680 research outputs found
Collective Effects in Linear Spectroscopy of Dipole-Coupled Molecular Arrays
We present a consistent analysis of linear spectroscopy for arrays of nearest
neighbor dipole-coupled two-level molecules that reveals distinct signatures of
weak and strong coupling regimes separated for infinite size arrays by a
quantum critical point. In the weak coupling regime, the ground state of the
molecular array is disordered, but in the strong coupling regime it has
(anti)ferroelectric ordering. We show that multiple molecular excitations
(odd/even in weak/strong coupling regime) can be accessed directly from the
ground state. We analyze the scaling of absorption and emission with system
size and find that the oscillator strengths show enhanced superradiant behavior
in both ordered and disordered phases. As the coupling increases, the single
excitation oscillator strength rapidly exceeds the well known Heitler-London
value. In the strong coupling regime we show the existence of a unique spectral
transition with excitation energy that can be tuned by varying the system size
and that asymptotically approaches zero for large systems. The oscillator
strength for this transition scales quadratically with system size, showing an
anomalous one-photon superradiance. For systems of infinite size, we find a
novel, singular spectroscopic signature of the quantum phase transition between
disordered and ordered ground states. We outline how arrays of ultra cold
dipolar molecules trapped in an optical lattice can be used to access the
strong coupling regime and observe the anomalous superradiant effects
associated with this regime.Comment: 12 pages, 7 figures main tex
Broken symmetry and the variation of critical properties in the phase behaviour of supramolecular rhombus tilings
The degree of randomness, or partial order, present in two-dimensional
supramolecular arrays of isophthalate tetracarboxylic acids is shown to vary
due to subtle chemical changes such as the choice of solvent or small
differences in molecular dimensions. This variation may be quantified using an
order parameter and reveals a novel phase behaviour including random tiling
with varying critical properties as well as ordered phases dominated by either
parallel or non-parallel alignment of neighbouring molecules, consistent with
long-standing theoretical studies. The balance between order and randomness is
driven by small differences in the intermolecular interaction energies, which
we show, using numerical simulations, can be related to the measured order
parameter. Significant variations occur even when the energy difference is much
less than the thermal energy highlighting the delicate balance between entropic
and energetic effects in complex self-assembly processes
Direct observation of molecular arrays in the organized smooth endoplasmic reticulum
RIGHTS : This article is licensed under the BioMed Central licence at http://www.biomedcentral.com/about/license which is similar to the 'Creative Commons Attribution Licence'. In brief you may : copy, distribute, and display the work; make derivative works; or make commercial use of the work - under the following conditions: the original author must be given credit; for any reuse or distribution, it must be made clear to others what the license terms of this work are.Abstract Background Tubules and sheets of endoplasmic reticulum perform different functions and undergo inter-conversion during different stages of the cell cycle. Tubules are stabilized by curvature inducing resident proteins, but little is known about the mechanisms of endoplasmic reticulum sheet stabilization. Tethering of endoplasmic reticulum membranes to the cytoskeleton or to each other has been proposed as a plausible way of sheet stabilization. Results Here, using fluorescence microscopy we show that the previously proposed mechanisms, such as membrane tethering via GFP-dimerization or coiled coil protein aggregation - do not explain the formation of the calnexin-induced organized smooth endoplasmic reticulum membrane stacks. We also show that the LINC complex proteins known to serve a tethering function in the nuclear envelope are excluded from endoplasmic reticulum stacks. Finally, using cryo-electron microscopy of vitreous sections methodology that preserves cellular architecture in a hydrated, native-like state, we show that the sheet stacks are highly regular and may contain ordered arrays of macromolecular complexes. Some of these complexes decorate the cytosolic surface of the membranes, whereas others appear to span the width of the cytosolic or luminal space between the stacked sheets. Conclusion Our results provide evidence in favour of the hypothesis of endoplasmic reticulum sheet stabilization by intermembrane tethering.Published versio
Electromechanical Imaging of Biological Systems with Sub-10 nm Resolution
Electromechanical imaging of tooth dentin and enamel has been performed with
sub-10 nm resolution using piezoresponse force microscopy. Characteristic
piezoelectric domain size and local protein fiber ordering in dentin have been
determined. The shape of a single collagen fibril in enamel is visualized in
real space and local hysteresis loops are measured. Because of the ubiquitous
presence of piezoelectricity in biological systems, this approach is expected
to find broad application in high-resolution studies of a wide range of
biomaterials.Comment: 12 pages, 4 figures, submitted for publication in Appl. Phys. Let
Construction, analysis, ligation, and self-assembly of DNA triple crossover complexes
This paper extends the study and prototyping of unusual DNA motifs, unknown in nature, but founded
on principles derived from biological structures. Artificially designed DNA complexes show promise as building
blocks for the construction of useful nanoscale structures, devices, and computers. The DNA triple crossover
(TX) complex described here extends the set of experimentally characterized building blocks. It consists of
four oligonucleotides hybridized to form three double-stranded DNA helices lying in a plane and linked by
strand exchange at four immobile crossover points. The topology selected for this TX molecule allows for the
presence of reporter strands along the molecular diagonal that can be used to relate the inputs and outputs of
DNA-based computation. Nucleotide sequence design for the synthetic strands was assisted by the application
of algorithms that minimize possible alternative base-pairing structures. Synthetic oligonucleotides were purified,
stoichiometric mixtures were annealed by slow cooling, and the resulting DNA structures were analyzed by
nondenaturing gel electrophoresis and heat-induced unfolding. Ferguson analysis and hydroxyl radical
autofootprinting provide strong evidence for the assembly of the strands to the target TX structure. Ligation
of reporter strands has been demonstrated with this motif, as well as the self-assembly of hydrogen-bonded
two-dimensional crystals in two different arrangements. Future applications of TX units include the construction
of larger structures from multiple TX units, and DNA-based computation. In addition to the presence of reporter
strands, potential advantages of TX units over other DNA structures include space for gaps in molecular arrays,
larger spatial displacements in nanodevices, and the incorporation of well-structured out-of-plane components
in two-dimensional arrays
Charge Fluctuation Forces Between Stiff Polyelectrolytes in Salt Solution: Pairwise Summability Re-examined
We formulate low-frequency charge-fluctuation forces between charged
cylinders - parallel or skewed - in salt solution: forces from dipolar van der
Waals fluctuations and those from the correlated monopolar fluctuations of
mobile ions. At high salt concentrations forces are exponentially screened. In
low-salt solutions dipolar energies go as or ; monopolar
energies vary as or , where is the minimal separation
between cylinders. However, pairwise summability of rod-rod forces is easily
violated in low-salt conditions. Perhaps the most important result is not the
derivation of pair potentials but rather the demonstration that some of these
expressions may not be used for the very problems that originally motivated
their derivation.Comment: 8 pages and 1 fig in ps forma
Structures and Textures of Crystalline Phases and Lyotropic Mesophases of Alkylhenzenesulfonates
The molecular arrays in crystalline n-alkylbenzenesulfonates (CS-C12) were determined by X-ray diffraction and found to be lamellar bilayers with the basic lamellar thickness of 30.1-36.9 A for single-tailed, and 22.25-30.0A for C12 double-tailed surfactants, depending on the length of surfactant molecule. In the colloid dispersions of metal dodecylbenzenesulfonates as well as in the systems of double-tailed C12-isomers, microscopic investigations of liquid crystal textures showed the characteristic lyotropic nematic, lamellar, pseudo-isotropic and transient phases with batonnents
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