Molecular organization in molluscan operculae.

Operculin is a glycine-rich protein present as the major component of gastropod operculae. X-ray and infrared studies of operculins lead to the conclusion that operculae contain antiparallel-chain pleated sheets oriented so that the plane of the pleated sheet is parallel to the plane of the operculum, which is a flat ovoidal or circular sheet. Partial hydrolysis gives evidence of repeating Asp-Gly-Asp and Asp-Ala-Asp sequences as well as of regions rich in Gly.     

Structure of polyglycine I: a comparison of the antiparallel pleated and antiparallel rippled sheets

Packing energy calculations are made for two possible sheet structures of polyglycine I, i.e. the antiparallel pleated and rippled sheets. They indicate that the rippled sheet is the more stable structure and that its calculated lattice parameters are close to those experimentally determined. Furthermore, the results on the packing of pleated sheets of polyglycine improve understanding of the well-known model of silk fibroin structure of Marsh et al. (1955). They also suggest that the sheet structures of l-polypeptides with short side-chains should pack in monoclinic unit cells rather than the orthorhombic ones which are observed. A possible origin of this discrepancy is discussed.     

L-alanyl-L-alanyl-L-alanine: parallel pleated sheet arrangement in unhydrated crystal structure, and comparisons with the antiparallel sheet structure

The tripeptide L-alanyl-L-alanyl-L-alanine has been crystallized from a water/dimethylformamide solution in an unhydrated form, with cell dimensions a = 11.849, b = 10.004, c = 9.862 A, beta = 101.30 degrees, monoclinic space group P21 with 4 molecules per cell (2 independent molecules in the asymmetric unit). The structure was determined by direct methods and refined to a discrepancy index R = 0.057. The tri-L-alanine molecules are packed in a parallel pleated sheet arrangement with unusually long amide nitrogen-carbonyl oxygen contacts within sheets. Comparisons are made with the antiparallel pleated sheet structure of tri-L-alanine hemihydrate, previously crystallized from the same solvent system.     

Strain-based estimation of time-dependent transmural myocardial architecture in the ovine heart

Left ventricular myofibers are connected by an extensive extracellular collagen matrix to form myolaminar sheets. Histological cardiac tissue studies have previously observed a pleated transmural distribution of sheets in the ovine heart, alternating sign of the sheet angle from epicardium to endocardium. The present study investigated temporal variations in myocardial fiber and sheet architecture during the cardiac cycle. End-diastolic histological measurements made at subepicardium, midwall, and subendocardium at an anterior-basal and a lateral-equatorial region of the ovine heart, combined with transmural myocardial Lagrangian strains, showed that the sheet angle but not the fiber angle varied temporally throughout the cardiac cycle. The magnitude of the sheet angle decreased during systole at all transmural depths at the anterior-basal site and at midwall and subendocardium depths at the lateral-equatorial site, making the sheets more parallel to the radial axis. These results support a previously suggested accordion-like wall-thickening mechanism of the myocardial sheets.     

A DFT study of structure and stability of pleated and rippled cross‐β sheets with hydrophobic sidechains

The rippled cross‐β sheet, a topography, in which mirror‐image peptides are arranged with alternating chirality into a periodic two‐dimensional network, is burgeoning as a new design principle for materials and biomedical applications. Experiments by the Schneider, Nilsson, and Raskatov labs have independently shown diverse racemic mixtures of aggregation‐prone peptide of different sizes to favor the rippled over the pleated topography. Yet, systematic ab initio studies are lacking, and the field is yet to develop rules that would enable the design of new rippled cross‐β frameworks from first principles. Here, DFT calculations were performed on a set of model systems, designed to begin understanding the impact that bulky, hydrophobic sidechains have upon the formation of pleated and rippled cross‐β frameworks. It is hoped that this study will help stimulate the development of a predictive, general framework to enable rational design of rippled cross‐β sheets in the future.     

桂西黔南国家重点保护植物的地理分布、保护现状及对策

基于实地考察、文献资料和标本数据,借助GIS技术构建桂西黔南生物多样性保护优先区的39种国家重点保护植物空间地理分布格局,结合30m空间分辨率遥感影像解译的植被与地表覆盖数据,研究该优先区内的国家重点保护物种的生境与植被类型的关系及保护现状,并就优先区国家重点保护植物的地理分布格局分析优先区以外的保护空缺。此外,对优先区近二十年来的植被与地表覆盖特征的时空变化进行分析研究,结果优先区的人居用地、湿地(包括水库)面积分别增加67.75km2和371.11km2,森林面积相对稳定,灌丛和草丛面积分别减少230.69km2和174.70km2。该地区的国家重点保护植物正面临着生境退化和丧失的威胁。在此基础上对国家重点保护植物今后的保护及监测提出了对策和建议,为生物多样性研究、保护空缺分析、植被时空变化研究以及动态监测等方面的工作提供借鉴。     

Structure of apoproteins in insect lipophorin

The two polypeptide chains of cockroach and locust lipophorins were separated and their amino acid compositions were determined. Circular dichroic spectra of the lipophorins and apolipophorin from 190 to 250 nm showed a single trough at 218 nm and a peak at 194 nm. Infrared spectra of the lipophorins in D2O showed a strong peak at 1625 cm-1 and a weak shoulder at 1693 cm-1 corresponding to v (pi, 0) and nu (0, pi) of antiparallel pleated sheet. The resonance frequency splitting delta nu = nu (0, pi) -nu (pi, 0) was 68 cm-1, which was larger than that of ordinary globular proteins containing antiparallel pleated sheet. From circular dichroic and infrared spectra it was concluded that lipophorins contained polypeptides rich in antiparallel pleated sheet with longer unbroken extensions than the case for ordinary globular proteins. Partial proteolytic digestion study of lipophorins with trypsin, chymotrypsin, and subtilisin showed that the larger apolipophorin (AL1) was exposed to the surface of the particle and the smaller apolipophorin (AL2) lay protected from the attack of the enzymes. Crosslinked products between AL1 and AL2 were readily obtained when dimethylsuberimidate or dimethyladipimidate was added to the lipophorin solution, without giving lipophorin dimers, suggesting that the two chains were located within 11 A from each other. Such structural features of insect lipoprotein were compared with other insect lipophorins and the human serum low-density lipoprotein (LDL). Similarities between lipophorins and LDL were found in the molecular weight, amino acid compositions, and the secondary structure of major apoproteins.     

Molecular systematics and biogeography of the amphibious genus Littorella (Plantaginaceae)

Littorella (Plantaginaceae) is a disjunct, amphibious genus represented by three closely related species. Littorella uniflora occurs in Europe including Iceland and the Azores, L. americana is found in temperate North America, and L. australis grows in temperate South America. Littorella has been recognized in numerous floristic treatments, but its status as a genus has recently been questioned. Rahn (Botanical Journal of the Linnean Society 120: 145-198, 1996) proposed a new phylogeny for Plantaginaceae based on morphological, embryological, and chemical data in which he reduced Littorella to a subgenus of Plantago. This article compares the phylogeny proposed by Rahn to one based on DNA sequence data from the internal transcribed spacer (ITS) region. In our analysis, Littorella forms a strongly supported monophyletic clade sister to Plantago and its recognition at the generic rank appears warranted. Littorella australis is sister to L. americana, and this clade is sister to the European L. uniflora. This more distant relationship between L. uniflora and L. americana provides support for maintaining both taxa at the specific rank and suggests a European origin for Littorella. Our studies also indicate that the monotypic genus Bougueria is deeply nested within Plantago and that its inclusion within Plantago as proposed by Rahn appears justified.     

Secondary structure of histones in solution

The secondary structure of histones H2B and H3 from calf thymus has been quantitatively studied in heavy water solutions in a wide range of histone concentrations, pD, and concentrations of sodium chloride by an infrared spectroscopy method. Also, the interactions between molecules of different histones in equimolar mixtures H2A-H2B, H2A-H3, H2A-H4, H2B-H3, H2B-H4, H3-H4, and H2A-H2B-H3-H4 have been investigated using the same method. For H2B and H3 conditions favourable for aggregation have been shown to induce the formation of pleated sheet structure. When the pD and concentration of NaCl are in a physiological range, the secondary structure of H2B and H3 contains about 15% of alpha-helix, 4% of parallel pleated sheet structure, 14% of antipatallel pleated sheet structure in H2B and 18% in H3. For mixtures in all cases, except H2A-H4, there is an interaction between molecules of different histones followed by a reduction of the antiparallel pleated sheet structure content. The data on the secondary structure of histones in different states (under self-association, in mixtures, in nucleosomes, and in chromatin) have been discussed and it is suggested that: 1) the secondary structure of histones in chromatin is essentially similar to that in the state of self-association; 2) in the core nucleosome particle the quantity of DNA (in nucleotide pairs), and the quantities of alpha-helix and antiparallel pleated sheet structure (in peptide groups) satisfy the relation 1 : 1 : 1.     

Determination of intermolecular distance for a model peptide of Bombyx mori silk fibroin, GAGAG, with rotational echo double resonance

Rotational echo double resonance NMR spectroscopy is applied for the determination of the distance of intermolecular chains of pentapeptide, GAGAG (G: Gly, A: Ala), a model typical of the crystalline domain in Bombyx mori silk fibroin. 1:4 mixture of G[1-(13)C]AGAG and GAG[(15)N]AG with antiparallel beta-sheet structure was used to determine the distance of intermolecular hydrogen bonding between adjacent molecules within pleated sheet and the (13)C-(15)N interatomic distance was determined to be 4.3 A. On the other hand, 1:4 mixture of GAG[1-(13)C]AG and GAG[(15)N]AG gave information on the interpleated sheet arrangement. When we assumed the same distances between two interpleated sheets, the distance was calculated to be 5.3 A and the angle (15)N-(13)C-(15)N was 180 degrees.     

Far-infrared spectra of sequential copolymers of amino acids with alkyl group side chains

Far-infrared spectra were measured for the sequential copolymers of amino acids with alkyl group side chains. The analysis of the spectra showed that (L -Ala-L -Ala-Gly)_n, (L -Ala-Gly)_n, (L -Ala-Gly-Gly)_n, (L -Val-L -Ala-L -Ala)_n, and (L -Val-L -Ala)_n, have the antiparallel pleated sheet structures and that the backbone conformations of (L -Val-L -Val-L -Ala)_n and (L -Val-L -Val-Gly)_n are the same as that of poly-L -valine. The far-infrared bands characteristic of the antiparallel pleated sheet structure were assigned on the basis of the result of the normal coordinate analysis of poly-L -alanine with this structure. The intersheet and interchain spacings of the sequential copolymers with the antiparallel pleated sheet structure were determined from the x-ray powder-diffraction patterns of these samples.     

Logical analysis of the mechanism of protein folding II. The nucleation process

Regions of secondary structure are predicted, without using information about the conformation of the protein itself, and compared with crystallographic assignments for seven proteins of recently published sequence and conformation (Table 1). It is observed in Table 3 that the prediction of helices is good (78.7% for %cor.ass.3), except for proteins having large antiparallel pleated sheets, and the prediction of β-structure is quite good (51.2% for %cor.ass.3) except for helix-rich proteins.The prediction of secondary structure from sequence, and a survey of all protein structures analysed so far by X-ray crystallography, suggest that nuceleation starts in almost all cases from interactions in the medium range between the regions having helical potential (α-candidate) and β-structural potential (β-candidate), which are very close to each other but separated by at least three hydrophilic or neutral residues in four consecutive residues on the polypeptide chain. Predictability of loops or turns is enhanced to 71.3% (%cor.ass.2) from 64.4% by taking into account the contiguous α-β interactions. Such a medium-range interaction is called here a probable nucleus. There are a lot of nuclei in large proteins such as carboxypeptidase Aα, while there exists at least one in small proteins like the trypsin inhibitor, Moreover, such an interaction could be a transitionary state towards a helix-rich protein, and towards a helix-deficient protein having a large antiparallel pleated sheet β-structure as well.The analysis of the relation between probable nuclei with regard to their mutual spatial proximity strongly suggests that the topological pathway of the polypeptide chain in three-dimensional space might be decided by the long-range interactions between an α-candidate and a β-candidate. An empirical rule is observed that almost all parallel pleated sheets are accompanied by helices in their neighbourhood. An accumulation of chemical facts, such as complementation experiments, combinations of disulphide bonds, etc., seems also to be elucidated by the proposed mechanism of protein folding.     

A computational study of the aerodynamic performance of a dragonfly wing section in gliding flight

A comprehensive computational fluid-dynamics-based study of a pleated wing section based on the wing of Aeshna cyanea has been performed at ultra-low Reynolds numbers corresponding to the gliding flight of these dragonflies. In addition to the pleated wing, simulations have also been carried out for its smoothed counterpart (called the 'profiled' airfoil) and a flat plate in order to better understand the aerodynamic performance of the pleated wing. The simulations employ a sharp interface Cartesian-grid-based immersed boundary method, and a detailed critical assessment of the computed results was performed giving a high measure of confidence in the fidelity of the current simulations. The simulations demonstrate that the pleated airfoil produces comparable and at times higher lift than the profiled airfoil, with a drag comparable to that of its profiled counterpart. The higher lift and moderate drag associated with the pleated airfoil lead to an aerodynamic performance that is at least equivalent to and sometimes better than the profiled airfoil. The primary cause for the reduction in the overall drag of the pleated airfoil is the negative shear drag produced by the recirculation zones which form within the pleats. The current numerical simulations therefore clearly demonstrate that the pleated wing is an ingenious design of nature, which at times surpasses the aerodynamic performance of a more conventional smooth airfoil as well as that of a flat plate. For this reason, the pleated airfoil is an excellent candidate for a fixed wing micro-aerial vehicle design.     

Side-chain control of beta-peptide secondary structures.

As one of the most important families of non-natural polymers with the propensity to form well-defined secondary structures, the beta-peptides are attracting increasing attention. The compounds incorporating beta-amino acid residues have found various applications in medicinal chemistry and biochemistry. The conformational pool of beta-peptides comprises several periodic folded conformations, which can be classified as helices, and nonpolar and polar strands. The latter two are prone to form pleated sheets. The numerous studies that have been performed on the side-chain dependence of the stability of the folded structures allow certain conclusions concerning the principles of design of the beta-peptide foldamers. The folding propensity is influenced by local torsional, side-chain to backbone and long-range side-chain interactions. Although beta-peptide foldamers are sensitive to solvent, the systematic choice of the side-chain pattern and spatiality allows the design of the desired specific secondary structure. The application of beta-peptide foldamers may open up new directions in the synthesis of highly organized artificial tertiary structures with biochemical functions.     

Role of African protected areas in maintaining connectivity for large mammals

The African protected area (PA) network has the potential to act as a set of functionally interconnected patches that conserve meta-populations of mammal species, but individual PAs are vulnerable to habitat change which may disrupt connectivity and increase extinction risk. Individual PAs have different roles in maintaining connectivity, depending on their size and location. We measured their contribution to network connectivity (irreplaceability) for carnivores and ungulates and combined it with a measure of vulnerability based on a 30-year trend in remotely sensed vegetation cover (Normalized Difference Vegetation Index). Highly irreplaceable PAs occurred mainly in southern and eastern Africa. Vegetation cover change was generally faster outside than inside PAs and particularly so in southern Africa. The extent of change increased with the distance from PAs. About 5% of highly irreplaceable PAs experienced a faster vegetation cover loss than their surroundings, thus requiring particular conservation attention. Our analysis identified PAs at risk whose isolation would disrupt the connectivity of the PA network for large mammals. This is an example of how ecological spatial modelling can be combined with large-scale remote sensing data to investigate how land cover change may affect ecological processes and species conservation.     

基于土地覆盖和NDVI变化的拉萨河流域生境质量评估

气候变化和人类活动导致的土地覆盖和植被变化都会对生境质量产生影响。青藏高原是众多珍稀高原动植物的栖息地,具有重要的生物多样性维持价值。拉萨河流域是青藏高原经济最发达、人口最密集的核心地区,人类活动对生境质量带来的胁迫和压力持续增加。为揭示近些年来土地覆盖和植被变化对拉萨河流域生境质量的影响,选择生长季NDVI作为植被变化的指示因子,通过对不同植被类型各年份的生境适宜度进行修正,利用In VEST模型评估了拉萨河流域1990—2015年的生境质量时空变化。研究结果表明,1990—2015年拉萨河流域土地覆盖变化整体相对较小,其中人工表面和湿地面积增幅相对较大,分别为82.65%和32.40%;土地覆盖变化的转移方向主要为稀疏草地转化为草原和草甸、耕地转化为人工表面以及冰川/积雪转化为荒地。植被变化方面,1990—2000年,除流域中上游的裸岩、裸土地区和念青唐古拉山地区外,流域NDVI整体有较显著上升;而2000年以后略有下降。从生境质量的空间分布来看,高质量生境主要分布在流域下游、念青唐古拉山南侧河谷地区以及拉萨河源头等地区,低质量生境主要分布在拉萨市市辖区及周边、林周县县城及周边,以及流域中上游的荒地等地区。从时间变化上来看,1990—2000年,拉萨河流域整体生境质量指数从0.51上升到0.57; 2010年和2015年整体生境质量指数分别为0.56和0.55,较2000年略有下降。相比于土地覆盖变化,NDVI对生境质量变化的影响更为显著。     

The Risk of Peat Erosion from Climate Change: Land Management Combinations—An Assessment with Bayesian Belief Networks

Modeling risk factors to soils is constrained by the lack of key data and understanding that explicitly and quantitatively link specific threats to risk. Peat erosion results from the complex interaction of climatic, topographic, and anthropogenic influences acting over a long period of time. With numerous contemporary factors operating to perpetuate the erosion processes, it is often difficult to identify with certainty what actually are the initial and subsequent drivers of erosion. In this situation, expert opinion forms a vital source of information. Here we demonstrate how Bayesian Belief Networks (BBN) can be used to combine quantitative data from the National Soils Inventory of Scotland (NSIS) with qualitative expert knowledge to estimate risk of peat erosion in Scotland. This model was used to identify the main factors associated with peat erosion. It was shown that climatic variables (increased temperature, decreased precipitation) are the most important risk factors for perpetuating peatland erosion. However, the BBN approach also indicated that maintaining good vegetation cover is a significant mitigating factor. It would follow that land management practices that impact negatively on vegetation cover would also exacerbate peatland erosion given a hot dry climate.     

Impacts of an invasive snail (Tarebia granifera) on nutrient cycling in tropical streams: the role of riparian deforestation in Trinidad, West Indies

Non-native species and habitat degradation are two major catalysts of environmental change and often occur simultaneously. In freshwater systems, degradation of adjacent terrestrial vegetation may facilitate introduced species by altering resource availability. Here we examine how the presence of intact riparian cover influences the impact of an invasive herbivorous snail, Tarebia granifera, on nitrogen (N) cycling in aquatic systems on the island of Trinidad. We quantified snail biomass, growth, and N excretion in locations where riparian vegetation was present or removed to determine how snail demographics and excretion were related to the condition of the riparian zone. In three Neotropical streams, we measured snail biomass and N excretion in open and closed canopy habitats to generate estimates of mass- and area-specific N excretion rates. Snail biomass was 2 to 8 times greater and areal N excretion rates ranged from 3 to 9 times greater in open canopy habitats. Snails foraging in open canopy habitat also had access to more abundant food resources and exhibited greater growth and mass-specific N excretion rates. Estimates of ecosystem N demand indicated that snail N excretion in fully closed, partially closed, and open canopy habitats supplied 2%, 11%, and 16% of integrated ecosystem N demand, respectively. We conclude that human-mediated riparian canopy loss can generate hotspots of snail biomass, growth, and N excretion along tropical stream networks, altering the impacts of an invasive snail on the biogeochemical cycling of N.     

荒漠草原区柠条固沙人工林地表草本植被季节变化特征

研究荒漠草原人工林固沙区地表草本植被季节变化特征及其和柠条林龄的关系,对于分析柠条人工林地表草本植物的季节适应性和制订合理的人工林管理措施均具有重要的科学意义。选择6、15、24年生和36年生柠条人工林为研究对象,通过调查每个样地5月、8月和10月地表草本植物密度、物种数、盖度和高度,分析了荒漠草原区柠条人工固沙林生长过程中地表草本植被季节变化特征及其影响因素。结果表明,地表草本植物物种数在柠条林龄6和15a时受季节改变的影响较小,在24a之后受到季节变化的显著影响(P0.05)。地表草本植物密度在柠条林龄6a时受季节改变的影响较小,但在15a之后季节变化显著影响地表草本植物个体数分布(P0.05),而且在10月具有最多的地表草本植物个体数。地表草本植被盖度和高度均受到季节变化的显著影响(P0.05),而受林龄的影响较小;不同年龄林地地表草本植被盖度和高度均表现为10月和8月较高,5月较低。研究表明,荒漠草原柠条人工林固沙区,柠条林发育生长和灌木形态的改变不仅影响土壤营养条件,而且还可以调控由于季节改变而引起的土壤温湿度变化,柠条林龄和季节更替二者交互作用,共同影响地表草本植被的季节变化特征。     

Ecohydrological feedback mechanisms in arid rangelands: Simulating the impacts of topography and land use

The interaction between ecological and hydrological processes is particularly important in arid and semi-arid regions. Often the interaction between these processes is not completely understood and they are studied separately. We developed a grid-based computer model simulating the dynamics of the four most common vegetation types (perennial grass, annuals, dwarf shrubs and shrubs) and related hydrological processes in the region studied. Eco-hydrological interactions gain importance in rangelands with increasing slope, where vegetation cover obstructs run-off and decreases evaporation from the soil. Overgrazing can influence these positive feedback mechanisms. In this study, we first show that model predictions of cover and productivity of the vegetation types are realistic by comparing them with estimates obtained from field surveys. Then, we apply a realistic range in slope angle combined with two land use regimes (light versus heavy grazing intensity).Our simulation results reveal that hydrological processes and associated productivity are strongly affected by slope, whereas the magnitude of this impact depends on overgrazing. Under low stocking rates, undisturbed vegetation is maintained and run-off and evaporation remain low on flat plains and gentle slope. On steep slopes, run-off and evaporation become larger, while water retention potential decreases, which leads to reduced productivity. Overgrazing, however, reduces vegetation cover and biomass production and the landscape's ability to conserve water decreases even on flat plains and gentle slopes.Generally, the abundance of perennial grasses and shrubs decreases with increasing slope and grazing. Dominance is shifted towards shrubs and annuals. As a management recommendation we suggest that different vegetation growth forms should not only be regarded as forage producers but also as regulators of ecosystem functioning. Particularly on sloping range lands, a high percentage of cover by perennial vegetation insures that water is retained in the system.