Axial Optical Traps: A New Direction for Optical Tweezers

AbstractOptical tweezers have revolutionized our understanding of the microscopic world. Axial optical tweezers, which apply force to a surface-tethered molecule by directly moving either the trap or the stage along the laser beam axis, offer several potential benefits when studying a range of novel biophysical phenomena. This geometry, although it is conceptually straightforward, suffers from aberrations that result in variation of the trap stiffness when the distance between the microscope coverslip and the trap focus is being changed. Many standard techniques, such as back-focal-plane interferometry, are difficult to employ in this geometry due to back-scattered light between the bead and the coverslip, whereas the noise inherent in a surface-tethered assay can severely limit the resolution of an experiment. Because of these complications, precision force spectroscopy measurements have adapted alternative geometries such as the highly successful dumbbell traps. In recent years, however, most of the difficulties inherent in constructing a precision axial optical tweezers have been solved. This review article aims to inform the reader about recent progress in axial optical trapping, as well as the potential for these devices to perform innovative biophysical measurements

An Ultrastable and Dense Single-Molecule Click Platform for Sensing Protein–Deoxyribonucleic Acid Interactions

An ultrastable, highly dense single-molecule assay ideal for observing protein–DNA interactions is demonstrated. Stable click tethered particle motion leverages next generation click-chemistry to achieve an ultrahigh density of surface tethered reporter particles, and has low non-specific interactions, is stable at elevated temperatures to at least 45 °C, and is compatible with Mg2+, an important ionic component of many regulatory protein–DNA interactions. Prepared samples remain stable, with little degradation, for &gt;6 months in physiological buffers. These improvements enable the authors to study previously inaccessible sequence and temperature-dependent effects on DNA binding by the bacterial protein, histone-like nucleoid-structuring protein, a global transcriptional regulator found in Escherichia coli. This greatly improved assay can directly be translated to accelerate existing tethered particle-based, single-molecule biosensing applications.</p

Leaving a covenantal religion: Orthodox Jewish disaffiliation from an immigration psychology perspective

This study explored psychological variables associated with disaffiliation from Orthodox Judaism (a covenantal community), and subsequent wellness. A web-based survey (N = 206) assessed factors previously used to study immigrants: push (distress within origin community), pull (toward destination community), and goal attainment. Psychological wellness, perceived stress, overall health, and loneliness were also assessed. Findings included: (1) strong pull toward opportunities for physical and ideological autonomy; (2) those who experienced more push toward disaffiliation, reported decreased current wellness; (3) goal attainment was associated with increased wellbeing; (4) significant differences in the experiences of disaffiliation between men and women; (5) most who disaffiliated left religion altogether; those who remained religious decreased their participation, and few joined non-Jewish faith communities. Results demonstrate that this immigration paradigm can be adapted to advance research on individuals who disaffiliate from covenantal religious communities

Neuronal Shot Noise and Brownian 1/f21/f^2 Behavior in the Local Field Potential

We demonstrate that human electrophysiological recordings of the local field potential (LFP) from intracranial electrodes, acquired from a variety of cerebral regions, show a ubiquitous $1/f^2$ scaling within the power spectrum. We develop a quantitative model that treats the generation of these fields in an analogous way to that of electronic shot noise, and use this model to specifically address the cause of this $1/f^2$ Brownian noise. The model gives way to two analytically tractable solutions, both displaying Brownian noise: 1) uncorrelated cells that display sharp initial activity, whose extracellular fields slowly decay and 2) rapidly firing, temporally correlated cells that generate UP-DOWN states

Fluctuation conductivity in the presence of columnar defects

Includes bibliographical references (pages [38]-39)Introducing columnar defects into high Tc superconductors by heavy ion irradiation is known to be the most effective way to suppress dissipation in the vortex state. It was found tha t even above Tc columnar defects still have a noticeable influence on the conductivity. To account for this effect we calculated the correction to the fluctuation conductivity due to the defects using a time-dependent Ginzburg-Landau approach, with the defects modeled as cylindrical regions with suppressed Tc. For zero magnetic field and for fields far from Hc2) parallel to the columns, we calculated the induced correction with logarithmic accuracy and found tha t it does not depend on the defect strength in the vicinity of Tc. For fields close to Hc2 and parallel to the columnar, defects we obtained the correction exactly within the lowest Landau level approximation (LLL). We analyzed the dependence of the conductivity on the tilting angle of field within a LLL approximation and found that the defect-induced angular dependence is opposite to the anisotropy-induced dependence. We were also able to obtain a logarithmically accurate dependence of the conductivity for large tilting angles. The angular dependencies found can be used to extract quantitative information about parameters of columnar defects.M.S. (Master of Science

View to the U: An eye on UTM research

This is an audio recording from the podcast series "View to the U: An eye on UTM research".On this episode of VIEW to the U podcast, we continue this season of “Adventures in Research” with Professor Josh Milstein from the Department of Chemical & Physical Sciences at UofT Mississauga and in the Department of Physics at UofT St. George. Josh has a couple of stories to tell about his time in academia, and we also talk about podcasts he’s listening to, books he’s reading, and how the The Big Bang Theory – the TV show, not the actual theory – helped Josh’s mom realize that her son might have a future in science. Prior to coming to UTM, Josh completed his PhD at University of Colorado at Boulder in 2004. He has held a number of prestigious appointments including as a Royal Society Fellow at the University of Oxford from 2004-06, a Sloan-Swartz Research fellowship at the California Institute of Technology from 2006-08, and he was a Senior Research Fellow at the University of Michigan in Ann Arbor from 2008-11. Josh Milstein joined the faculty at UTM/UofT in 2011

Data from: Simulation assisted analysis of the intrinsic stiffness for short DNA molecules imaged with scanning atomic force microscopy

Studying the mechanical properties of short segments of dsDNA can provide insight into various biophysical phenomena, from DNA looping to the organization of nucleosomes. Scanning atomic force microscopy (AFM) is able to acquire images of single DNA molecules with near-basepair resolution. From many images, one may use equilibrium statistical mechanics to quantify the intrinsic stiffness (or persistence length) of the DNA. However, this approach is highly dependent upon both the correct microscopic polymer model and a correct image analysis of DNA contours. These complications have led to significant debate over the flexibility of dsDNA at short length scales. We first show how to extract accurate measures of DNA contour lengths by calibrating to DNA traces of simulated AFM data. After this calibration, we show that DNA adsorbed on an aminopropyl-mica surface behaves as a worm-like chain (WLC) for contour lengths as small as ~20 nm. We also show that a DNA binding protein can modify the mechanics of the DNA from that of a WLC

Simulation Assisted Analysis of the Intrinsic Stiffness for Short DNA Molecules Imaged with Scanning Atomic Force Microscopy.

Studying the mechanical properties of short segments of dsDNA can provide insight into various biophysical phenomena, from DNA looping to the organization of nucleosomes. Scanning atomic force microscopy (AFM) is able to acquire images of single DNA molecules with near-basepair resolution. From many images, one may use equilibrium statistical mechanics to quantify the intrinsic stiffness (or persistence length) of the DNA. However, this approach is highly dependent upon both the correct microscopic polymer model and a correct image analysis of DNA contours. These complications have led to significant debate over the flexibility of dsDNA at short length scales. We first show how to extract accurate measures of DNA contour lengths by calibrating to DNA traces of simulated AFM data. After this calibration, we show that DNA adsorbed on an aminopropyl-mica surface behaves as a worm-like chain (WLC) for contour lengths as small as ~20 nm. We also show that a DNA binding protein can modify the mechanics of the DNA from that of a WLC