SU(2) Cosmological Solitons

We present a class of numerical solutions to the SU(2) nonlinear $\sigma$-model coupled to the Einstein equations with cosmological constant $\Lambda\geq 0$ in spherical symmetry. These solutions are characterized by the presence of a regular static region which includes a center of symmetry. They are parameterized by a dimensionless ``coupling constant'' $\beta$, the sign of the cosmological constant, and an integer ``excitation number'' $n$. The phenomenology we find is compared to the corresponding solutions found for the Einstein-Yang-Mills (EYM) equations with positive $\Lambda$ (EYM$\Lambda$). If we choose $\Lambda$ positive and fix $n$, we find a family of static spacetimes with a Killing horizon for $0 \leq \beta < \beta_{max}$. As a limiting solution for $\beta = \beta_{max}$ we find a {\em globally} static spacetime with $\Lambda=0$, the lowest excitation being the Einstein static universe. To interpret the physical significance of the Killing horizon in the cosmological context, we apply the concept of a trapping horizon as formulated by Hayward. For small values of $\beta$ an asymptotically de Sitter dynamic region contains the static region within a Killing horizon of cosmological type. For strong coupling the static region contains an ``eternal cosmological black hole''.Comment: 20 pages, 6 figures, Revte

Crystallization and preliminary X-ray analysis of the 12S form of phosphofructokinase from Saccharomyces cerevisiae

The tetrameric 12S form of yeast phosphofructokinase, obtained by limited proteolytic cleavage of the native enzyme, was crystallized under a variety of conditions. The crystals have been characterized in the X-ray beam and are suitable for crystallographic studies

Atomic resolution structure of CAG RNA repeats: structural insights and implications for the trinucleotide repeat expansion diseases

CAG repeats occur predominantly in the coding regions of human genes, which suggests their functional importance. In some genes, these sequences can undergo pathogenic expansions leading to neurodegenerative polyglutamine (poly-Q) diseases. The mutant transcripts containing expanded CAG repeats possibly contribute to pathogenesis in addition to the well-known pathogenic effects of mutant proteins. We have analysed two crystal forms of RNA duplexes containing CAG repeats: (GGCAGCAGCC)2. One of the structures has been determined at atomic resolution (0.95 Å) and the other at 1.9 Å. The duplexes include non-canonical A–A pairs that fit remarkably well within a regular A-helix. All the adenosines are in the anti-conformation and the only interaction within each A–A pair is a single C2-H2···N1 hydrogen bond. Both adenosines in each A–A pair are shifted towards the major groove, although to different extents; the A which is the H-bond donor stands out more (the ‘thumbs-up’ conformation). The main effect on the helix conformation is a local unwinding. The CAG repeats and the previously examined CUG structures share a similar pattern of electrostatic charge distribution in the minor groove, which could explain their affinity for the pathogenesis-related MBNL1 protein

Spider chitin: An Ultrafast Microwave-Assisted Method for Chitin Isolation from Caribena versicolor Spider Molt Cuticle

Chitin, as a fundamental polysaccharide in invertebrate skeletons, continues to be actively investigated, especially with respect to new sources and the development of effective methods for its extraction. Recent attention has been focused on marine crustaceans and sponges; however, the potential of spiders (order Araneae) as an alternative source of tubular chitin has been overlooked. In this work, we focused our attention on chitin from up to 12 cm-large Theraphosidae spiders, popularly known as tarantulas or bird-eating spiders. These organisms “lose” large quantities of cuticles during their molting cycle. Here, we present for the first time a highly effective method for the isolation of chitin from Caribena versicolor spider molt cuticle, as well as its identification and characterization using modern analytical methods. We suggest that the tube-like molt cuticle of this spider can serve as a naturally prefabricated and renewable source of tubular chitin with high potential for application in technology and biomedicine. © 2019 by the authors

X-ray Crystal and Ab Initio Structures of 3 ,5 -di-O-Acetyl-N(4)-Hydroxy-2 -Deoxycytidine and Its 5-Fluoro Analogue: Models of the N(4)-OH-dCMP and N(4)-OH-FdCMP Molecules Interacting with Thymidylate Synthase

The crystal and molecular structures of the 3 ,5 -di-O-acetyl-N (4)-hydroxy-2 -deoxycytidine molecule and its 5-fluoro congener have been determined by X-ray single crystal diffraction. The 3 ,5 -di-O-acetyl-N (4)-hydroxy-5-fluoro-2 -deoxycytidine molecule crystallizes in the space group C2 with the following unit cell parameters: a = 21.72Å, b = 8.72Å, c = 8.61Å, and β = 90.42 • . 3 ,5 -di-O-acetyl-N (4)-hydroxy-2 -deoxycytidine also belongs to the monoclinic space group C2 and the unit cell parameters are: a = 39.54Å, b = 8.72Å, c = 22.89Å, and β = 95.26 • . The nonfluorine analogue demonstrates a rare example of crystal structure with five symmetry-independent molecules in the unit cell. All the molecules in both crystal structures have the sugar residue anti oriented with respect to the base, as well as have the N(4)-OH residue in cis conformation relatively to the N(3)-nitrogen atom. In addition to the molecular geometries from X-ray experiment, the optimized molecular geometries have been obtained with the use of theoretical ab initio calculations at the RHF/6-31G(d) level. The corresponding geometric parameters in the molecules of 3 ,5 -di-O-acetyl-N (4)-hydroxy-2 -deoxycytidine and its 5-fluoro congener have been compared. The differences including the C(5) C(6) bond shortening and C(4) C(5) C(6) angle widening in the fluorine analogue are discussed in this paper in relation to the molecular mechanism of enzyme, thymidylate synthase, inhibition by N (4)-hydroxy-2 -deoxycytidine monophosphate and its 5-fluoro congener. KEY WORDS: Crystal structure; ab initio structure; nucleoside analogues of N (4)-OH-dCMP and N (4)-OHFdCMP; thymidylate synthase

Crystal Structure of Mouse Thymidylate Synthase in Tertiary Complex with dUMP and Raltitrexed Reveals N-Terminus Architecture and Two Different Active Site Conformations

The crystal structure of mouse thymidylate synthase (mTS) in complex with substrate dUMP and antifolate inhibitor Raltitrexed is reported. The structure reveals, for the first time in the group of mammalian TS structures, a well-ordered segment of 13 N-terminal amino acids, whose ordered conformation is stabilized due to specific crystal packing. The structure consists of two homodimers, differing in conformation, one being more closed (dimer AB) and thus supporting tighter binding of ligands, and the other being more open (dimer CD) and thus allowing weaker binding of ligands. This difference indicates an asymmetrical effect of the binding of Raltitrexed to two independent mTS molecules. Conformational changes leading to a ligand-induced closing of the active site cleft are observed by comparing the crystal structures of mTS in three different states along the catalytic pathway: ligand-free, dUMP-bound, and dUMP- and Raltitrexed-bound. Possible interaction routes between hydrophobic residues of the mTS protein N-terminal segment and the active site are also discussed

Broken symmetry between RNA enantiomers in a crystal lattice

Explaining the origin of the homochirality of biolog-ical molecules requires a mechanism of disruptingthe natural equilibrium between enantiomers and am-plifying the initial imbalance to significant levels. Au-thors of existing models have sought an explanationin the parity-breaking weak nuclear force, in some se-lectively acting external factor, or in random fluctua-tions that subsequently became amplified by an auto-catalytic process. We have obtained crystals in whichL- and D-enantiomers of short RNA duplexes assem-ble in an asymmetric manner. These enantiomersmake different lattice contacts and have different ex-posures to water and metal ions present in the crys-tal. Apparently, asymmetry between enantiomers canarise upon their mutual interactions and then prop-agate via crystallization. Asymmetric racemic com-pounds are worth considering as possible factors insymmetry breaking and enantioenrichment that tookplace in the early biosphere