Direct Ultraviolet Imaging and Spectroscopy of Betelgeuse

Direct images of Betelgeuse were obtained over a span of 4 years with the Faint Object Camera on the Hubble Space Telescope. These images reveal the extended ultraviolet continuum emission (about 2 times the optical diameter), the varying overall ultraviolet flux levels and a pattern of bright surface continuum features that change in position and appearance over several months or less. Concurrent photometry and radial velocity measures support the model of a pulsating star, first discovered in the ultraviolet from IUE. Spatially resolved HST spectroscopy reveals a larger extention in chromospheric emissions of Mg II as well as the rotation of the supergiant. Changing localized subsonic flows occur in the low chromosphere that can cover a substantial fraction of the stellar disk and may initiate the mass outflow.Comment: 9 pages, 5 figures, Betelgeuse Workshop, November 2012, Paris. To be published in the European Astronomical Society Publications Series, 2013, Editors: Pierre Kervella, Thibaut Le Bertre & Guy Perri

Molecular architecture of softwood revealed by solid-state NMR

Economically important softwood from conifers is mainly composed of the polysaccharides cellulose, galactoglucomannan and xylan, and the phenolic polymer, lignin. The interactions between these polymers lead to wood mechanical strength and must be overcome in biorefining. Here, we use 13C multidimensional solid-state NMR to analyse the polymer interactions in never-dried cell walls of the softwood, spruce. In contrast to some earlier softwood cell wall models, most of the xylan binds to cellulose in the two-fold screw conformation. Moreover, galactoglucomannan alters its conformation by intimately binding to the surface of cellulose microfibrils in a semi-crystalline fashion. Some galactoglucomannan and xylan bind to the same cellulose microfibrils, and lignin is associated with both of these cellulose-bound polysaccharides. We propose a model of softwood molecular architecture which explains the origin of the different cellulose environments observed in the NMR experiments. Our model will assist strategies for improving wood usage in a sustainable bioeconomy

Different stellar rotation in the two main sequences of the young globular cluster NGC1818: first direct spectroscopic evidence

We present a spectroscopic analysis of main sequence (MS) stars in the young globular cluster NGC1818 (age~40 Myrs) in the Large Magellanic Cloud. Our photometric survey on Magellanic Clouds clusters has revealed that NGC1818, similarly to the other young objects with age 600 Myrs, displays not only an extended MS Turn-Off (eMSTO), as observed in intermediate-age clusters (age~1-2 Gyrs), but also a split MS. The most straightforward interpretation of the double MS is the presence of two stellar populations: a sequence of slowly-rotating stars lying on the blue-MS and a sequence of fast rotators, with rotation close to the breaking speed, defining a red-MS. We report the first direct spectroscopic measurements of projected rotational velocities vsini for the double MS, eMSTO and Be stars of a young cluster. The analysis of line profiles includes non-LTE effects, required for correctly deriving v sini values. Our results suggest that: (i) the mean rotation for blue- and red-MS stars is vsini=71\pm10 km/s (sigma=37 km/s) and vsini=202\pm23 km/s (sigma=91 km/s), respectively; (ii) eMSTO stars have different vsini, which are generally lower than those inferred for red-MS stars, and (iii) as expected, Be stars display the highest vsini values. This analyis supports the idea that distinct rotational velocities play an important role in the appearence of multiple stellar populations in the color-magnitude diagrams of young clusters, and poses new constraints to the current scenarios.Comment: 16 pages, 1 table, 9 figures. Accepted for publication in AJ (11/07/2018

Folding of xylan onto cellulose fibrils in plant cell walls revealed by solid-state NMR

Exploitation of plant lignocellulosic biomass is hampered by our ignorance of the molecular basis for its properties such as strength and digestibility. Xylan, the most prevalent non-cellulosic polysaccharide, binds to cellulose microfibrils. The nature of this interaction remains unclear, despite its importance. Here we show that the majority of xylan, which forms a threefold helical screw in solution, flattens into a twofold helical screw ribbon to bind intimately to cellulose microfibrils in the cell wall. $^{13}$C solid-state magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, supported by in silico predictions of chemical shifts, shows both two- and threefold screw xylan conformations are present in fresh Arabidopsis stems. The twofold screw xylan is spatially close to cellulose, and has similar rigidity to the cellulose microfibrils, but reverts to the threefold screw conformation in the cellulose-deficient irx3 mutant. The discovery that induced polysaccharide conformation underlies cell wall assembly provides new principles to understand biomass properties.This work was part supported by BBSRC Grant BB/G016240/1 via The BBSRC Sustainable Bioenergy Cell Wall Sugars Programme. ODB and ERdA are grateful to CNPq for financial support for this work via Grants # 159341/2011-6 and 206278/2014-4. ACP is grateful to the Royal Society for a Newton International Fellowship. PD is supported by the Leverhulme Trust grant for the Centre for Natural Material Innovation. The UK 850 MHz solid-state NMR Facility used in this research was funded by EPSRC and BBSRC, as well as the University of Warwick including via part funding through Birmingham Science City Advanced Materials Projects 1 and 2 supported by Advantage West Midlands (AWM) and the European Regional Development Fund (ERDF). (Contract reference PR140003 for work after 5 January 2015). DFT calculations of NMR parameters were performed at the Centre for Scientific Computing at the University of Warwick

Structural and electronic determinants of lytic polysaccharide monooxygenase reactivity on polysaccharide substrates

Lytic polysaccharide monooxygenases (LPMOs) are industrially important copper-dependent enzymes that oxidatively cleave polysaccharides. We studied two closely related AA9-family LPMOs from Lentinus similis (LsAA9A) and Collariella virescens (CvAA9A). LsAA9A and CvAA9A cleave a range of polysaccharides, including cellulose, xyloglucan, mixed-linkage glucan, and glucomannan. LsAA9A additionally cleaves isolated xylan substrates, the first LPMO to show such activity. Insights into the determinants of specificity come from the structures of CvAA9A and of LsAA9A bound to cellulosic and non-cellulosic oligosaccharides. EPR spectra further reveal differences in copper co-ordination on binding of xylan compared to glucans. LsAA9A activity is notably less sensitive to reducing agent potential on xylan when compared to other substrates, suggesting a different mechanistic pathway for the cleavage of xylan. These data show that AA9 LPMOs can display different apparent substrate specificities dependent upon both productive protein:carbohydrate interactions across a binding surface and also electronic considerations at the copper active site.Travel to synchrotrons was supported by the Danish Ministry of Higher Education and Science through the Instrument Center DANSCATT and the European Community’s Seventh Framework Programme (FP7/2007-2013) under BioStruct-X (grant agreement 283570). This work was supported by the UK Biotechnology and Biological Sciences Research Council (grant numbers BB/L000423/1 to P.D. and P.H.W., and BB/L021633/1 to P.H.W.) and the Danish Council for Strategic Research (grant numbers 12-134923 to L.L.L. and 12-134922 to K.S.J.)

Probing the molecular architecture of Arabidopsis thaliana secondary cell walls using two- and three-dimensional (13)C solid state nuclear magnetic resonance spectroscopy.

The plant secondary cell wall is a thickened polysaccharide and phenolic structure, providing mechanical strength to cells, particularly in woody tissues. It is the main feedstock for the developing bioenergy and green chemistry industries. Despite the role that molecular architecture (the arrangement of biopolymers relative to each other, and their conformations) plays in dictating biomass properties, such as recalcitrance to breakdown, it is poorly understood. Here, unprocessed dry (13)C-labeled stems from the model plant Arabidopsis thaliana were analyzed by a variety of (13)C solid state magic angle spinning nuclear magnetic resonance methods, such as one-dimensional cross-polarization and direct polarization, two-dimensional refocused INADEQUATE, RFDR, PDSD, and three-dimensional DARR, demonstrating their viability for the study of native polymer arrangements in intact secondary cell walls. All carbon sites of the two main glucose environments in cellulose (previously assigned to microfibril surface and interior residues) are clearly resolved, as are carbon sites of the other major components of the secondary cell wall: xylan and lignin. The xylan carbon 4 chemical shift is markedly different from that reported previously for solution or primary cell wall xylan, indicating significant changes in the helical conformation in these dried stems. Furthermore, the shift span indicates that xylan adopts a wide range of conformations in this material, with very little in the 31 conformation typical of xylan in solution. Additionally, spatial connections of noncarbohydrate species were observed with both cellulose peaks conventionally assigned as "surface" and as "interior" cellulose environments, raising questions about the origin of these two cellulose signals.This work was supported by BBSRC Grant BB/G016240/1, via The BBSRC Sustainable Bioenergy Cell Wall Sugars Programme. The UK 850 MHz solid state NMR Facility was funded by EPSRC Grant EP/F017901/1 and the BBSRC, as well as the University of Warwick, including via partial funding through Birmingham Science City Advanced Materials Projects 1 and 2, by Advantage West Midlands (AWM) and the European Regional Development Fund (ERDF).This is the final published version. It first appeared at http://pubs.acs.org/doi/abs/10.1021/bi501552k

Combined ultraviolet studies of astronomical sources

Topics addressed include: Cygnus Loop; P Cygni profiles in dwarf novae; YY Gem; nova shells; HZ Herculis; activity cycles in cluster giants; Alpha Ori; metal deficient giant stars; ultraviolet spectra of symbiotic stars detected by the Very Large Array; time variability in symbiotic stars; blue galaxies; and quasistellar objects with X-ray spectra