1,178 research outputs found
An information-bearing seed for nucleating algorithmic self-assembly
Self-assembly creates natural mineral, chemical, and biological structures of great complexity. Often, the same starting materials have the potential to form an infinite variety of distinct structures; information in a seed molecule can determine which form is grown as well as where and when. These phenomena can be exploited to program the growth of complex supramolecular structures, as demonstrated by the algorithmic self-assembly of DNA tiles. However, the lack of effective seeds has limited the reliability and yield of algorithmic crystals. Here, we present a programmable DNA origami seed that can display up to 32 distinct binding sites and demonstrate the use of seeds to nucleate three types of algorithmic crystals. In the simplest case, the starting materials are a set of tiles that can form crystalline ribbons of any width; the seed directs assembly of a chosen width with >90% yield. Increased structural diversity is obtained by using tiles that copy a binary string from layer to layer; the seed specifies the initial string and triggers growth under near-optimal conditions where the bit copying error rate is 17 kb of sequence information. In sum, this work demonstrates how DNA origami seeds enable the easy, high-yield, low-error-rate growth of algorithmic crystals as a route toward programmable bottom-up fabrication
HmuY Haemophore and Gingipain Proteases Constitute a Unique Syntrophic System of Haem Acquisition by Porphyromonas gingivalis
Haem (iron protoporphyrin IX) is both an essential growth factor and virulence regulator for the periodontal pathogen Porphyromonas gingivalis, which acquires it mainly from haemoglobin via the sequential actions of the R- and K-specific gingipain proteases. The haem-binding lipoprotein haemophore HmuY and its cognate receptor HmuR of P. gingivalis, are responsible for capture and internalisation of haem. This study examined the role of the HmuY in acquisition of haem from haemoglobin and the cooperation between HmuY and gingipain proteases in this process. Using UV-visible spectroscopy and polyacrylamide gel electrophoresis, HmuY was demonstrated to wrest haem from immobilised methaemoglobin and deoxyhaemoglobin. Haem extraction from oxyhaemoglobin was facilitated after oxidation to methaemoglobin by pre-treatment with the P. gingivalis R-gingipain A (HRgpA). HmuY was also capable of scavenging haem from oxyhaemoglobin pre-treated with the K-gingipain (Kgp). This is the first demonstration of a haemophore working in conjunction with proteases to acquire haem from haemoglobin. In addition, HmuY was able to extract haem from methaemalbumin, and could bind haem, either free in solution or from methaemoglobin, even in the presence of serum albumin
Convection, Thermal Bifurcation, and the Colors of A stars
Broad-band ultraviolet photometry from the TD-1 satellite and low dispersion
spectra from the short wavelength camera of IUE have been used to investigate a
long-standing proposal of Bohm-Vitense that the normal main sequence A- and
early-F stars may divide into two different temperature sequences: (1) a high
temperature branch (and plateau) comprised of slowly rotating convective stars,
and (2) a low temperature branch populated by rapidly rotating radiative stars.
We find no evidence from either dataset to support such a claim, or to confirm
the existence of an "A-star gap" in the B-V color range 0.22 <= B-V <= 0.28 due
to the sudden onset of convection. We do observe, nonetheless, a large scatter
in the 1800--2000 A colors of the A-F stars, which amounts to ~0.65 mags at a
given B-V color index. The scatter is not caused by interstellar or
circumstellar reddening. A convincing case can also be made against binarity
and intrinsic variability due to pulsations of delta Sct origin. We find no
correlation with established chromospheric and coronal proxies of convection,
and thus no demonstrable link to the possible onset of convection among the A-F
stars. The scatter is not instrumental. Approximately 0.4 mags of the scatter
is shown to arise from individual differences in surface gravity as well as a
moderate spread (factor of ~3) in heavy metal abundance and UV line blanketing.
A dispersion of ~0.25 mags remains, which has no clear and obvious explanation.
The most likely cause, we believe, is a residual imprecision in our correction
for the spread in metal abundances. However, the existing data do not rule out
possible contributions from intrinsic stellar variability or from differential
UV line blanketing effects owing to a dispersion in microturbulent velocity.Comment: 40 pages, 14 figures, 1 table, AAS LaTex, to appear in The
Astrophysical Journa
Bounds for the time to failure of hierarchical systems of fracture
For years limited Monte Carlo simulations have led to the suspicion that the
time to failure of hierarchically organized load-transfer models of fracture is
non-zero for sets of infinite size. This fact could have a profound
significance in engineering practice and also in geophysics. Here, we develop
an exact algebraic iterative method to compute the successive time intervals
for individual breaking in systems of height in terms of the information
calculated in the previous height . As a byproduct of this method,
rigorous lower and higher bounds for the time to failure of very large systems
are easily obtained. The asymptotic behavior of the resulting lower bound leads
to the evidence that the above mentioned suspicion is actually true.Comment: Final version. To appear in Phys. Rev. E, Feb 199
Probabilistic Approach to Time-Dependent Load-Transfer Models of Fracture
A probabilistic method for solving time-dependent load-transfer models of
fracture is developed. It is applicable to any rule of load redistribution,
i.e, local, hierarchical, etc. In the new method, the fluctuations are
generated during the breaking process (annealed randomness) while in the usual
method, the random lifetimes are fixed at the beginning (quenched disorder).
Both approaches are equivalent.Comment: 13 pages, 4 figures. To appear in Phys.Rev.
Method for separating single-wall carbon nanotubes and compositions thereof
The invention relates to a process for sorting and separating a mixture of (n, m) type single-wall carbon nanotubes according to (n, m) type. A mixture of (n, m) type single-wall carbon nanotubes is suspended such that the single-wall carbon nanotubes are individually dispersed. The nanotube suspension can be done in a surfactant-water solution and the surfactant surrounding the nanotubes keeps the nanotube isolated and from aggregating with other nanotubes. The nanotube suspension is acidified to protonate a fraction of the nanotubes. An electric field is applied and the protonated nanotubes migrate in the electric fields at different rates dependent on their (n, m) type. Fractions of nanotubes are collected at different fractionation times. The process of protonation, applying an electric field, and fractionation is repeated at increasingly higher pH to separated the (n, m) nanotube mixture into individual (n, m) nanotube fractions. The separation enables new electronic devices requiring selected (n, m) nanotube types
Population Synthesis in the Blue IV: Accurate Model Predictions for Lick Indices and UBV Colors in Single Stellar Populations
[Abridged] We present new model predictions for 16 Lick absorption line
indices from Hdelta through Fe5335, and UBV colors for single stellar
populations (SPs) with ages ranging between 1 and 15 Gyr, [Fe/H] ranging from
-1.3 to +0.3, and variable abundance ratios. We develop a method to estimate
mean ages and abundances of Fe, C, N, Mg, and Ca that explores the sensitivity
of the various indices to those parameters. When applied to high-S/N Galactic
cluster data, the models match the clusters' elemental abundances and ages with
high precision. Analyzing stacked SDSS spectra of early-type galaxies brighter
than Lstar, we find mean luminosity-weighted ages of the order of ~ 8 Gyr and
iron abundances slightly below solar. Abundance ratios, [X/Fe], are higher than
solar, and correlate positively with galaxy luminosity. Nitrogen is the element
whose abundance correlates the most strongly with luminosity, which seems to
indicate secondary enrichment. This result may impose a lower limit of 50-200
Myr to the time-scale of star formation in early-type galaxies. Unlike in the
case of clusters, in galaxies bluer Balmer lines yield younger ages than Hbeta.
This age discrepancy is stronger for lower luminosity galaxies. We examine four
scenarios to explain this trend. The most likely is the presence of small
amounts of a young/intermediate-age SP component. Two-component models provide
a better match to the data when the mass fraction of the young component is a
few %. This result implies that star formation has been extended in early-type
galaxies, and more so in less massive galaxies, lending support to the
``downsizing'' scenario. It also implies that SP synthesis models are capable
of constraining not only the mean ages of SPs in galaxies, but also their age
spread.Comment: To appear in the Astrophysical Journal Supplement Series. 55 Pages,
using emulateapj5.sty. Full version, containing all (enlarged) figures can be
found at http://www.astro.virginia.edu/~rps7v/Models/ms.pdf . A number of
useful tables in the Appendix can be obtained in advance of publication by
request to the autho
Selective functionalization of carbon nanotubes
The present invention is directed toward methods of selectively functionalizing carbon nanotubes of a specific type or range of types, based on their electronic properties, using diazonium chemistry. The present invention is also directed toward methods of separating carbon nanotubes into populations of specific types or range(s) of types via selective functionalization and electrophoresis, and also to the novel compositions generated by such separations
Bulk Cutting of Carbon Nanotubes Using Electron Beam Irradiation
According to some embodiments, the present invention provides a method for attaining short carbon nanotubes utilizing electron beam irradiation, for example, of a carbon nanotube sample. The sample may be pretreated, for example by oxonation. The pretreatment may introduce defects to the sidewalls of the nanotubes. The method is shown to produces nanotubes with a distribution of lengths, with the majority of lengths shorter than 100 tun. Further, the median length of the nanotubes is between about 20 nm and about 100 nm
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