Monday, September 29, 2014

Heterogeneous Interaction of SiO2 with N2O5: Aerosol Flow Tube and Single Particle Optical Levitation–Raman Spectroscopy Studies

M. J. Tang, J. C. J. Camp, L. Rkiouak, J. McGregor, I. M. Watson, R. A. Cox, M. Kalberer, A. D. Ward and F. D. Pope

Silica (SiO2) is an important mineral present in atmospheric mineral dust particles, and the heterogeneous reaction of N2O5 on atmospheric aerosol is one of the major pathways to remove nitrogen oxides from the atmosphere. The heterogeneous reaction of N2O5 with SiO2 has only been investigated by two studies previously, and the reported uptake coefficients differ by a factor of >10. In this work two complementary laboratory techniques were used to study the heterogeneous reaction of SiO2 particles with N2O5 at room temperature and at different relative humidities (RHs). The uptake coefficients of N2O5, γ(N2O5), were determined to be (7.2 ± 0.6) × 10–3 (1σ) at 7% RH and (5.3 ± 0.8) × 10–3 (1σ) at 40% RH for SiO2 particles, using the aerosol flow tube technique. We show that γ(N2O5) determined in this work can be reconciled with the two previous studies by accounting for the difference in geometric and BET derived aerosol surface areas. To probe the particle phase chemistry, individual micrometer sized SiO2 particles were optically levitated and exposed to a continuous flow of N2O5 at different RHs, and the composition of levitated particles was monitored online using Raman spectroscopy. This study represents the first investigation into the heterogeneous reactions of levitated individual SiO2 particles as a surrogate for mineral dust. Relative humidity was found to play a critical role: while no significant change of particle composition was observed by Raman spectroscopy during exposure to N2O5 at RH of <2%, increasing the RH led to the formation of nitrate species on the particle surface which could be completely removed after decreasing the RH back to <2%. This can be explained by the partitioning of HNO3 between the gas and adsorbed phases. The atmospheric implications of this work are discussed.


Common intermediates and kinetics, but different energetics, in the assembly of SNARE proteins

Sylvain Zorman, Aleksander A Rebane, Lu Ma, Guangcan Yang, Matthew A Molski, Jeff Coleman, Frederic Pincet, James E Rothman, Yongli Zhang

Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) are evolutionarily conserved machines that couple their folding/assembly to membrane fusion. However, it is unclear how these processes are regulated and function. To determine these mechanisms, we characterized the folding energy and kinetics of four representative SNARE complexes at a single-molecule level using high-resolution optical tweezers. We found that all SNARE complexes assemble by the same step-wise zippering mechanism: slow N-terminal domain (NTD) association, a pause in a force-dependent half-zippered intermediate, and fast C-terminal domain (CTD) zippering. The energy release from CTD zippering differs for yeast (13 kBT) and neuronal SNARE complexes (27 kBT), and is concentrated at the C-terminal part of CTD zippering. Thus, SNARE complexes share a conserved zippering pathway and polarized energy release to efficiently drive membrane fusion, but generate different amounts of zippering energy to regulate fusion kinetics.


Hierarchical organization of chiral rafts in colloidal membranes

Prerna Sharma, Andrew Ward, T. Gibaud, Michael F. Hagan & Zvonimir Dogic

Liquid–liquid phase separation is ubiquitous in suspensions of nanoparticles, proteins and colloids. It has an important role in gel formation, protein crystallization and perhaps even as an organizing principle in cellular biology. With a few notable exceptions, liquid–liquid phase separation in bulk proceeds through the continuous coalescence of droplets until the system undergoes complete phase separation. But when colloids, nanoparticles or proteins are confined to interfaces, surfaces or membranes, their interactions differ fundamentally from those mediated by isotropic solvents, and this results in significantly more complex phase behaviour. Here we show that liquid–liquid phase separation in monolayer membranes composed of two dissimilar chiral colloidal rods gives rise to thermodynamically stable rafts that constantly exchange monomeric rods with the background reservoir to maintain a self-limited size. We visualize and manipulate rafts to quantify their assembly kinetics and to show that membrane distortions arising from the rods’ chirality lead to long-range repulsive raft–raft interactions. Rafts assemble into cluster crystals at high densities, but they can also form bonds to yield higher-order structures. Taken together, our observations demonstrate a robust membrane-based pathway for the assembly of monodisperse membrane clusters that is complementary to existing methods for colloid assembly in bulk suspensions. They also reveal that chiral inclusions in membranes can acquire long-range repulsive interactions, which might more generally have a role in stabilizing assemblages of finite size.


Friday, September 26, 2014

Combined holographic optical trapping and optical image processing using a single diffractive pattern displayed on a spatial light modulator

Alexander Jesacher, Stefan Bernet, and Monika Ritsch-Marte

We demonstrate simultaneous holographic optical trapping and optical image processing using a single-phase diffraction pattern displayed on a liquid crystal spatial light modulator (SLM). The ability of modern SLMs to provide multiorder phase shifts represents a degree of freedom that allows the calculation of diffraction patterns that act in precisely defined but different ways on light beams of different wavelengths. We exploit this property to calculate a single-phase hologram that shapes multiple optical traps at 785 nm while performing double-helix point spread function engineering at 532 nm. Both channels are independent to a large degree and have efficiencies of about 75% compared to the ideal diffractive patterns.


Kinesin processivity is gated by phosphate release

Bojan Milic, Johan O. L. Andreasson, William O. Hancock, and Steven M. Block

Kinesin-1 is a motor protein central to intracellular transport. Prevailing models of the kinesin mechanochemical cycle—which invoke docking of the neck linker domain upon ATP binding—fail to explain the remarkable processivity of kinesin, which represents a competition between dissociation from the microtubule and continuation of the stepping cycle. We show that kinesin dissociation, which characterizes the end of a processive run, is gated by phosphate release following ATP hydrolysis. The structural change driving kinesin motility, likely neck linker docking, is therefore completed only upon hydrolysis. Our results offer insights into gating mechanisms and necessitate revisions to existing models of the kinesin cycle.


Wednesday, September 24, 2014

Rapidly Exploring Random Tree Algorithm-Based Path Planning for Robot-Aided Optical Manipulation of Biological Cells

Tao Ju; Shuang Liu; Jie Yang; Dong Sun

In numerous cellular applications, cells are transported to specific positions or extracted from complex cell solutions. Therefore, an efficient cell transportation path planner for these applications is important for avoiding collisions with other cells or obstacles. In this paper, a path planning approach to transporting cells using a robot-aided optical manipulation system is presented. Optical tweezers functions as a special end-effector in transporting a target cell to the desired position along the generated path. The path planner is designed based on the rapidly exploring random trees (RRT) algorithm for calculating a collision-free path for cell transportation. Both static and dynamic path planners are developed. For the dynamic path planner, an online monitoring strategy is employed to dynamically avoid collisions with randomly appeared obstacles caused by environmental influence such as the Brownian movement of microparticles. Experiments of transporting yeast cells are performed to demonstrate the effectiveness of the proposed approach. Note to Practitioners - Manipulations of cells and other microparticles represent an essential process for most cell-based bioengineering applications, such as cytopathology, cell sociology, and cytotaxonomy. Cell transportation, which is treated as a typical cell manipulation task, has recently received considerable attention because of its wide applications. This paper presents a novel approach to applying RRT-based path planner to cell transportation with a robot-aided optical manipulation system. The research outcome provides a unique solution to achieving cell transportation automatically and efficiently.


Volatility and Oxidative Aging of Aqueous Maleic Acid Aerosol Droplets and the Dependence on Relative Humidity

Benjamin J. Dennis-Smither, Frances H. Marshall, Rachael E. H. Miles, Thomas C. Preston, and Jonathan P. Reid

The microphysical structure and heterogeneous oxidation by ozone of single aerosol particles containing maleic acid (MA) has been studied using aerosol optical tweezers and cavity enhanced Raman spectroscopy. The evaporation rate of MA from aqueous droplets has been measured over a range of relative humidities and the pure component vapor pressure determined to be (1.7 ± 0.2) × 10–3 Pa. Variation in the refractive index (RI) of an aqueous MA droplet with relative humidity (RH) allowed the subcooled liquid RI of MA to be estimated as 1.481 ± 0.001. Measurements of the hygroscopic growth are shown to be consistent with equilibrium model predictions from previous studies. Simultaneous measurements of the droplet composition, size, and refractive index have been made during ozonolysis at RHs in the range 50–80%, providing insight into the volatility of organic products, changes in the droplet hygroscopicity, and optical properties. Exposure of the aqueous droplets to ozone leads to the formation of products with a wide range of volatilities spanning from involatile to volatile. Reactive uptake coefficients show a weak dependence on ozone concentration, but no dependence on RH or salt concentration. The time evolving RI depends significantly on the RH at which the oxidation proceeds and can even show opposing trends; while the RI increases with ozone exposure at low relative humidity, the RI decreases when the oxidation proceeds at high relative humidity. The variations in RI are broadly consistent with a framework for predicting RIs for organic components published by Cappa et al. ( J. Geophys. Res. 2011, 116, D15204). Once oxidized, particles are shown to form amorphous phases on drying rather than crystallization, with slow evaporation kinetics of residual water.


Intermediates Stabilized by Tryptophan Pairs Exist in Trpzip Beta-Hairpins

Zhongbo Yu, Sangeetha Selvam, and Hanbin Mao
Transitions of protein secondary structures, such as alpha-helices and beta-hairpins, are often too small and too fast to follow by many single-molecular approaches. Here we describe new population deconvolution methods to investigate the mechanical unfolding/refolding events in Trpzip β-hairpins that are tethered between two optically trapped polystyrene particles through click chemistry. The application of force to the Trpzip peptides shifted population distribution, which allowed us to identify intermediates from regular force–extension curves of the peptides after population deconvolution analysis. Comparison of the intermediates between the Trpzip2 and Trpzip4 peptides suggests the intermediates are likely stabilized by the tryptophan pair stacking. We anticipate the method of population deconvolution described here can offer a unique capability to investigate fast transitions in small biological structures.