130,611 research outputs found
Pseudopeptides designed to form supramolecular helixes: The role of the stereogenic centers
No full textThe two epimers Boc-L-Phe-D-Oxd-(S)-Î23-hPhg-OBn (1) and Boc-L-Phe-D-Oxd-(R)-Î23-hPhg-OBn (2) have been prepared by standard methods in solution, and their conformation was analyzed both in solution and in the solid state. While in solution 1 shows a random coil structure, 2 tends to assume a Î3-turn conformation that is nearly retained in the solid state. On the other hand, in the solid state molecules of 1 associate generating a helix that involves the formation of elongated crystals with hexagonal cross-section. This effect is not observed in the crystals formed by Boc-L-Phe-D-Oxd-(R)-Î23-hPhg-OBn 2. © 2009 American Chemical Society
Nanofibers from oxazolidi-2-one containing hybrid foldamers: what is the right molecular size?
No full textA series of oligomers of the type Boc-(L-Phe-D-Oxd)n-OBn (Boc = tert-butoxycarbonyl; Oxd = 4-methyl-5carboxy oxazolidin-2-one; Bn = benzyl) were prepared for n = 2-5. The shortest oligomer, Boc-(L-Phe-D-Oxd)2-OBn, aggregates and forms a fiber-like material with an anti-parallel β-sheet struc- ture in which the oligopeptide units are connected to each other by only one intermolecular hydrogen bond. The longer oligomers exhibit structural heterogeneity. They start to organize into secondary structures by the formation of intramolecular hydrogen bonds at the pentamer level. Microscopy and diffraction of the oligomers indicated a crystalline character for only the shorter ones. © 2009 Wiley-VCH Verlag GmbH & Co
The Interaction of Lipid Modified Pseudopeptides with Lipid Membranes
No full textWe have studied the structure of two lipopeptides based on the simple dipeptide building block L-Phe-D-Oxd. These peptides have been reported previously to form fiber-like materials. The lipopeptides synthesized here had the structures Cn2H(2n+1)CO-L-Phe-D-Oxd-OBn or Cn2H(2n+1)CO-D-Phe-L-Oxd-OBn with n = 5 or 11. Addition of the N-terminal lipid modification did not cause a major disturbance of the structures these molecules form. The lipid modifications themselves showed highly rigid structures as inferred from solid-state 2H NMR. The peptide backbone showed 13C NMR chemical shifts in agreement with -sheet secondary structure. Addition of a lipid modification to the N-terminus is a common motif in biology to attach proteins to the membrane. Therefore, we also investigated the lipopeptides in the pre¬sence of synthetic POPC bilayers. Two different molecular species were detected under these circumstan¬ces: (i) lipopeptide monomers that showed chain order parameters similar to those of the host membrane, (ii) lipopeptide aggregates that exhibited very similar structures and dynamics as the crystalline aggre¬gates. Overall, the lipopeptides showed a well defined and rigid secondary structure that is in agreement with fibrillar aggregates previously detected for those peptides without the lipid modification
AMYLOIDS: SELF-ASSEMBLY OF OXAZOLIDIN-2-ONES CONTAINING HYBRID PSEUDOPEPTIDES
No full textWith the aim of obtaining new molecules that could form nanomaterials through intermolecular aggregations in the solid state, we prepared some oligomers of the Boc-(L-Phe-D-Oxd)n-OR series with the expectation that the presence of phenyl groups could favour aggregation by means of NH hydrogen bonds and/or pi-stacking interactions. We can show that fibers or fibrils are formed dependent on the length of the foldamer chain. The chemical properties of these compounds have been examined in the solid phase utilizing X-ray diffraction, solid state NMR, SEM, and TEM microscopy. Theoretical calculations have been performed to explain the results
MeSH term explosion and author rank improve expert recommendations
No full textInformation overload is an often-cited phenomenon that reduces the productivity, efficiency and efficacy of scientists. One challenge for scientists is to find appropriate collaborators in their research. The literature describes various solutions to the problem of expertise location, but most current approaches do not appear to be very suitable for expert recommendations in biomedical research. In this study, we present the development and initial evaluation of a vector space model-based algorithm to calculate researcher similarity using four inputs: 1) MeSH terms of publications; 2) MeSH terms and author rank; 3) exploded MeSH terms; and 4) exploded MeSH terms and author rank. We developed and evaluated the algorithm using a data set of 17,525 authors and their 22,542 papers. On average, our algorithms correctly predicted 2.5 of the top 5/10 coauthors of individual scientists. Exploded MeSH and author rank outperformed all other algorithms in accuracy, followed closely by MeSH and author rank. Our results show that the accuracy of MeSH term-based matching can be enhanced with other metadata such as author rank
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
"Closing the R&D Gap, Evaluating the Sources of R&D Spending"
No full textBoth spending and tax policies have been implemented in the United States with the goal of stimulating private sector research and development (R&D). Karier questions whether current R&D policy, especially the research and experimentation tax credit, can contribute to closing the gap between nondefense expenditures on R&D in the United States and such expenditures in other countries, such as Japan and Germany. He also explores possible changes to our current R&D policy to make it more effective.
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