437 research outputs found
Design and characterization of programmable DNA nanotubes
DNA self-assembly provides a programmable bottom-up approach for the synthesis of complex structures from nanoscale components. Although nanotubes are a fundamental form encountered in tile-based DNA self-assembly, the factors governing tube structure remain poorly understood. Here we report and characterize a new type of nanotube made from DNA double-crossover molecules (DAE-E tiles). Unmodified tubes range from 7 to 20 nm in diameter (4 to 10 tiles in circumference), grow as long as 50 μm with a persistence length of ~4 μm, and can be programmed to display a variety of patterns. A survey of modifications (1) confirms the importance of sticky-end stacking, (2) confirms the identity of the inside and outside faces of the tubes, and (3) identifies features of the tiles that profoundly affect the size and morphology of the tubes. Supported by these results, nanotube structure is explained by a simple model based on the geometry and energetics of B-form DNA
Protein folding disorders: Toward a basic biological paradigm
Mechanistic 'physics' models of protein folding fail to account for the observed spectrum of protein folding and aggregation disorders, suggesting that a more appropriately biological paradigm will be needed for understanding the etiology, prevention, and treatment of these diseases
On the link pattern distribution of quarter-turn symmetric FPL configurations
We present new conjectures on the distribution of link patterns for
fully-packed loop (FPL) configurations that are invariant, or almost invariant,
under a quarter turn rotation, extending previous conjectures of Razumov and
Stroganov and of de Gier. We prove a special case, showing that the link
pattern that is conjectured to be the rarest does have the prescribed
probability. As a byproduct, we get a formula for the enumeration of a new
class of quasi-symmetry of plane partitions.Comment: 12 pages, 6 figures. Submitted to FPSAC 200
Limits of economy and fidelity for programmable assembly of size-controlled triply-periodic polyhedra
We propose and investigate an extension of the Caspar-Klug symmetry
principles for viral capsid assembly to the programmable assembly of
size-controlled triply-periodic polyhedra, discrete variants of the Primitive,
Diamond, and Gyroid cubic minimal surfaces. Inspired by a recent class of
programmable DNA origami colloids, we demonstrate that the economy of design in
these crystalline assemblies -- in terms of the growth of the number of
distinct particle species required with the increased size-scale (e.g.
periodicity) -- is comparable to viral shells. We further test the role of
geometric specificity in these assemblies via dynamical assembly simulations,
which show that conditions for simultaneously efficient and high-fidelity
assembly require an intermediate degree of flexibility of local angles and
lengths in programmed assembly. Off-target misassembly occurs via incorporation
of a variant of disclination defects, generalized to the case of hyperbolic
crystals. The possibility of these topological defects is a direct consequence
of the very same symmetry principles that underlie the economical design,
exposing a basic tradeoff between design economy and fidelity of programmable,
size controlled assembly.Comment: 15 pages, 5 figures, 6 supporting movies (linked), Supporting
Appendi
Symmetry and Asymmetry in Quasicrystals or Amorphous Materials
About forty years after its discovery, it is still common to read in the literature that quasicrystals (QCs) occupy an intermediate position between amorphous materials and periodic crystals. However, QCs exhibit high-quality diffraction patterns containing a collection of discrete Bragg reflections at variance with amorphous phases. Accordingly, these materials must be properly regarded as long-range ordered materials with a symmetry incompatible with translation invariance. This misleading conceptual status can probably arise from the use of notions borrowed from the amorphous solids framework (such us tunneling states, weak interference effects, variable range hopping, or spin glass) in order to explain certain physical properties observed in QCs. On the other hand, the absence of a general, full-fledged theory of quasiperiodic systems certainly makes it difficult to clearly distinguish the features related to short-range order atomic arrangements from those stemming from long-range order correlations. The contributions collected in this book aim at gaining a deeper understanding on the relationship between the underlying structural order and the resulting physical properties in several illustrative aperiodic systems, including the border line between QCs and related complex metallic alloys, hierarchical superlattices, electrical transmission lines, nucleic acid sequences, photonic quasicrystals, and optical devices based on aperiodic order designs
The Panchromatic Hubble Andromeda Treasury
The Panchromatic Hubble Andromeda Treasury (PHAT) is an on-going HST
Multicycle Treasury program to image ~1/3 of M31's star forming disk in 6
filters, from the UV to the NIR. The full survey will resolve the galaxy into
more than 100 million stars with projected radii from 0-20 kpc over a
contiguous 0.5 square degree area in 828 orbits, producing imaging in the F275W
and F336W filters with WFC3/UVIS, F475W and F814W with ACS/WFC, and F110W and
F160W with WFC3/IR. The resulting wavelength coverage gives excellent
constraints on stellar temperature, bolometric luminosity, and extinction for
most spectral types. The photometry reaches SNR=4 at F275W=25.1, F336W=24.9,
F475W=27.9, F814W=27.1, F110W=25.5, and F160W=24.6 for single pointings in the
uncrowded outer disk; however, the optical and NIR data are crowding limited,
and the deepest reliable magnitudes are up to 5 magnitudes brighter in the
inner bulge. All pointings are dithered and produce Nyquist-sampled images in
F475W, F814W, and F160W. We describe the observing strategy, photometry,
astrometry, and data products, along with extensive tests of photometric
stability, crowding errors, spatially-dependent photometric biases, and
telescope pointing control. We report on initial fits to the structure of M31's
disk, derived from the density of RGB stars, in a way that is independent of
the assumed M/L and is robust to variations in dust extinction. These fits also
show that the 10 kpc ring is not just a region of enhanced recent star
formation, but is instead a dynamical structure containing a significant
overdensity of stars with ages >1 Gyr. (Abridged)Comment: 48 pages including 22 pages of figures. Accepted to the Astrophysical
Journal Supplements. Some figures slightly degraded to reduce submission siz
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