Asymptotic Symmetries of Gravity and Higher-Spin Theories

The present work aims to propose a higher-spin generalization of the connection, recently pointed out, between infinite-dimensional asymptotic symmetries of gravity/QED and soft theorems in particle physics. To this purpose, we first set the stage for the current debate on the topic by reviewing the main results about asymptotically flat spaces, together with Weinberg's celebrated soft theorems. We then show that, under a specific choice of falloff conditions, it is indeed possible to retrieve an infinite-dimensional asymptotic symmetry group for any integer spin, whose corresponding Ward identities are equivalent to Weinberg's soft factorization theorem. In addition, we also address the definition of asymptotic symmetries in higher-dimensional spacetimes. To tackle this problem we provide a geometric argument supporting the existence of an infinite-dimensional asymptotic symmetry group in any dimension

Crystallographic and Fluorescence Studies of the Interaction of Haloalkane Dehalogenase with Halide Ions. Studies with Halide Compounds Reveal a Halide Binding Site in the Active Site

Haloalkane dehalogenase from Xanthobacter autotrophicus GJ10 catalyzes the conversion of 1,2-dichloroethane to 2-chloroethanol and chloride without use of oxygen or cofactors. The active site is situated in an internal cavity, which is accesible from the solvent, even in the crystal. Crystal structures of the dehalogenase enzyme complexed with iodoacetamide, chloroacetamide, iodide, and chloride at pH 6.2 and 8.2 revealed a halide binding site between the ring NH's of two tryptophan residues, Trp-125 and Trp-175, located in the active site. The halide ion lies on the intersection of the planes of the rings of the tryptophans. The binding of iodide and chloride to haloalkane dehalogenase caused a strong decrease in protein fluorescence. The decrease could be fitted to a modified form of the Stern-Volmer equation, indicating the presence of fluorophors of different accessibilities. Halide binding was much stronger at pH 6.0 than at pH 8.2. Assuming ligand binding to Trp-125 and Trp-175 as the sole cause of fluorescence quenching, dissociation constants at pH 6.0 with chloride and iodide were calculated to be 0.49 +/- 0.04 and 0.074 +/- 0.007 mM, respectively. Detailed structural investigation showed that the halide binding site probably stabilizes the halide product as well as the negatively charged transition state occurring during the formation of the covalent intermediate

The cyclization mechanism of cyclodextrin glycosyltransferase (CGTase) as revealed by a gamma-cyclodextrin-CGTase complex at 1.8-angstrom resolution

The enzyme cyclodextrin glycosyltransferase is closely related to alpha-amylases but has the unique ability to produce cyclodextrins (circular alpha(1-->4)-linked glucoses) from starch. To characterize this specificity we determined a 1.8-Angstrom structure of an E257Q/D229N mutant cyclodextrin glycosyltransferase in complex with its product gamma-cyclodextrin, which reveals for the first time how cyclodextrin is competently bound. Across subsites -2, -1, and fl, the cyclodextrin ring binds in a twisted mode similar to linear sugars, giving rise to deformation of its circular symmetry. At subsites -3 and +2, the cyclodextrin binds in a manner different from linear sugars. Sequence comparisons and site-directed mutagenesis experiments support the conclusion that subsites -3 and +2 confer the cyclization activity in addition to subsite -6 and Tyr-195, On this basis, a role of the individual residues during the cyclization reaction cycle is proposed

Crystal structure of quinohemoprotein alcohol dehydrogenase from Comamonas testosteroni - Structural basis for substrate oxidation and electron transfer

Quinoprotein alcohol dehydrogenases are redox enzymes that participate in distinctive catabolic pathways that enable bacteria to grow on various alcohols as the sole source of carbon and energy. The x-ray structure of the quinohemoprotein alcohol dehydrogenase from Comamonas testosteroni has been determined at 1.44 Angstrom resolution. It comprises two domains. The N-terminal domain has a beta-propeller fold and binds one pyrrolo-quinoliue quinone cofactor and one calcium ion in the active site. A tetrahydrofuran-2-carboxylic acid molecule is present in the substrate-binding cleft. The position of this oxidation product provides valuable information on the amino acid residues involved in the reaction mechanism and their function. The C-terminal domain is an a-helical type I cytochrome c with His(608) and Met(647) as heme-iron ligands. This is the first reported structure of an electron transfer system between a quinoprotein alcohol dehydrogenase and cytochrome c. The shortest distance between pyrroloquinoline quinone and heme c is 12.9 Angstrom, one of the longest physiological edge-to-edge distances yet determined between two redox centers. A highly unusual disulfide bond between two adjacent cysteines bridges the redox centers. It appears essential for electron transfer. A water channel delineates a possible pathway for proton transfer from the active site to the solvent.</p

A Numerical Study of the Salinity Structure of a Shallow Bay - Case of Copano Bay, TX

The Gulf of Mexico has 39 estuaries, in which most of them are characterized as bar-built, shallow bay estuaries. Located at the northwest Gulf of Mexico, the Mission Aransas Estuarine Research Reserve is an area with 750 km^2 with 6 bays. The second largest bay is named Copano Bay, an area with 200 km^2 that has two main river sources, from Mission River and Aransas River, which are the only source of fresh water to the system. The bay is opened at one tidal channel at the south that exchanges salty water with Aransas Bay. As part of the monitoring system for Copano Bay, we used the two stations located at the east and west sides of the bay to understand the temporal variability of salinity in the bay. Because the salinity pattern is not as well defined as the temperature profile, we used a 3D hydrodynamic model (ROMS) to analyze how changes in river discharge, precipitation and winds will affect the bay. After running the simulations for 5 years, from January/2010 to December/2014, we found that the salinity of the bay is controlled by flooding events on the upper bay and by tides on the channel side. During ’wet years’ (2010 and 2015), the salinity is kept in a range between 10 gkg^-1 and 25 gkg^-1. For ’dry years’, where the discharge is low, the salinity was kept in a range of 30 gkg^-1 to 45 gkg^-1, considered hypersaline conditions. The year of 2011, considered a ’transition year’, had the lowest river discharge and precipitation, causing the salinity to increase at a constant rate. By comparing the east and west sides, we saw that the east side is barely influenced by river discharge, responding mostly to the tides, while the west side is mostly influenced by the river discharge. The flooding events are responsible for an increase in vertical and horizontal stratification. A closer look at local events showed the water column took longer to stabilize, after a change in wind due to a storm or front, under hypersaline conditions than under normal years