Lukáš Chvátal, Oto Brzobohatý, Pavel Zemánek
We present theoretical results related to the optical binding of two nanoparticles (NPs) in a standing wave created by a retro-reflected wide Gaussian beam. Recent experimental results demonstrated that this geometry enables easy confinement and spatial self-arrangement of NPs. Since the NPs are not usually of the same size, we investigate the influence of variations in NPs size on their stable spatial confinement in this type of optical trap.
DOI
Wednesday, March 4, 2015
Prediction of metallic nano-optical trapping forces by finite element-boundary integral method
Xiao-Min Pan, Kai-Jiang Xu, Ming-Lin Yang, and Xin-Qing Sheng
The hybrid of finite element and boundary integral (FE-BI) method is employed to predict nano-optical trapping forces of arbitrarily shaped metallic nanostructures. A preconditioning strategy is proposed to improve the convergence of the iterative solution. Skeletonization is employed to speed up the design and optimization where iteration has to be repeated for each beam configuration. The radiation pressure force (RPF) is computed by vector flux of the Maxwell’s stress tensor. Numerical simulations are performed to validate the developed method in analyzing the plasmonic effects as well as the optical trapping forces. It is shown that the proposed method is capable of predicting the trapping forces of complex metallic nanostructures accurately and efficiently.
DOI
The hybrid of finite element and boundary integral (FE-BI) method is employed to predict nano-optical trapping forces of arbitrarily shaped metallic nanostructures. A preconditioning strategy is proposed to improve the convergence of the iterative solution. Skeletonization is employed to speed up the design and optimization where iteration has to be repeated for each beam configuration. The radiation pressure force (RPF) is computed by vector flux of the Maxwell’s stress tensor. Numerical simulations are performed to validate the developed method in analyzing the plasmonic effects as well as the optical trapping forces. It is shown that the proposed method is capable of predicting the trapping forces of complex metallic nanostructures accurately and efficiently.
DOI
Tuesday, March 3, 2015
Direct measurement of axial optical forces
Gregor Thalhammer, Lisa Obmascher, and Monika Ritsch-Marte
Direct measurement of optical forces based on recording the change of momentum between the in- and outgoing light does not have specific requirements on particle size or shape, or on beam shape. Thus this approach overcomes many of the limitations of force measurements based on position measurements, which require frequent calibration. In this work we validate the achievable accuracy for direct force measurements in the axial direction for a single beam optical tweezers setup, based on numerical simulations and experimental investigations of situations, where the true force is known. We find that for typical experimental situations a good accuracy with an error of less than 1 % of the maximum force can be achieved, independent of particle size or refractive index, provided that the total amount of light scattered in the backward direction is also taken into account, which is easy to accomplish experimentally. Due to the inherent particle shape independence of the direct force measurement method, these findings support that it provides accurate results for 3D force measurements for particles of arbitrary shape.
DOI
Direct measurement of optical forces based on recording the change of momentum between the in- and outgoing light does not have specific requirements on particle size or shape, or on beam shape. Thus this approach overcomes many of the limitations of force measurements based on position measurements, which require frequent calibration. In this work we validate the achievable accuracy for direct force measurements in the axial direction for a single beam optical tweezers setup, based on numerical simulations and experimental investigations of situations, where the true force is known. We find that for typical experimental situations a good accuracy with an error of less than 1 % of the maximum force can be achieved, independent of particle size or refractive index, provided that the total amount of light scattered in the backward direction is also taken into account, which is easy to accomplish experimentally. Due to the inherent particle shape independence of the direct force measurement method, these findings support that it provides accurate results for 3D force measurements for particles of arbitrary shape.
DOI
Effect of the object 3D shape on the viscoelastic testing in optical tweezers
Lingyao Yu and Yunlong Sheng
Viscoelastic testing of biological cells has been performed with the optical tweezers and stretcher. Historically, the cells were modeled by the spring-dashpot network or the power-law models, which can however characterize only the homogeneous, isotropic viscoelastic material, but not the 3D cell itself. Our mechanical and finite element analyses show that the cell elongations are different significantly for different cell 3D shapes in the creep testing. In the dynamic testing the loss tangent, which is measurable directly in the experiment, is not sensitive to the cell shape. However, the stress-strain hysteresis loop still depends on the cell 3D shape.
DOI
Viscoelastic testing of biological cells has been performed with the optical tweezers and stretcher. Historically, the cells were modeled by the spring-dashpot network or the power-law models, which can however characterize only the homogeneous, isotropic viscoelastic material, but not the 3D cell itself. Our mechanical and finite element analyses show that the cell elongations are different significantly for different cell 3D shapes in the creep testing. In the dynamic testing the loss tangent, which is measurable directly in the experiment, is not sensitive to the cell shape. However, the stress-strain hysteresis loop still depends on the cell 3D shape.
DOI
Plasmonic optical trapping of soft nanomaterials such as polymer chains and DNA: micro-patterning formation
Tatsuya Shoji, Yasuyuki Tsuboi
Localized surface plasmons exert a strong radiation force on nanoparticles in the vicinity of noble metal nanostructures, resulting in optical trapping. Such plasmon-based optical trapping is one of the hot topics in the field of nanophotonics and can be applied to molecular manipulation techniques. In this review paper, we describe the plasmon-based optical trapping of polymer chains and DNA. In addition, we describe the future outlook for this trapping method.
DOI
Localized surface plasmons exert a strong radiation force on nanoparticles in the vicinity of noble metal nanostructures, resulting in optical trapping. Such plasmon-based optical trapping is one of the hot topics in the field of nanophotonics and can be applied to molecular manipulation techniques. In this review paper, we describe the plasmon-based optical trapping of polymer chains and DNA. In addition, we describe the future outlook for this trapping method.
DOI
Molecular Weight Characterization of Globular Proteins using Optical Nanotweezers
Skyler Wheaton and Reuven Gordon
We trap a set of molecular weight standard globular proteins using a double nanohole optical trap. The root mean squared variation of the trapping laser transmission intensity gives a linear dependence with the molecular weight, showing the potential for analysis of globular proteins. The characteristic time of the autocorrelation of the trapping laser intensity variations scales with a -2/3 power dependence with the volume of the particle. A hydrodynamic laser tweezer model is used to explain these dependencies. Since this is a single particle technique that operates in solution and can be used to isolate an individual particle, we believe that it provides an interesting alternative to existing analysis methods and shows promise to expand the capabilities of protein related studies to the single particle level.
DOI
We trap a set of molecular weight standard globular proteins using a double nanohole optical trap. The root mean squared variation of the trapping laser transmission intensity gives a linear dependence with the molecular weight, showing the potential for analysis of globular proteins. The characteristic time of the autocorrelation of the trapping laser intensity variations scales with a -2/3 power dependence with the volume of the particle. A hydrodynamic laser tweezer model is used to explain these dependencies. Since this is a single particle technique that operates in solution and can be used to isolate an individual particle, we believe that it provides an interesting alternative to existing analysis methods and shows promise to expand the capabilities of protein related studies to the single particle level.
DOI
Monday, March 2, 2015
Red Blood Cell Aging During Storage, Studied Using Optical Tweezers Experiment
Justyna Czerwinska, Stefan Michael Wolf, Hanieh Mohammadi, Sylvia Jeney
This paper presents experimental and numerical studies of erythrocyte stretching, with a focus on the aging of red blood cells in an in vitro environment during storage. The experimental studies were performed using optical tweezers. The laser beam was used to pull and stretch a cell sedimented on a flat surface. A force calibration was obtained via a comparison of the experimental data with results from finite element simulations of the cell stretching. The experiments were performed using blood samples from blood bank donations made by three donors. The experiments were performed over 21 days of storage, and the estimate erythrocyte membrane shear modulus during this period increased from 2.5 to 13 μN/m.
DOI
This paper presents experimental and numerical studies of erythrocyte stretching, with a focus on the aging of red blood cells in an in vitro environment during storage. The experimental studies were performed using optical tweezers. The laser beam was used to pull and stretch a cell sedimented on a flat surface. A force calibration was obtained via a comparison of the experimental data with results from finite element simulations of the cell stretching. The experiments were performed using blood samples from blood bank donations made by three donors. The experiments were performed over 21 days of storage, and the estimate erythrocyte membrane shear modulus during this period increased from 2.5 to 13 μN/m.
DOI
A balance between membrane elasticity and polymerization energy sets the shape of spherical clathrin coats
Mohammed Saleem, Sandrine Morlot, Annika Hohendahl, John Manzi, Martin Lenz & Aurélien Roux
In endocytosis, scaffolding is one of the mechanisms to create membrane curvature by moulding the membrane into the spherical shape of the clathrin cage. However, the impact of membrane elastic parameters on the assembly and shape of clathrin lattices has never been experimentally evaluated. Here, we show that membrane tension opposes clathrin polymerization. We reconstitute clathrin budding in vitro with giant unilamellar vesicles (GUVs), purified adaptors and clathrin. By changing the osmotic conditions, we find that clathrin coats cause extensive budding of GUVs under low membrane tension while polymerizing into shallow pits under moderate tension. High tension fully inhibits polymerization. Theoretically, we predict the tension values for which transitions between different clathrin coat shapes occur. We measure the changes in membrane tension during clathrin polymerization, and use our theoretical framework to estimate the polymerization energy from these data. Our results show that membrane tension controls clathrin-mediated budding by varying the membrane budding energy.
DOI
In endocytosis, scaffolding is one of the mechanisms to create membrane curvature by moulding the membrane into the spherical shape of the clathrin cage. However, the impact of membrane elastic parameters on the assembly and shape of clathrin lattices has never been experimentally evaluated. Here, we show that membrane tension opposes clathrin polymerization. We reconstitute clathrin budding in vitro with giant unilamellar vesicles (GUVs), purified adaptors and clathrin. By changing the osmotic conditions, we find that clathrin coats cause extensive budding of GUVs under low membrane tension while polymerizing into shallow pits under moderate tension. High tension fully inhibits polymerization. Theoretically, we predict the tension values for which transitions between different clathrin coat shapes occur. We measure the changes in membrane tension during clathrin polymerization, and use our theoretical framework to estimate the polymerization energy from these data. Our results show that membrane tension controls clathrin-mediated budding by varying the membrane budding energy.
DOI
Ribosome Excursions during mRNA Translocation Mediate Broad Branching of Frameshift Pathways
Shannon Yan, Jin-Der Wen, Carlos Bustamante, Ignacio Tinoco Jr.
Programmed ribosomal frameshifting produces alternative proteins from a single transcript. −1 frameshifting occurs on Escherichia coli’s dnaX mRNA containing a slippery sequence AAAAAAG and peripheral mRNA structural barriers. Here, we reveal hidden aspects of the frameshifting process, including its exact location on the mRNA and its timing within the translation cycle. Mass spectrometry of translated products shows that ribosomes enter the −1 frame from not one specific codon but various codons along the slippery sequence and slip by not just −1 but also −4 or +2 nucleotides. Single-ribosome translation trajectories detect distinctive codon-scale fluctuations in ribosome-mRNA displacement across the slippery sequence, representing multiple ribosomal translocation attempts during frameshifting. Flanking mRNA structural barriers mechanically stimulate the ribosome to undergo back-and-forth translocation excursions, broadly exploring reading frames. Both experiments reveal aborted translation around mutant slippery sequences, indicating that subsequent fidelity checks on newly adopted codon position base pairings lead to either resumed translation or early termination.
DOI
Programmed ribosomal frameshifting produces alternative proteins from a single transcript. −1 frameshifting occurs on Escherichia coli’s dnaX mRNA containing a slippery sequence AAAAAAG and peripheral mRNA structural barriers. Here, we reveal hidden aspects of the frameshifting process, including its exact location on the mRNA and its timing within the translation cycle. Mass spectrometry of translated products shows that ribosomes enter the −1 frame from not one specific codon but various codons along the slippery sequence and slip by not just −1 but also −4 or +2 nucleotides. Single-ribosome translation trajectories detect distinctive codon-scale fluctuations in ribosome-mRNA displacement across the slippery sequence, representing multiple ribosomal translocation attempts during frameshifting. Flanking mRNA structural barriers mechanically stimulate the ribosome to undergo back-and-forth translocation excursions, broadly exploring reading frames. Both experiments reveal aborted translation around mutant slippery sequences, indicating that subsequent fidelity checks on newly adopted codon position base pairings lead to either resumed translation or early termination.
DOI
Kinesin-8 Motors Improve Nuclear Centering by Promoting Microtubule Catastrophe
Matko Glunčić, Nicola Maghelli, Alexander Krull, Vladimir Krstić, Damien Ramunno-Johnson, Nenad Pavin, and Iva M. Tolić
In fission yeast, microtubules push against the cell edge, thereby positioning the nucleus in the cell center. Kinesin-8 motors regulate microtubule catastrophe; however, their role in nuclear positioning is not known. Here we develop a physical model that describes how kinesin-8 motors affect nuclear centering by promoting a microtubule catastrophe. Our model predicts the improved centering of the nucleus in the presence of motors, which we confirmed experimentally in living cells. The model also predicts a characteristic time for the recentering of a displaced nucleus, which is supported by our experiments where we displaced the nucleus using optical tweezers.
DOI
In fission yeast, microtubules push against the cell edge, thereby positioning the nucleus in the cell center. Kinesin-8 motors regulate microtubule catastrophe; however, their role in nuclear positioning is not known. Here we develop a physical model that describes how kinesin-8 motors affect nuclear centering by promoting a microtubule catastrophe. Our model predicts the improved centering of the nucleus in the presence of motors, which we confirmed experimentally in living cells. The model also predicts a characteristic time for the recentering of a displaced nucleus, which is supported by our experiments where we displaced the nucleus using optical tweezers.
DOI
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