268 research outputs found
Proline provides site-specific flexibility for in vivo collagen.
Fibrillar collagens have mechanical and biological roles, providing tissues with both tensile strength and cell binding sites which allow molecular interactions with cell-surface receptors such as integrins. A key question is: how do collagens allow tissue flexibility whilst maintaining well-defined ligand binding sites? Here we show that proline residues in collagen glycine-proline-hydroxyproline (Gly-Pro-Hyp) triplets provide local conformational flexibility, which in turn confers well-defined, low energy molecular compression-extension and bending, by employing two-dimensional 13C-13C correlation NMR spectroscopy on 13C-labelled intact ex vivo bone and in vitro osteoblast extracellular matrix. We also find that the positions of Gly-Pro-Hyp triplets are highly conserved between animal species, and are spatially clustered in the currently-accepted model of molecular ordering in collagen type I fibrils. We propose that the Gly-Pro-Hyp triplets in fibrillar collagens provide fibril "expansion joints" to maintain molecular ordering within the fibril, thereby preserving the structural integrity of ligand binding sites.BBSRC, EPSRC, Raymond and Beverly Sackler Fund for Physics of Medicine, Wellcome Trust, ER
Direct Observation of Proton-Neutron Short-Range Correlation Dominance in Heavy Nuclei
We measured the triple coincidence A(e,e′n p) and A(e,e′ p p) reactions on carbon, aluminum, iron, and lead targets at Q2 \u3e1.5  (GeV/c)2, xB \u3e 1.1 and missing momentum \u3e400  MeV/c. This was the first direct measurement of both proton-proton (pp) and neutron-proton (np) short-range correlated (SRC) pair knockout from heavy asymmetric nuclei. For all measured nuclei, the average proton-proton (pp) to neutron-proton (np) reduced cross-section ratio is about 6%, in agreement with previous indirect measurements. Correcting for single-charge exchange effects decreased the SRC pairs ratio to ∼3%, which is lower than previous results. Comparisons to theoretical generalized contact formalism (GCF) cross-section calculations show good agreement using both phenomenological and chiral nucleon-nucleon potentials, favoring a lower pp to np pair ratio. The ability of the GCF calculation to describe the experimental data using either phenomenological or chiral potentials suggests possible reduction of scale and scheme dependence in cross-section ratios. Our results also support the high-resolution description of high-momentum states being predominantly due to nucleons in SRC pairs
Scheme for the generation of an entangled four-photon W-state
We present a scheme to produce an entangled four-photon W-state by using
linear optical elements. The symmetrical setup of linear optical elements
consists of four beam splitters, four polarization beam splitters and four
mirrors. A photon EPR-pair and two single photons are required as the input
modes. The projection on the W-state can be made by a four-photon coincidence
measurement. Further, we show that by means of a horizontally oriented
polarizer in front of one detector the W-state of three photons can be
generated.Comment: titile is changed, to appear in PR
The CLAS12 Backward Angle Neutron Detector (BAND)
The Backward Angle Neutron Detector (BAND) of CLAS12 detects neutrons emitted
at backward angles of to , with momenta between
and MeV/c. It is positioned 3 meters upstream of the target, consists of
rows and layers of cm by cm scintillator bars, and read
out on both ends by PMTs to measure time and energy deposition in the
scintillator layers. Between the target and BAND there is a 2 cm thick lead
wall followed by a 2 cm veto layer to suppress gammas and reject charged
particles. This paper discusses the component-selection tests and the detector
assembly. Timing calibrations (including offsets and time-walk) were performed
using a novel pulsed-laser calibration system, resulting in time resolutions
better than ps (150 ps) for energy depositions above 2 MeVee (5 MeVee).
Cosmic rays and a variety of radioactive sources were used to calibration the
energy response of the detector. Scintillator bar attenuation lengths were
measured. The time resolution results in a neutron momentum reconstruction
resolution, \% for neutron momentum MeV/c.
Final performance of the BAND with CLAS12 is shown, including electron-neutral
particle timing spectra and a discussion of the off-time neutral contamination
as a function of energy deposition threshold.Comment: 17 pages, 25 figures, 3 tables. Accepted for publication in NIM-
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