456 research outputs found
Direct Imaging of Protein Organization in an Intact Bacterial Organelle Using High-Resolution Atomic Force Microscopy
The function of bioenergetic membranes is strongly influenced by the spatial arrangement of their constituent membrane proteins. Atomic force microscopy (AFM) can be used to probe protein organization at high resolution, allowing individual proteins to be identified. However, previous AFM studies of biological membranes have typically required that curved membranes are ruptured and flattened during sample preparation, with the possibility of disruption of the native protein arrangement or loss of proteins. Imaging native, curved membranes requires minimal tip–sample interaction in both lateral and vertical directions. Here, long-range tip–sample interactions are reduced by optimizing the imaging buffer. Tapping mode AFM with high-resonance-frequency small and soft cantilevers, in combination with a high-speed AFM, reduces the forces due to feedback error and enables application of an average imaging force of tens of piconewtons. Using this approach, we have imaged the membrane organization of intact vesicular bacterial photosynthetic “organelles”, chromatophores. Despite the highly curved nature of the chromatophore membrane and lack of direct support, the resolution was sufficient to identify the photosystem complexes and quantify their arrangement in the native state. Successive imaging showed the proteins remain surprisingly static, with minimal rotation or translation over several-minute time scales. High-order assemblies of RC-LH1-PufX complexes are observed, and intact ATPases are successfully imaged. The methods developed here are likely to be applicable to a broad range of protein-rich vesicles or curved membrane systems, which are an almost ubiquitous feature of native organelles
From Monochrome to Technicolor: Simple Generic Approaches to Multicomponent Protein Nanopatterning Using Siloxanes with Photoremovable Protein-Resistant Protecting Groups.
We show that sequential protein deposition is possible by photodeprotection of films formed from a tetraethylene-glycol functionalized nitrophenylethoxycarbonyl-protected aminopropyltriethoxysilane (NPEOC-APTES). Exposure to near-UV irradiation removes the protein-resistant protecting group, and allows protein adsorption onto the resulting aminated surface. The protein resistance was tested using proteins with fluorescent labels and microspectroscopy of two-component structures formed by micro- and nanopatterning and deposition of yellow and green fluorescent proteins (YFP/GFP). Nonspecific adsorption onto regions where the protecting group remained intact was negligible. Multiple component patterns were also formed by near-field methods. Because reading and writing can be decoupled in a near-field microscope, it is possible to carry out sequential patterning steps at a single location involving different proteins. Up to four different proteins were formed into geometric patterns using near-field lithography. Interferometric lithography facilitates the organization of proteins over square cm areas. Two-component patterns consisting of 150 nm streptavidin dots formed within an orthogonal grid of bars of GFP at a period of ca. 500 nm could just be resolved by fluorescence microscopy
The one-dimensional Bose-Hubbard Model with nearest-neighbor interaction
We study the one-dimensional Bose-Hubbard model using the Density-Matrix
Renormalization Group (DMRG).For the cases of on-site interactions and
additional nearest-neighbor interactions the phase boundaries of the
Mott-insulators and charge density wave phases are determined. We find a direct
phase transition between the charge density wave phase and the superfluid
phase, and no supersolid or normal phases. In the presence of nearest-neighbor
interaction the charge density wave phase is completely surrounded by a region
in which the effective interactions in the superfluid phase are repulsive. It
is known from Luttinger liquid theory that a single impurity causes the system
to be insulating if the effective interactions are repulsive, and that an even
bigger region of the superfluid phase is driven into a Bose-glass phase by any
finite quenched disorder. We determine the boundaries of both regions in the
phase diagram. The ac-conductivity in the superfluid phase in the attractive
and the repulsive region is calculated, and a big superfluid stiffness is found
in the attractive as well as the repulsive region.Comment: 19 pages, 30 figure
Quantum corrections to the dynamics of interacting bosons: beyond the truncated Wigner approximation
We develop a consistent perturbation theory in quantum fluctuations around
the classical evolution of a system of interacting bosons. The zero order
approximation gives the classical Gross-Pitaevskii equations. In the next order
we recover the truncated Wigner approximation, where the evolution is still
classical but the initial conditions are distributed according to the Wigner
transform of the initial density matrix. Further corrections can be
characterized as quantum scattering events, which appear in the form of a
nonlinear response of the observable to an infinitesimal displacement of the
field along its classical evolution. At the end of the paper we give a few
numerical examples to test the formalism.Comment: published versio
Phase Diagram of Bosonic Atoms in Two-Color Superlattices
We investigate the zero temperature phase diagram of a gas of bosonic atoms
in one- and two-color standing-wave lattices in the framework of the
Bose-Hubbard model. We first introduce some relevant physical quantities;
superfluid fraction, condensate fraction, quasimomentum distribution, and
matter-wave interference pattern. We then discuss the relationships between
them on the formal level and show that the superfluid fraction, which is the
relevant order parameter for the superfluid to Mott-insulator transition,
cannot be probed directly via the matter wave interference patterns. The formal
considerations are supported by exact numerical solutions of the Bose-Hubbard
model for uniform one-dimensional systems. We then map out the phase diagram of
bosons in non-uniform lattices. The emphasis is on optical two-color
superlattices which exhibit a sinusoidal modulation of the well depth and can
be easily realized experimentally. From the study of the superfluid fraction,
the energy gap, and other quantities we identify new zero-temperature phases,
including a localized and a quasi Bose-glass phase, and discuss prospects for
their experimental observation.Comment: 18 pages, 17 figures, using REVTEX
Microfluidic and Nanofluidic Cavities for Quantum Fluids Experiments
The union of quantum fluids research with nanoscience is rich with
opportunities for new physics. The relevant length scales in quantum fluids,
3He in particular, are comparable to those possible using microfluidic and
nanofluidic devices. In this article, we will briefly review how the physics of
quantum fluids depends strongly on confinement on the microscale and nanoscale.
Then we present devices fabricated specifically for quantum fluids research,
with cavity sizes ranging from 30 nm to 11 microns deep, and the
characterization of these devices for low temperature quantum fluids
experiments.Comment: 12 pages, 3 figures, Accepted to Journal of Low Temperature Physic
Probing Sub-Micron Forces by Interferometry of Bose-Einstein Condensed Atoms
We propose a technique, using interferometry of Bose-Einstein condensed
alkali atoms, for the detection of sub-micron-range forces. It may extend
present searches at 1 micron by 6 to 9 orders of magnitude, deep into the
theoretically interesting regime of 1000 times gravity. We give several
examples of both four-dimensional particles (moduli), as well as
higher-dimensional particles -- vectors and scalars in a large bulk-- that
could mediate forces accessible by this technique.Comment: 32 pages, 5 figures, RevTeX4, expanded discussion of interactions,
references added, to appear in PR
Spin squeezing and pairwise entanglement for symmetric multiqubit states
We show that spin squeezing implies pairwise entanglement for arbitrary
symmetric multiqubit states. If the squeezing parameter is less than or equal
to 1, we demonstrate a quantitative relation between the squeezing parameter
and the concurrence for the even and odd states. We prove that the even states
generated from the initial state with all qubits being spin down, via the
one-axis twisting Hamiltonian, are spin squeezed if and only if they are
pairwise entangled. For the states generated via the one-axis twisting
Hamiltonian with an external transverse field for any number of qubits greater
than 1 or via the two-axis counter-twisting Hamiltonian for any even number of
qubits, the numerical results suggest that such states are spin squeezed if and
only if they are pairwise entangled.Comment: 6 pages. Version 3: Small corrections were mad
Ginzburg-Landau theory of vortices in a multi-gap superconductor
The Ginzburg-Landau functional for a two-gap superconductor is derived within
the weak-coupling BCS model. The two-gap Ginzburg-Landau theory is, then,
applied to investigate various magnetic properties of MgB2 including an upturn
temperature dependence of the transverse upper critical field and a core
structure of an isolated vortex. Orientation of vortex lattice relative to
crystallographic axes is studied for magnetic fields parallel to the c-axis. A
peculiar 30-degree rotation of the vortex lattice with increasing strength of
an applied field observed by neutron scattering is attributed to the multi-gap
nature of superconductivity in MgB2.Comment: 11 page
Brownian motion of a charged particle in electromagnetic fluctuations at finite temperature
The fluctuation-dissipation theorem is a central theorem in nonequilibrium
statistical mechanics by which the evolution of velocity fluctuations of the
Brownian particle under a fluctuating environment is intimately related to its
dissipative behavior. This can be illuminated in particular by an example of
Brownian motion in an ohmic environment where the dissipative effect can be
accounted for by the first-order time derivative of the position. Here we
explore the dynamics of the Brownian particle coupled to a supraohmic
environment by considering the motion of a charged particle interacting with
the electromagnetic fluctuations at finite temperature. We also derive
particle's equation of motion, the Langevin equation, by minimizing the
corresponding stochastic effective action, which is obtained with the method of
Feynman-Vernon influence functional. The fluctuation-dissipation theorem is
established from first principles. The backreaction on the charge is known in
terms of electromagnetic self-force given by a third-order time derivative of
the position, leading to the supraohmic dynamics. This self-force can be argued
to be insignificant throughout the evolution when the charge barely moves. The
stochastic force arising from the supraohmic environment is found to have both
positive and negative correlations, and it drives the charge into a fluctuating
motion. Although positive force correlations give rise to the growth of the
velocity dispersion initially, its growth slows down when correlation turns
negative, and finally halts, thus leading to the saturation of the velocity
dispersion. The saturation mechanism in a suparohmic environment is found to be
distinctly different from that in an ohmic environment. The comparison is
discussed.Comment: accepter by Foundation of Physics, for IARD 6, 200
- …