fiber optical tweezers setup. One 3D trap and two 2D traps were formed at different vertical levels with adjustable separations and positions. We demonstrated that this fiber-based trapping system can be used as a simple block to perform multiple functions, such as particle grouping, separation, and stacking. Moreover, we found that multiple beads can be trapped and stacked up in three dimensions. Compared with those formed with objective-based optical tweezers, the multiple traps presented here are small in size and independent of the objective or the substrate, and hence hold the promise to be integrated in microfluidic systems. This fiber-based multiple traps can be used for on-chip parallel manipulation, particle separation, and characterization of interactions of colloidal and biological systems.

Singular optical manipulation of birefringent elastic media using nonsingular beams

Etienne Brasselet

It is shown that nonsingular light beams can generate singular birefringent patterns in homogeneous birefringent elastic media. These orientational defects of the optical-axis spatial distribution originate from an optical torque driven by a nonzero longitudinal field component. Singular radial and spin-dependent azimuthal light-induced elastic distortion patterns are described and experimentally observed in a uniform liquid-crystal film in the course of a focused circularly polarized Gaussian beam.

Stable optical trapping of latex nanoparticles with ultrashort pulsed illumination

Arijit Kumar De, Debjit Roy, Aveek Dutta, and Debabrata Goswami

Here we report how ultrafast pulsed illumination at low average power results in a stable three-dimensional (3D) optical trap holding latex nanoparticles which is otherwise not possible with continuous wave lasers at the same power level. The gigantic peak power of a femtosecond pulse exerts a huge instantaneous gradient force that has been predicted theoretically earlier and implemented for microsecond pulses in a different context by others. In addition, the resulting two-photon fluorescence allows direct observation of trapping events by providing intrinsic 3D resolution.

Experimental analysis of recoil effects induced by fluorescence photons

Alexander Zhdanov, Satish Rao, Andrey Fedyanin, and Dmitri Petrov

The momentum transfer to a scatterer from fluorescence photons was detected using an optical system that permits one to simultaneously measure the radiation force exerted on and fluorescence emission from the scatterer. The core of this technique is a partially metal covered dielectric bead optically trapped in a liquid with dye molecules. Fluorescence emission from the volume that includes the bead is measured simultaneously with the Brownian motion of the bead. The perturbed motion of the bead is a result of photon momentum transfer from the fluorescence of the dye to the trapped scatterer. The bead position fluctuations indicate the presence of the fluorescence and its bleaching nature. The results demonstrate the capability of the photonic force microscopy technique to be a complement to spectroscopy in the study of optical processes.

Physical methods and molecular biology

I. N. Serdyuk

The review is devoted to describing the current state of physical and chemical methods used for studying the structural and functional bases of vital processes. Special attention is focused on the physical methods that have opened a new page in the research on the structure of biological macromolecules. They include primarily the methods of detecting and manipulating single molecules using optical and magnetic tweezers. New physical methods, such as 2D infrared spectroscopy, fluorescence correlation spectroscopy, and magnetic resonance microscopy are also analyzed briefly in the review. The path that physics and biology have passed for the last 55 years shows that there is no single method providing all necessary information on macromolecules and their interactions. Each method provides its view of the system in space and time. All physical methods are complementary. It is complementarity that is the fundamental idea justifying the existence in practice of all physical methods the description of which was the aim of the review.

Thursday, November 12, 2009

Translocation of RecA-Coated Double-Stranded DNA through Solid-State Nanopores

R. M. M. Smeets, S. W. Kowalczyk, A. R. Hall, N. H. Dekker and C. Dekker

We report translocation of double-stranded DNA (dsDNA) molecules that are coated with RecA protein through solid-state nanopores. Translocation measurements show current-blockade events with a wide variety in time duration (10^−4−10^−1 s) and conductance blockade values (3−14 nS). Large blockades (11.4 ± 0.7 nS) are identified as being caused by translocations of RecA−dsDNA filaments. We confirm these results through a variety of methods, including changing molecular length and using an optical tweezer system to deliver bead-functionalized molecules to the nanopore. We further distinguish two different regimes of translocation: a low-voltage regime (less than 150 mV) in which the event rate increases exponentially with voltage, and a high-voltage regime in which it remains constant. Our results open possibilities for a variety of future experiments with (partly) protein-coated DNA molecules, which is interesting for both fundamental science and genomic screening applications.

Irregular spin angular momentum transfer from light to small birefringent particles

M. Rothmayer, D. Tierney, E. Frins, W. Dultz, and H. Schmitzer

The transfer of spin angular momentum from photons to small particles is a key experiment of quantum physics. The particles rotate clockwise or counterclockwise depending on the polarization of the light beam which holds them in an optical trap. We show that even perfectly disk shaped particles will in general not rotate with a constant angular speed. The particles will periodically accelerate and decelerate their rotational motion due to a varying spin angular momentum transfer from the light. Using the Poincaré sphere we derive the equation of motion of a birefringent plate and verify the results by measuring the time dependent rotation of small crystals of Hg(I) iodide and 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) in the trap of polarized optical tweezers. For small ellipticities of the polarized light in the tweezers the plate stops in a fixed orientation relative to the axes of the light ellipse. We discuss the origin of this halt and propose an application of small birefringent plates as self-adjusting optical retarders in micro-optics.

Nanomechanics of biomolecules: focus on DNA

Y. Eugene Pak, Dae Shick Kim, Mohana Marimuthu and Sanghyo Kim

Nano-mechanical measurements and manipulations at the single-cell and single-molecular levels using the atomic force microscope (AFM) and optical tweezers are presenting fascinating opportunities to the researchers in bioscience and biotechnology. Single molecule biophysics technologies, due to their capability to detect transient states of molecules and biomolecular complexes, are the methods of choice for studies in DNA structure and dynamics, DNA-DNA and DNA-protein interactions, and viral DNA packaging. The aim of this review is to describe the recent developments of scientific tools and the knowledge gained in single molecule DNA mechanics such as DNA elasticity, electrostatics, condensation and interactions of DNA with surrounding fluids during its hydrodynamic flow.

Elimination of a zero-order beam induced by a pixelated spatial light modulator for holographic projection

Hao Zhang, Jinghui Xie, Juan Liu, and Yongtian Wang

A technique is proposed theoretically and verified experimentally to eliminate a zero-order beam caused by a pixelated phase-only spatial light modulator (SLM) for holographic projection. The formulas for determination of the optical field in the Fourier plane are deduced, and the influence of the pixelated structure of a SLM on the intensity of the zero-order beam is numerically investigated. Two currently existing techniques are studied and a new one is presented. These three techniques are used separately to eliminate the zero-order interruption, and the optical performances of the reconstructed patterns are compared. The new technique results in higher reconstruction quality and diffraction efficiency. A short animated movie is illuminated for holographic projection display. The experimental results show that the zero-order beam can be efficiently eliminated by the new technique. It is believed that this technique can be used in various optical systems that are based on pixelated phase-only SLMs, such as holographic optical tweezers and optical testing systems.

Monitoring of Swelling and Degrading Behavior of Alginate Beads using Optical Tweezers

Lan Jin, You-Chan Hong, Jin-Woo Pyo, Hanwook Song, Ji Yoon Kang, Sang Woo Lee, Dae Sung Yoon, Beop-Min Kim & Kwangsoo No

Calcium alginate beads are widely used in drug delivery studies due to their high biocompatibility and the simple gelatinization process. It is well known that the alginate bead size changes in solutions with time, increasing initially and decreasing at a later stage. Therefore, it is essential to monitor or predict the size change of the beads since it affects the drug delivery efficiency significantly. We used the optical tweezers, a non-contact method, to investigate the temporal changes of the alginate beads in solutions instead of using the traditional drying and weighing technique. Responses to alginate concentration or external stimuli such as pH were also studied. The power spectrum method was utilized to estimate the trapping forces on the beads, which is related to the particle size changes. The results of our experiment indicate that the optical tweezers technique can continuously monitor the swelling and degrading of an alginate bead in an aqueous medium over hours which poses a high potential for drug encapsulation and release efficiency studies in the future.

DNA based molecular motors

Jens Michaelis, Adam Muschielok, Joanna Andrecka, Wolfgang Kügel and Jeffrey R. Moffitt

Most of the essential cellular processes such as polymerisation reactions, gene expression and regulation are governed by mechanical processes. Controlled mechanical investigations of these processes are therefore required in order to take our understanding of molecular biology to the next level. Single-molecule manipulation and force spectroscopy have over the last 15 years been developed into extremely powerful techniques. Applying these techniques to the investigation of proteins and DNA molecules has led to a mechanistic understanding of protein function on the level of single molecules. As examples for DNA based molecular machines we will describe single-molecule experiments on RNA polymerases as well as on the packaging of DNA into a viral capsid—a process that is driven by one of the most powerful molecular motors.

Tuesday, November 10, 2009

Thermo-optical resonance locking of an optically trapped salt-water microdroplet

Marc Guillon, Rachael E H Miles, Jonathan P Reid and David McGloin

We demonstrate that it is possible to lock the radius of an optically trapped salt-water microdroplet to then=1 whispering gallery resonances (WGRs) at the trapping laser wavelength. The optical properties of the droplet are determined using stimulated Raman scattering. The droplet is in thermodynamic equilibrium with the surrounding vapour and the proposed locking mechanism consists of a balance between bulk heating and WGR heating. Raman measurements allow the size parameter (nka) of the droplet to be determined with a precision of ~10−5 and the resonance linewidth to be estimated.

Monday, November 9, 2009

Optical position clamping with predictive control

Heikki Ojala, Anders Korsbäck, Anders E. Wallin, and Edward Hæggström

We increase the effective stiffness of optical tweezers by position clamping a polystyrene bead with a predictive feedback control algorithm. This algorithm mitigates the effect of feedback loop delay. Hence, higher gain than with proportional control can be employed, which results in higher effective trap stiffness, without trap instability. In experiments (initial trap stiffness 0.056 pN/nm with a 1.78 µm diameter polystyrene bead), predictive control increased the effective trap stiffness by 55% relative to proportional control. We also derive theoretical expressions for the power spectra of the bead position controlled by our algorithm.

The αβ T Cell Receptor Is an Anisotropic Mechanosensor

Sun Taek Kim, Koh Takeuchi, Zhen-Yu J. Sun, Maki Touma, Carlos E. Castro, Amr Fahmy, Matthew J. Lang, Gerhard Wagner and Ellis L. Reinherz

Thymus-derived lymphocytes protect mammalian hosts against virus- or cancer-related cellular alterations through immune surveillance, eliminating diseased cells. In this process, T cell receptors (TCRs) mediate both recognition and T cell activation via their dimeric αβ, CD3ϵγ, CD3ϵδ, and CD3ζζ subunits using an unknown structural mechanism. Here, site-specific binding topology of anti-CD3 monoclonal antibodies (mAbs) and dynamic TCR quaternary change provide key clues. Agonist mAbs footprint to the membrane distal CD3ϵ lobe that they approach diagonally, adjacent to the lever-like Cβ FG loop that facilitates antigen (pMHC)-triggered activation. In contrast, a non-agonist mAb binds to the cleft between CD3ϵ and CD3γ in a perpendicular mode and is stimulatory only subsequent to an external tangential but not a normal force (∼50 piconewtons) applied via optical tweezers. Specific pMHC but not irrelevant pMHC activates a T cell upon application of a similar force. These findings suggest that the TCR is an anisotropic mechanosensor, converting mechanical energy into a biochemical signal upon specific pMHC ligation during immune surveillance. Activating anti-CD3 mAbs mimic this force via their intrinsic binding mode. A common TCR quaternary change rather than conformational alterations can better facilitate structural signal initiation, given the vast array of TCRs and their specific pMHC ligands.

Monday, November 2, 2009

Measurement of axial and transverse trapping stiffness of optical tweezers in air using a radially polarized beam

Masaki Michihata, Terutake Hayashi, and Yasuhiro Takaya

The trapping efficiency and stiffness of optical tweezers using radial polarization are evaluated; the ray-tracing method and a proposed measurement method are used for numerical and experimental analyses, respectively. The maximum axial trapping efficiency with radial polarization is 1.84 times that with linear polarization, while the maximum transverse trapping efficiency decreases by 0.58 times. Further, the axial and transverse trapping efficiencies are found to be 1.19 times larger and 0.83 times smaller, respectively, than the values with linear polarization. From the experiments, the axial and transverse stiffness values are 1.2 times larger and 0.8 times smaller, respectively, with radial polarization. Hence, radial polarization enhances the axial trapping properties while reducing the transverse trapping properties.

Rotation Detection in Light-Driven Nanorotors

P. H. Jones, F. Palmisano, F. Bonaccorso, P. G. Gucciardi, G. Calogero, A. C. Ferrari and O. M. Marago

We analyze the rotational dynamics of light driven nanorotors, made of nanotube bundles and gold nanorods aggregates, with nonsymmetric shapes, trapped in optical tweezers. We identify two different regimes depending on dimensions and optical properties of the nanostructures. These correspond to alignment with either the laser propagation axis or the dominant polarization direction, or rotational motions caused by either unbalanced radiation pressure or polarization torque. By analyzing the motion correlations of the trapped nanostructures, we measure with high accuracy both the optical trapping parameters and the rotation frequency induced by the radiation pressure. Our results pave the way to improved all-optical detection, control over rotating nanomachines, and rotation detection in nano-optomechanics.