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961.
Zhang J Markiewicz MJ Mowery BP Weisblum B Stahl SS Gellman SH 《Biomacromolecules》2012,13(2):323-331
Nylon-3 polymers contain β-amino-acid-derived subunits and can be viewed as higher homologues of poly(α-amino acids). This structural relationship raises the possibility that nylon-3 polymers offer a platform for development of new materials with a variety of biological activities, a prospect that has recently begun to receive experimental support. Nylon-3 homo- and copolymers can be prepared via anionic ring-opening polymerization of β-lactams, and use of an N-acyl-β-lactam as coinitiator in the polymerization reaction allows placement of a specific functional group, borne by the N-acyl-β-lactam, at the N-terminus of each polymer chain. Controlling the unit at the C-termini of nylon-3 polymer chains, however, has been problematic. Here we describe a strategy for specifying C-terminal functionality that is based on the polymerization mechanism. After the anionic ring-opening polymerization is complete, we introduce a new β-lactam, approximately 1 equiv relative to the expected number of polymer chains. Because the polymer chains bear a reactive imide group at their C-termini, this new β-lactam should become attached at this position. If the terminating β-lactam bears a distinctive functional group, that functionality should be affixed to most or all C-termini in the reaction mixture. We use the new technique to compare the impact of N- and C-terminal placement of a critical hydrophobic fragment on the biological activity profile of nylon-3 copolymers. The synthetic advance described here should prove to be generally useful for tailoring the properties of nylon-3 materials. 相似文献
962.
de Medeiros Modolon S Otsuka I Fort S Minatti E Borsali R Halila S 《Biomacromolecules》2012,13(4):1129-1135
The preparation of biocompatible nanocarriers that have potential applications in the cosmetic and health industries is highly desired. The self-assembly of amphiphilic block copolymers displaying biosourced polysaccharides at the surface is one of the most promising approaches. In the continuity of our works related to the preparation of "hybrid" amphiphilic oligosaccharide-based block copolymers, we present here the design of a new generation of self-assembled nanoparticles composed entirely of oligosaccharide-based amphiphilic block co-oligomers (BCO). These systems are defined by a covalent linkage of the two saccharidic blocks through their reducing end units, resulting in a sweet "head-to-head" connection. As an example, we have prepared and studied a BCO in which the hydrophilic part is composed of a free maltoheptaosyl derivative clicked to a hydrophobic part composed of a peracetylated maltoheptaosyl derivative. This amphiphilic BCO self-assembles to form spherical micelles in water with an average diameter of 30 nm. The efficient enzymatic hydrolysis of the maltoheptaose that constitutes the shell of the micelles was followed by light scattering and colorimetric methods. 相似文献
963.
Boyer S Brown SD Collins RA Cruickshank RH Lefort MC Malumbres-Olarte J Wratten SD 《PloS one》2012,7(5):e38215
DNA barcoding remains a challenge when applied to diet analyses, ancient DNA studies, environmental DNA samples and, more generally, in any cases where DNA samples have not been adequately preserved. Because the size of the commonly used barcoding marker (COI) is over 600 base pairs (bp), amplification fails when the DNA molecule is degraded into smaller fragments. However, relevant information for specimen identification may not be evenly distributed along the barcoding region, and a shorter target can be sufficient for identification purposes. This study proposes a new, widely applicable, method to compare the performance of all potential 'mini-barcodes' for a given molecular marker and to objectively select the shortest and most informative one. Our method is based on a sliding window analysis implemented in the new R package SPIDER (Species IDentity and Evolution in R). This method is applicable to any taxon and any molecular marker. Here, it was tested on earthworm DNA that had been degraded through digestion by carnivorous landsnails. A 100 bp region of 16 S rDNA was selected as the shortest informative fragment (mini-barcode) required for accurate specimen identification. Corresponding primers were designed and used to amplify degraded earthworm (prey) DNA from 46 landsnail (predator) faeces using 454-pyrosequencing. This led to the detection of 18 earthworm species in the diet of the snail. We encourage molecular ecologists to use this method to objectively select the most informative region of the gene they aim to amplify from degraded DNA. The method and tools provided here, can be particularly useful (1) when dealing with degraded DNA for which only small fragments can be amplified, (2) for cases where no consensus has yet been reached on the appropriate barcode gene, or (3) to allow direct analysis of short reads derived from massively parallel sequencing without the need for bioinformatic consolidation. 相似文献
964.
Background
We aimed to evaluate the incidence of type 1 diabetes mellitus in children <15 years of age (yr) in the Auckland region (New Zealand) over 20 years (1990–2009).Methods
We performed a retrospective review of all patients <15 yr diagnosed with type 1 diabetes, from an unselected complete regional cohort.Results
There were 884 new cases of type 1 diabetes, and age at diagnosis rose from 7.6 yr in 1990/1 to 8.9 yr in 2008/9 (r2 = 0.31, p = 0.009). There was a progressive increase in type 1 diabetes incidence among children <15 yr (p<0.0001), reaching 22.5 per 100,000 in 2009. However, the rise in incidence did not occur evenly among age groups, being 2.5-fold higher in older children (10–14 yr) than in the youngest group (0–4 yr). The incidence of new cases of type 1 diabetes was highest in New Zealand Europeans throughout the study period in all age groups (p<0.0001), but the rate of increase was similar in New Zealand Europeans and Non-Europeans. Type 1 diabetes incidence and average annual increase were similar in both sexes. There was no change in BMI SDS shortly after diagnosis, and no association between BMI SDS and age at diagnosis.Conclusions
There has been a steady increase in type 1 diabetes incidence among children <15 yr in Auckland over 20 years. Contrary to other studies, age at diagnosis has increased and the greatest rise in incidence occurred in children 10–14 yr. There was little change in BMI SDS in this population, providing no support for the ‘accelerator hypothesis’. 相似文献965.
Building cognitive abilities often requires sustained engagement with effortful tasks. We demonstrate that beliefs about willpower-whether willpower is viewed as a limited or non-limited resource-impact sustained learning on a strenuous mental task. As predicted, beliefs about willpower did not affect accuracy or improvement during the initial phases of learning; however, participants who were led to view willpower as non-limited showed greater sustained learning over the full duration of the task. These findings highlight the interactive nature of motivational and cognitive processes: motivational factors can substantially affect people's ability to recruit their cognitive resources to sustain learning over time. 相似文献
966.
967.
Westerterp M Gourion-Arsiquaud S Murphy AJ Shih A Cremers S Levine RL Tall AR Yvan-Charvet L 《Cell Stem Cell》2012,11(2):195-206
Intact cholesterol homeostasis helps to maintain hematopoietic stem and multipotential progenitor cell (HSPC) quiescence. Mice with defects in cholesterol efflux pathways due to deficiencies of the ATP binding cassette transporters ABCA1 and ABCG1 displayed a dramatic increase in HSPC mobilization and extramedullary hematopoiesis. Increased extramedullary hematopoiesis was associated with elevated serum levels of G-CSF due to generation of IL-23 by splenic macrophages and dendritic cells. This favored hematopoietic lineage decisions toward granulocytes rather than macrophages in the bone marrow leading to impaired support for osteoblasts and decreased Cxcl12/SDF-1 production by mesenchymal progenitors. Greater HSPC mobilization and extramedullary hematopoiesis were reversed by raising HDL levels in Abca1(-/-)Abcg1(-/-) and Apoe(-/-) mice or in a mouse model of myeloproliferative neoplasm mediated by Flt3-ITD mutation. Our data identify a role of cholesterol efflux pathways in the control of HSPC mobilization. This may translate into therapeutic strategies for atherosclerosis and hematologic malignancies. 相似文献
968.
969.
Dana M. Blumenthal Andrew P. Norton Samuel E. Cox Erik M. Hardy Glen E. Liston Lisa Kennaway D. Terrance Booth Justin D. Derner 《Biological invasions》2012,14(2):395-404
Identifying environments where invasive plants are most invasive is key to understanding causes of invasion and developing
effective management strategies. In mixed-grass prairie, invasive plants are often successful in relatively wet, nitrogen-rich
areas, and areas protected from grazing. Dalmatian toadflax, a common invader of mixed-grass prairie, can also be favored
by high water and nitrogen availability, but is thought to be relatively unpalatable to cattle, and therefore favored by grazing.
We used spatially-adjusted model selection techniques to quantify relationships between toadflax cover (measured using very
high-resolution aerial imagery), and relative snow deposition (estimated with a blowing snow model), slope, aspect, soil texture,
and grazing intensity (estimated by proximity to water tanks). Toadflax was common throughout the 400 ha study site, occurring
in 742 of 1,861 images. Toadflax cover was high on steeper slopes, particularly those with southern aspects. These two topographic
variables were more effective in explaining toadflax distribution than modeled snow deposition, suggesting that factors other
than snow deposition cause toadflax invasion on south-facing slopes. Toadflax cover was also high in areas further from water
tanks, indicating that grazing may inhibit toadflax invasion. More broadly, this result suggests that grazing can reduce invasion
of even relatively unpalatable species in ecosystems with long evolutionary histories of grazing. 相似文献