1,203 research outputs found
On the prevalence of bridged macrocyclic pyrroloindolines formed in regiodivergent alkylations of tryptophan.
A Friedel-Crafts alkylation is described that efficiently transforms tryptophan-containing peptides into macrocycles of varying ring connectivity. Factors are surveyed that influence the distribution of regioisomers, with a focus on indole C3-alkylations leading to bridged endo-pyrroloindolines. We probe the stability and stereochemistry of these pyrroloindolines, study their rearrangement to C2-linked indolic macrocycles, and demonstrate a scalable, stereoselective synthesis of this compound class. Placing the macrocyclization in sequence with further template-initiated annulation leads to extraordinary polycyclic products and further demonstrates the potential for this chemistry to drive novel peptidomimetic lead discovery programs
Correlation Between the Deuteron Characteristics and the Low-energy Triplet np Scattering Parameters
The correlation relationship between the deuteron asymptotic normalization
constant, , and the triplet np scattering length, , is
investigated. It is found that 99.7% of the asymptotic constant is
determined by the scattering length . It is shown that the linear
correlation relationship between the quantities and
provides a good test of correctness of various models of nucleon-nucleon
interaction. It is revealed that, for the normalization constant and
for the root-mean-square deuteron radius , the results obtained with the
experimental value recommended at present for the triplet scattering length
are exaggerated with respect to their experimental counterparts. By
using the latest experimental phase shifts of Arndt et al., we obtain, for the
low-energy scattering parameters (, , ) and for the
deuteron characteristics (, ), results that comply well with
experimental data.Comment: 19 pages, 1 figure, To be published in Physics of Atomic Nucle
Probing Stereoselectivity in Ring-Opening Metathesis Polymerization Mediated by Cyclometalated Ruthenium-Based Catalysts: A Combined Experimental and Computational Study
The microstructures of polymers produced by ring-opening metathesis polymerization (ROMP) with cyclometalated Ru-carbene metathesis catalysts were investigated. A strong bias for a cis,syndiotactic microstructure with minimal head-to-tail bias was observed. In instances where trans errors were introduced, it was determined that these regions were also syndiotactic. Furthermore, hypothetical reaction intermediates and transition structures were analyzed computationally. Combined experimental and computational data support a reaction mechanism in which cis,syndio-selectivity is a result of stereogenic metal control, while microstructural errors are predominantly due to alkylidene isomerization via rotation about the RuâC double bond
Long-term photometry and periods for 261 nearby pulsating M giants
We present the results of a 5.5-year CCD photometric campaign that monitored
261 bright, southern, semi-regular variables with relatively precise Hipparcos
parallaxes. The data are supplemented with independent photoelectric
observations of 34 of the brightest stars, including 11 that were not part of
the CCD survey, and a previously unpublished long time-series of VZ Cam.
Pulsation periods and amplitudes are established for 247 of these stars, the
majority of which have not been determined before. All M giants with sufficient
observations for period determination are found to be variable, with 87% of the
sample (at S/N >= 7.5) exhibiting multi-periodic behaviour. The period ratios
of local SRVs are in excellent agreement with those in the Large Magellanic
Cloud. Apparent K-band magnitudes are extracted from multiple NIR catalogues
and analysed to determine the most reliable values. We review the effects of
interstellar and circumstellar extinction and calculate absolute K-band
magnitudes using revised Hipparcos parallaxes.Comment: 18 pages, 15 figures; accepted for publication in Monthly Notices of
the Royal Astronomical Societ
Giant magnetic anisotropy and tunnelling of the magnetization in Li2(Li1âxFex)N
Large magnetic anisotropy and coercivity are key properties of functional
magnetic materials and are generally associated with rare earth elements. Here
we show an extreme, uniaxial magnetic anisotropy and the emergence of magnetic
hysteresis in Li2(Li1-xFex)N. An extrapolated, magnetic anisotropy field of 220
Tesla and a coercivity field of over 11 Tesla at 2 Kelvin outperform all known
hard-ferromagnets and single-molecule magnets (SMMs). Steps in the hysteresis
loops and relaxation phenomena in striking similarity to SMMs are particularly
pronounced for x << 1 and indicate the presence of nano-scale magnetic centres.
Quantum tunnelling, in form of temperature-independent relaxation and
coercivity, deviation from Arrhenius behaviour and blocking of the relaxation,
dominates the magnetic properties up to 10 Kelvin. The simple crystal
structure, the availability of large single crystals, and the ability to vary
the Fe concentration make Li2(Li1-xFex)N (i) an ideal model system to study
macroscopic quantum effects at elevated temperatures and (ii) a basis for novel
functional magnetic materials.Comment: 19 pages, 5 figures, Supplementary Information attached, revised
version published in Nature Communication
O- vs. N-protonation of 1-dimethylaminonaphthalene-8-ketones: formation of a peri NâC bond or a hydrogen bond to the pi-electron density of a carbonyl group
X-ray crystallography and solid-state NMR measurements show that protonation of a series of 1-dimethylaminonaphthalene-8-ketones leads either to O protonation with formation of a long NâC bond (1.637â1.669 Ă
) between peri groups, or to N protonation and formation of a hydrogen bond to the Ï surface of the carbonyl group, the latter occurring for the larger ketone groups (C(O)R, R = t-butyl and phenyl). Solid state 15N MAS NMR studies clearly differentiate the two series, with the former yielding significantly more deshielded resonances. This is accurately corroborated by DFT calculation of the relevant chemical shift parameters. In the parent ketones X-ray crystallography shows that the nitrogen lone pair is directed towards the carbonyl group in all cases
Bayesian approach to cyclic activity of CF Oct
Bayesian statistical methods of Gregory-Loredo and the Bretthorst
generalization of the Lomb-Scargle periodogram have been applied for studying
activity cycles of the early K-type subgiant star CF Oct. We have used a ~45
year long dataset derived from archival photographic observations, published
photoelectric photometry, Hipparcos data series and All Sky Automated Survey
archive. We have confirmed the already known rotational period for the star of
20.16 d and have shown evidences that it has exhibited changes from 19.90 d to
20.45 d. This is an indication for stellar surface differential rotation.The
Bayesian magnitude and time--residual analysis reveals clearly at least one
long-term cycle. The cycle lenght's posterior distributions appear to be
multimodal with a pronounced peak at a period of 7.1 y with FWHM of 54 d for
time-residuals and at a period of 9.8 y with FWHM of 184 d for magitude data.
These results are consistent with the previously postulated cycle of 9+/-3
years.Comment: The paper contains 6 figures and 3 tables. Accepted for publication
in Monthly Notices of the Royal Astronomical Society Main Journa
Weak capture of protons by protons
The cross section for the proton weak capture reaction
is calculated with wave functions obtained from a number of modern, realistic
high-precision interactions. To minimize the uncertainty in the axial two-body
current operator, its matrix element has been adjusted to reproduce the
measured Gamow-Teller matrix element of tritium decay in model
calculations using trinucleon wave functions from these interactions. A
thorough analysis of the ambiguities that this procedure introduces in
evaluating the two-body current contribution to the pp capture is given. Its
inherent model dependence is in fact found to be very weak. The overlap
integral for the pp capture is predicted to be in the range
7.05--7.06, including the axial two-body current contribution, for all
interactions considered.Comment: 17 pages RevTeX (twocolumn), 5 postscript figure
High-latitude supergiants: anomalies in the spectrum of LN Hya in 2010
High-resolution echelle spectra taken with the 6-m telescope in 2003-2011 are
used to study features of the optical spectrum and the velocity field in the
atmosphere of the semiregular variable LN Hya in detail. The weak symmetric
photospheric absorptions indicate radial velocity variations from night to
night (by as much as 3 km/s), resulting from small pulsations. Peculiarities
and profile variations were found for strong lines of FeI, FeII, BaII, SiII,
etc. The profiles of these lines were asymmetric: their short-wave wings were
extended and their cores were either split or distorted by emission. During the
2010 observing season, the position and depth of the Halpha absorption
component, the intensities of the short and long-wave emission components, and
the intensity ratio of the latter components varied from spectrum to spectrum.
Weak emissions of neutral atoms (VI, MnI, CoI, NiI, FeI) appeared in the
spectrum of June 1, 2010. All these spectral peculiarities, recorded for the
first time, suggest that we have detected rapid changes in the physical
conditions in the upper atmospheric layers of LN Hya in 2010.Comment: 17 pages, 5 figures, 3 table
Neurology
Contains reports on eight research projects.United States Air Force (AF33(616)-7588, AF49(638)-1130)National Science Foundation (Grant G-16526)United States Army Chemical Corps (DA-18-108-405-Cml-942)United States Public Health Service (B-3055, B-3090)United States Navy, Office of Naval Research (Contract Nonr-1841(70)
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