Helical structures from an isotropic homopolymer model

We present Monte Carlo simulation results for square-well homopolymers at a series of bond lengths. Although the model contains only isotropic pairwise interactions, under appropriate conditions this system shows spontaneous chiral symmetry breaking, where the chain exists in either a left- or a right-handed helical structure. We investigate how this behavior depends upon the ratio between bond length and monomer radius.Comment: 10 pages, 3 figures, accepted for publication by Physical Review Letter

Phylogenetic placement and generic re-circumscriptions of the multilocular genera Arenifera, Octopoma and Schlechteranthus (Aizoaceae: Ruschieae): Evidence from anatomical, morphological and plastid DNA data

"Ruschieae is the largest tribe in the highly speciose subfamily Ruschioideae (Aizoaceae). A generic-level phylogeny for the tribe was recently produced, providing new insights into relationships between the taxa. Octopoma and Arenifera are woody shrubs with multilocular capsules and are distributed across the Succulent Karoo. Octopoma was shown to be polyphyletic in the tribal phylogeny, but comprehensive sampling is required to confirm its polyphyly. Arenifera has not previously been sampled and therefore its phylogenetic placement in the tribe is uncertain. In this study, phylogenetic sampling for nine plastid regions (atpB-rbcL, matK, psbJ-petA, rpl16, rps16, trnD-trnT, trnL-F, trnQUUG-rps16, trnS-trnG) was expanded to include all species of Octopoma and Arenifera, to assess phylogenetic placement and relationships of these genera. Three phylogenetic analyses were carried out, maximum parsimony, maximum likelihood and Bayesian inference. Leaf anatomical sections were studied to further inform generic circumscriptions. The phylogenies showed Octopoma to be polyphyletic, with the type, O. octojuge, and the related O. nanum, resolved as sister to Zeuktophyllum and Smicrostigma, while the other species were placed in the Conophytum-clade. Arenifera was also shown to be polyphyletic, with the type, A. pillansii, placed in the xeromorphic-clade, and the remainder of the species recovered among the Octopoma species in the Conophytum-clade (forming the Octopoma subglobosum-Arenifera spinescens subclade). Generic affinities of the O. subglobosum-A. spinescens subclade were assessed in relation to the sister taxon Schlechteranthus. The leaf anatomy was found to be informative within the study group. Bladder cells were observed in Arenifera pillansii, a hypodermis in Little Karoo Octopoma (O. octojuge, O. nanum, O. quadrisepalum) and epidermal cells forming blunt papillae in Schlechteranthus and the O. subglobosum-A. spinescens subclade. Upon assessment of the anatomical, morphological and phylogenetic data, Schlechteranthus is here expanded to include the species in the O. subglobosum-A. spinescens subclade. Eight new combinations are made in Schlechteranthus. As a result, Arenifera is again monotypic and the circumscription of Octopoma is refined to include three species restricted to the Little Karoo. Two subgenera within Schlechteranthus s.l. (subg. Schlechteranthus, subg. Microphyllus) are erected to accommodate differences in leaf size, capsule size, closing body size and locule number."Web of Scienc

Structure and Aggregation of a Helix-Forming Polymer

We have studied the competition between helix formation and aggregation for a simple polymer model. We present simulation results for a system of two such polymers, examining the potential of mean force, the balance between inter and intramolecular interactions, and the promotion or disruption of secondary structure brought on by the proximity of the two molecules. In particular, we demonstrate that proximity between two such molecules can stabilize secondary structure. However, for this model, observed secondary structure is not stable enough to prevent collapse of the system into an unstructured globule.Comment: Accepted to the Journal of Chemical Physic

A taxonomic revision of chamaecrista (caesalpinioideae, cassieae, cassiinae) in Southern Africa

The southern African species of Chamaecrista were all treated under Cassia in Gordon-Gray’s treatment for the Flora of Southern Africa. However, given the subsequent generic recircumscriptions in the subtribe Cassiinae, and the expanded collections of these taxa, there is a need to revisit the taxonomy of the group. The present study aimed to conduct a detailed taxonomic revision of the species of Chamaecrista indigenous to the flora of southern Africa region. Extensive fieldwork was carried out to study the taxa in their natural environment and morphological characters were additionally studied using herbarium material. The revision presented here includes comprehensive descriptions, a key to the species, nomenclature, typifications, diagnostic characters with illustrations, and geographical distribution maps of all recognised taxa. Eleven species of Chamaecrista are recognised for the flora of southern Africa, two of which are described as new, i.e. Chamaecrista grandiglandulata and C. gordon-grayei. In addition, one new subspecies is described, namely C. gordon-grayei subsp. longipedicellata, while Chamaecrista comosa var. capriconia is raised to the rank of subspecies, viz. Chamaecrista comosa subsp. capriconia. The typifications published here include lectotypes designated for Chamaecrista plumosa and C. stricta, neotypes designated for C. capensis var. flavescens, C. comosa, and C. plumosa var. diffusa, and an isolectotype designated for C. stricta

Felicia douglasii (Asteraceae-Astereae), a distinctive new species from the Cape Floristic Region, South Africa

Felicia Cass. (Asteraceae) is the largest of the African members of tribe Astereae, and comprises ±90 species from southern and tropical Africa to Arabia. As currently circumscribed, it is rather weakly diagnosed by the herbaceous or shrubby habit, mostly radiate capitula with epaleate receptacle, ± concolorous rays, mostly bisexual disc florets, and eglandular cypselas with several scabrid or barbellate pappus bristles (Grau, 1973; Herman et al., 2000; Manning and Goldblatt, 2012)

A taxonomic revision of the Pteronia camphorate group (Astereae, Asteraceae)

As a first step towards a comprehensive revision of Pteronia (Asteraceae, Astereae), we present here a taxonomic treatment of the Pteronia camphorata group, in which we recognise three species. The group includes the type species P. camphorata (an important medicinal plant) and is easily distinguished from the rest of the genus by the glabrous rather than glandular or variously pilose cypselas. The lectotype of P. camphorata has involucral bracts with entire margins and long acuminate apices, both diagnostic characters for P. stricta. As such P. stricta is here reduced into synonymy with P. camphorata and the next available name, P. aspera Thunb, reinstated to accommodate the species heretofore treated as P. camphorata. Three varieties within Pteronia camphorata had been previously recognised (var. armata, var. laevigata and var. longifolia). The characters distinguishing var. armata were found to be continuous with the variation observed within var. longifolia. The third infraspecific taxon, var. laevigata, was however found to be distinct and is therefore recognised here as a new species, P. cederbergensis Bello, Magee & Boatwr. It is readily distinguished by the glabrous branches, opposite to sub-opposite somewhat succulent leaves, the 2-seriate pappus and larger, obclavate cypselas. Transverse sections of the leaves and cypselas were also examined and provide additional differences between the three recognised species.IS

Calibration and performance of the Galileo solid-state imaging system in Jupiter orbit

The solid-state imaging subsystem (SSI) on the National Aeronautics and Space Administration’s (NASA’s) Galileo Jupiter orbiter spacecraft has successfully completed its 2-yr primary mission exploring the Jovian system. The SSI has remained in remarkably stable calibration during the 8-yr flight, and the quality of the returned images is exceptional. Absolute spectral radiometric calibration has been determined to 4 to 6% across its eight spectral filters. Software and calibration files are available to enable radiometric, geometric, modulation transfer function (MTF), and scattered light image calibration. The charge-coupled device (CCD) detector endured the harsh radiation environment at Jupiter without significant damage and exhibited transient image noise effects at about the expected levels. A lossy integer cosine transform (ICT) data compressor proved essential to achieving the SSI science objectives given the low data transmission rate available from Jupiter due to a communication antenna failure. The ICT compressor does introduce certain artifacts in the images that must be controlled to acceptable levels by judicious choice of compression control parameter settings. The SSI team’s expertise in using the compressor improved throughout the orbital operations phase and, coupled with a strategy using multiple playback passes of the spacecraft tape recorder, resulted in the successful return of 1645 unique images of Jupiter and its satellites

A massive quiescent galaxy at redshift 4.658

A. C. Carnall thanks the Leverhulme Trust for their support via a Leverhulme Early Career Fellowship. R. J. McLure, J. S. Dunlop, D. J. McLeod, V. Wild, R. Begley, C. T. Donnan and M. L. Hamadouche acknowledge the support of the Science and Technology Facilities Council. F. Cullen acknowledges support from a UKRI Frontier Research Guarantee Grant (grant reference EP/X021025/1). A. Cimatti acknowledges support from the grant PRIN MIUR 2017 - 20173ML3WW 001.The extremely rapid assembly of the earliest galaxies during the first billion years of cosmic history is a major challenge for our understanding of galaxy formation physics (1; 2; 3; 4; 5). The advent of JWST has exacerbated this issue by confirming the existence of galaxies in significant numbers as early as the first few hundred million years (6; 7; 8). Perhaps even more surprisingly, in some galaxies, this initial highly efficient star formation rapidly shuts down, or quenches, giving rise to massive quiescent galaxies as little as 1.5 billion years after the Big Bang (9; 10), however, due to their faintness and red colour, it has proven extremely challenging to learn about these extreme quiescent galaxies, or to confirm whether any exist at earlier times. Here we report the spectroscopic confirmation of a massive quiescent galaxy, GS-9209, at redshift, z = 4.658, just 1.25 billion years after the Big Bang, using JWST NIRSpec. From these data we infer a stellar mass of M∗ = 3.8 ± 0.2 × 1010 M⊙, which formed over a ≃ 200 Myr period before this galaxy quenched its star formation activity at z=6.5+0.2−0.5, when the Universe was ≃ 800 million years old. This galaxy is both a likely descendent of the highest-redshift submillimetre galaxies and quasars, and a likely progenitor for the dense, ancient cores of the most massive local galaxies.PostprintPeer reviewe

On the possibility of magneto-structural correlations: detailed studies of di-nickel carboxylate complexes

A series of water-bridged dinickel complexes of the general formula [Ni<sub>2</sub>(μ<sub>2</sub>-OH<sub>2</sub>)(μ2- O<sub>2</sub>C<sup>t</sup>Bu)<sub>2</sub>(O<sub>2</sub>C<sup>t</sup>Bu)2(L)(L0)] (L = HO<sub>2</sub>C<sup>t</sup>Bu, L0 = HO<sub>2</sub>C<sup>t</sup>Bu (1), pyridine (2), 3-methylpyridine (4); L = L0 = pyridine (3), 3-methylpyridine (5)) has been synthesized and structurally characterized by X-ray crystallography. The magnetic properties have been probed by magnetometry and EPR spectroscopy, and detailed measurements show that the axial zero-field splitting, D, of the nickel(ii) ions is on the same order as the isotropic exchange interaction, J, between the nickel sites. The isotropic exchange interaction can be related to the angle between the nickel centers and the bridging water molecule, while the magnitude of D can be related to the coordination sphere at the nickel sites