2,311 research outputs found
Engineering photocycle dynamics. Crystal structures and kinetics of three photoactive yellow protein hinge-bending mutants
Crystallographic and spectroscopic analyses of three hinge-bending mutants of the photoactive yellow protein are described. Previous studies have identified Gly(47) and Gly(51) as possible hinge points in the structure of the protein, allowing backbone segments around the chromophore to undergo large concerted motions. We have designed, crystallized, and solved the structures of three mutants: G47S, G51S, and G47S/G51S. The protein dynamics of these mutants are significantly affected. Transitions in the photocycle, measured with laser induced transient absorption spectroscopy, show rates up to 6-fold different from the wild type protein and show an additive effect in the double mutant. Compared with the native structure, no significant conformational differences were observed in the structures of the mutant proteins. We conclude that the structural and dynamic integrity of the region around these mutations is of crucial importance to the photocycle and suggest that the hinge-bending properties of Gly(51) may also play a role in PAS domain proteins where it is one of the few conserved residues
Unicyclic Components in Random Graphs
The distribution of unicyclic components in a random graph is obtained
analytically. The number of unicyclic components of a given size approaches a
self-similar form in the vicinity of the gelation transition. At the gelation
point, this distribution decays algebraically, U_k ~ 1/(4k) for k>>1. As a
result, the total number of unicyclic components grows logarithmically with the
system size.Comment: 4 pages, 2 figure
Predicting Concentrations of Organic Chemicals in Fish by Using Toxicokinetic Models
Quantification of chemical toxicity continues to be generally based on measured external concentrations. Yet, internal chemical concentrations have been suggested to be a more suitable parameter. To better understand the relationship between the external and internal concentrations of chemicals in fish, and to quantify internal concentrations we compared three. toxicokinetic (TK) models with each other and with literature data of measured concentrations of 39 chemicals. Two one, compartment models, together with the physiologically based toxicokinetic (PBTK) model, in which we improved the treatment of lipids, were used to predict concentrations of organic chemicals in two fish species: rainbow trout (Oncorhynchus mykiss) and fathead minnow (Pimephales promelas). All models predicted the measured internal concentrations in fish within I order of magnitude for at least 68% of the chemicals. Furthermore, the PBTK model outperformed the one-compartment models with respect to simulating chemical concentrations in the whole body (at least 88% of internal concentrations were predicted within 1 order of magnitude using the PBTK model). All the models can be used to predict concentrations in different fish species without additional experiments. However, further development of TK models is required for polar, ionizable, and easily biotransformed compounds
PTPBR7 Binding Proteins in Myelinating Neurons of the Mouse Brain
Abstract Mouse protein tyrosine phosphatase PTPBR7 is a receptor-like, transmembrane protein that is localized on the surface of neuronal cells. Its protein phosphatase activity is reduced upon multimerization, and PTPBR7-deficient mice display motor coordination defects. Extracellular molecules that may influence PTPBR7 activity, however, remain to be determined. We here show that the PTPBR7 extracellular domain binds to highly myelinated regions in mouse brain, in particular the white matter tracks in cerebellum. PTPBR7 deficiency does not alter this binding pattern, as witnessed by RAP in situ staining of Ptprr -/-mouse brain sections. Additional in situ and in vitro experiments also suggest that sugar moieties of heparan sulphate and chondroitin sulphate glycosaminoglycans are not critical for PTPBR7 binding. Candidate binding proteins were affinity-purified exploiting the PTPBR7 extracellular domain and identified by mass spectrometric means. Results support the suggested link between PTPRR isoforms and cerebellar calcium ion homeostasis, and suggest an additional role in the process of cell-cell adhesion
Identification and tunable optical coherent control of transition-metal spins in silicon carbide
Color centers in wide-bandgap semiconductors are attractive systems for
quantum technologies since they can combine long-coherent electronic spin and
bright optical properties. Several suitable centers have been identified, most
famously the nitrogen-vacancy defect in diamond. However, integration in
communication technology is hindered by the fact that their optical transitions
lie outside telecom wavelength bands. Several transition-metal impurities in
silicon carbide do emit at and near telecom wavelengths, but knowledge about
their spin and optical properties is incomplete. We present all-optical
identification and coherent control of molybdenum-impurity spins in silicon
carbide with transitions at near-infrared wavelengths. Our results identify
spin for both the electronic ground and excited state, with highly
anisotropic spin properties that we apply for implementing optical control of
ground-state spin coherence. Our results show optical lifetimes of 60 ns
and inhomogeneous spin dephasing times of 0.3 s, establishing
relevance for quantum spin-photon interfacing.Comment: Updated version with minor correction, full Supplementary Information
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General relativistic corrections to the Sagnac effect
The difference in travel time of corotating and counter-rotating light waves
in the field of a central massive and spinning body is studied. The corrections
to the special relativistic formula are worked out in a Kerr field. Estimation
of numeric values for the Earth and satellites in orbit around it show that a
direct measurement is in the order of concrete possibilities.Comment: REVTex, accepted for publication on Phys. Rev.
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