134,862 research outputs found
Hecalus ghaurii Rao & Ramakrishnan
No full textHecalus ghaurii Rao & Ramakrishnan Material examined. Pakistan: 13, Sindh Prov., Tando Jam, 12.iii.07 (lost). Remarks. The identity of the above specimen was made possible from images made prior to the specimens’ loss.Published as part of Khatri, Imran & Webb, Michael D., 2010, The Deltocephalinae leafhoppers of Pakistan (Hemiptera, Cicadellidae), pp. 1-47 in Zootaxa 2365 on page 12, DOI: 10.5281/zenodo.19365
The Bloch-Wigner-Ramakrishnan polylogarithm function
No full textThe dilogarithm function Li\\sb 2(z)=\\sum\\sp∞\\sbn=1z\\sp n/n\\sp 2 (\\vert z\\vert lt;1) has many beautiful properties and plays a role in connection with problems in many parts of mathematics, most recently K-theory. This function cannot be extended in a one-valued way to the entire complex plane, but the modified function D(z)=\\cal I(Li\\sb 2(z)+\\log (1-z)\\log \\vert z\\vert) introduced by D. Wigner and S. Bloch can be extended as a continuous real-valued function on \\Bbb C which is real-analytic except at 0 and 1. For the higher polylogarithm functions Li\\sb m(z)=\\sum\\sp∞\\sbn=1z\\sp n/n\\sp m a similar one-valued modification D\\sb m(z) was defined in principle, but not written down in closed form or studied in detail, by D. Ramakrishnan. \\par In the present paper the function D\\sb m(z) is given explicitly and some of its properties are studied. It turns out that values of certain Kronecker double series related to special values of Hecke L-series for imaginary quadratic fields can be given in terms of the functions D\\sb m(z) (generalizing a result of S. Bloch involving D\\sb 2=D) and that the function D\\sb m is related to a certain Green's function for the quotient of the upper half-plane by the group of translations τ \\mapsto τ +n, n\\in \\Bbb Z. Most interestingly, a conjecture is presented according to which the value of the Dedekind zeta-function ζ\\sb F(s) as s=m for an arbitrary number field F and integer mgt;1 can be expressed in closed form in terms of finitely many values of D\\sb m(z) at arguments z\\in F
A NOTE ON SPECIAL VALUES OF CERTAIN DIRICHLET <i>L</i>-FUNCTIONS
No full textAbstractIn Gun and Ramakrishnan [‘On special values of certain Dirichlet L-functions’, Ramanujan J.15 (2008), 275–280], we gave expressions for the special values of certain Dirichlet L-function in terms of finite sums involving Jacobi symbols. In this note we extend our earlier results by giving similar expressions for two more special values of Dirichlet L-functions, namely L(−1,χm) and L(−2,χ−m′), where m,m′ are square-free integers with m≡1 mod 8 and m′≡3 mod 8 and χD is the Kronecker symbol . As a consequence, using the identities of Cohen [‘Sums involving the values at negative integers of L-functions of quadratic characters’, Math. Ann.217 (1975), 271–285], we also express the finite sums with Jacobi symbols in terms of sums involving divisor functions. Finally, we observe that the proof of Theorem 1.2 in Gun and Ramakrishnan (as above) is a direct consequence of Equation (24) in Gun, Manickam and Ramakrishnan [‘A canonical subspace of modular forms of half-integral weight’, Math. Ann.347 (2010), 899–916].</jats:p
MeSH term explosion and author rank improve expert recommendations
Get PDFInformation 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
Get PDFThe 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
Logic Based Modeling and Analysis of Workflows (Extended Abstract)
No full text) Hasan Davulcu Michael Kifer C.R. Ramakrishnan I.V. Ramakrishnan Department of Computer Science SUNY at Stony Brook, Stony Brook, NY 11794-4400 fdavulcu,kifer,cram,[email protected] To appear in PODS-98 June 1998, Seattle, Washington This work is partially supported by a DLA/DARPA contract and by the NSF grants IRI9404629, CCR-9510072, CCR-9705998, CCR-9711386, CCR-9404921, CDA-9303181, CDA-9504275 Triggers execute activity p If cond 6 holds then If cond 7 holds then execute activity q If event d occurs then If event g occurs then Global Constraints If d is ever executed then g must also be executed If e and i are both executed then e must happen before i If g and j are both executed then j must happen before g OR OR b d e f g h i j cond 1 cond 3 cond 4 Control Flow Graph cond 5 k AND cond 2 c a Figure 1: Frameworks for Specifying Workflows 1 Introduction A workflow is a collection of cooperating, coordinated activities designed to carry out a well-defined comple..
"Closing the R&D Gap, Evaluating the Sources of R&D Spending"
Get PDFBoth 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.
Controlling Conformation Of Macromolecules Using Non-Covalent Interaction And Micellization Behaviour Of Isomeric Phenyl Bearing Cationic Surfactants
Get PDFThis thesis contains investigations in two different areas, described under six chapters. Chapter 1 contains a broad introduction to the area of foldamers, while Chapters 2, 3, 4, and 5 deal with various novel classes of synthetic polymers which can form folded structures in solution utilizing different non-covalent interactions. Chapter 6 deals with a distinctly different topic, where the objective was to study the effect of phenyl ring location on the micellization properties of a series of isomeric cationic surfactants.
Synthetic polymers typically adopt a random coil conformation in solution, which is primarily an entropy driven process. So the generation of well-defined secondary structures in synthetic polymers requires specific intra-chain inter-segment interactions that will give adequate enthalpic contribution to overcome the entropic penalty associated with the formation of well-ordered conformations. During the past decade, various research groups have made significant effort to understand the essential design elements that could enable secondary structure formation in synthetic macromolecules through intra-chain inter-segment interactions, such as hydrogen bonding, solvophobic and solvophilic interaction, acid-base interaction, bond angle constraint, steric interaction, charge-transfer interaction, metal-ion complexation etc.1 Gellman2 first used the term “foldamer” to describe “any polymer with a strong tendency to adopt a specific compact conformation” which was more precisely defined by Moore and coworkers3 as “any oligomer that folds into a conformationally ordered state in solution, the structures of which are stabilized by a collection of non-covalent interactions between nonadjacent monomer units” and where the folded conformation is one of the various possible conformations.
Several classes of foldamers have been studied during the past decade; a majority of them are well-defined oligomers that possess relatively restricted conformational degrees of freedom. Relatively fewer studies have explored conformational control in flexible high molecular weight polymers that possess greater conformational freedom.4 A few years ago, Ghosh et al. designed a polymeric system wherein charge-transfer interactions between alternatively placed electron-rich and electron-deficient aromatic units, aided by metal-ion complexation and solvophobic interactions, causes the polymer chain to adopt a specific folded conformation.5 Such charge-transfer induced folding was first studied by Iverson and co-workers6 in well-defined oligomers and was later elaborated by Zhao et al.7 to generate alternate designs to fold oligomeric systems. In all these studies, the C-T interactions served not only to assist the folding process but it also served as a valuable spectroscopic signature to study the folding process.
The objectives of the present study are to develop simple synthetic strategies to generate different types of polymers that could be fold in solution using various noncovalent interactions. We have developed a simple synthetic strategy to design a new type of donor (1,5-dialkoxynaphthalene-DAN) containing polymer that carries a tertiary amine unit in the spacer segment, which could interact strongly with a suitably designed acceptor (pyromellitic diimide-PDI) bearing folding agent that carries a carboxylic acid group, as shown in Scheme 1.8 This acid-base interaction, brings the acceptor unit in a suitable position so as to form a C-T complex with the adjacent donors, resulting in the folding of the polymer chain. The folded conformation was studied using UV-vis and NMR spectroscopy and the folding propensities were rationalized using DFT studies. The highest association constant between the folding agent and the polymer was estimated to be around 1200 M-1.
Scheme 1. Schematic representation of folding aided by two-point interactions with a folding agent.
This value of association constant was not adequate to realize some potentially interesting properties in solid state. In an attempt to develop alternate systems, that could exhibit stronger propensity to fold, we designed a new type of cationic ionene,9 wherein electron-rich (DAN) and electron-deficient (PDI) aromatic units were included within the alkylene segments in an alternating fashion, as shown in Scheme 2.10 The charge-transfer (C-T) interaction between the donor and acceptor units in neighbouring segments of the ionene not only reinforced the transition to the collapsed nano-bundle form but also provides a useful spectroscopic handle to monitor the conformational change. The UV-visible spectra of these novel D-A ionene solutions at a fixed concentration in four different solvents, namely water, methanol, acetonitrile and DMSO, show different extents of charge-transfer interaction. The colour of the solution in water was deep-red, whereas in acetonitrile, it was light-yellow. The conformational transition could also be induced by titrating an acetonitrile solution of the ionene with increasing amounts of water causing a dramatic increase in the intensity of the charge-transfer band, which reflects the extent of collapse to the zig-zag state that brings the donor and acceptor units together. AFM studies confirmed the presence of flat pancake-like aggregates having nearly constant height of about 3-5 nm, which was in accordance with the estimated thickness of the postulated zig-zag structure.
Scheme 2. Schematic depiction of folding of D-A ionene (left), AFM micrograph showing pancake-like aggregates of D-A ionenes (right-top), a line scan depicting the heights and diameters of the aggregates along with a schematic depiction of the aggregate (right-bottom).
Scheme 3. Schematic representation of folding aided by interactions with a folding agent.
In order to explore this concept further, we designed a two component system wherein the solvophobically-driven collapse of a DAN-containing ionene chain in a polar solvent is reinforced by intercalation with a suitably designed electron-deficient acceptor-containing external folding agent. DAN containing ionene polymer chains in polar solvent form an accordion-type zig-zag structure that brings adjacent donor units in close proximity; this provided an ideal hydrophobic pocket for intercalation of suitably designed electron-deficient acceptor molecules, the additional driving motivation for the intercalation being the formation of a C-T complex as shown in Scheme 3.11 Several acceptor-bearing molecules were prepared by the derivatization of pyromellitic dianhydride and naphthalene tetracarboxylic dianhydride with two different oligoethylene glycol monomethyl ether monoamines. UV-vis spectroscopic studies were carried out by using a 1:1 mixture of the DAN-ionenes and different acceptor molecules in water/DMSO solvent mixtures. The intensity of the charge-transfer (C-T) band was seen to increase with the water content in the solvent mixture, thereby suggesting that the intercalation is indeed aided by solvophobic effects. The naphthalene diimide (NDI) bearing acceptor molecules consistently formed significantly stronger C-T complexes when compared to the pyromellitic diimide (PDI) bearing acceptor molecules, which is a reflection of the stronger π-stacking tendency of the former. The highest association constant between the folding agent and the polymer was estimated to be around 4519 M-1, which was a substantial improvement over the earlier reported values.9
With a slight modification in the pendant group, we designed a water-soluble DAN-containing ionene, which can intercalate hydrophobic electron-deficient molecules, like TNT (2,4,6-trinitrotoluene), within the hydrophobic interstices between DAN units (as shown in Scheme 4), causing a depletion in fluorescence from the DAN units; TNT at concentration as low as 30 nM could be detected in this manner.
Scheme 4. Schematic representation of folding of water soluble ionene and interactions with an electron-deficient hydrophobic moiety TNT.
Scheme 5. Schematic representation of folded D-A allyl ionene.
In light of the growing interest in single-chain polymeric nanoparticles, the fully collapsed D-A ionenes in water could be viewed as polymeric nanoparticles that are stitched together by reversible weak noncovalent interactions. In an attempt to transform the folded structure into a polymeric nanoparticle using covalent bonding, we designed D-A ionene that carries potentially polymerizable allyl units on the cationic head group instead of the dimethyl amine head group that was used in previous examples (as shown in Scheme 5). Preliminary studies showed that polymerization does not proceed readily; however, thiol-ene based clicking strategy enabled partial stitching of the folded segments, by the use of a suitably designed dithiol.
In the last section of this thesis, we examined the effect of phenyl ring location on the micellization properties of a series of isomeric cationic surfactants, wherein the phenyl ring location was varied from head to tail region (as shown in Scheme 6).12 Thus, cationic surfactants (S1-S5) bearing a long alkyl chain that carries a 1,4phenylene unit and a trimethyl ammonium headgroup was synthesized and their solution properties were examined. Micellization behavior was studied using conductivity, ITC (Isothermal Titration Calorimetry), SANS (Small-Angle Neutron Scattering) and NMR. These present studies demonstrated that the presence of a large rigid ring near the hydrocarbon tail-end of the surfactant leads to a dramatic change in the micelle structure; the driving motivation to form micelles in such systems is greatly reduced and the micelles that are formed are relatively smaller and contain significantly fewer surfactants. NMR studies of micellar solutions of these surfactants indicate that the variation in the phenyl ring location may also help to probe the microenvironment at various depths within the micellar aggregates.
Scheme 6. Structures of the various surfactant molecules carrying the 1,4-dioxyphenylene unit at different locations within hydrophobic segment (left), variation of CMC values (right).
References
(1) Foldamers - structure, properties, and applications, edited by Stefan Hecht and Ivan Huc, Wiley-VCH, 2007.
(2) Gellman, S. H. Acc. Chem. Res. 1998, 31, 173.
(3) Hill, D. J.; Mio, M. J.; Prince, R. B.; Huges, T. S.; Moore, J. S. Chem. Rev. 2001, 101, 3893.
(4) (a) Wang, W.; Li, L. S.; Helms, G.; Zhou, H. H.; Li, A. D. Q. J. Am. Chem. Soc. 2003, 125, 1120. (b) Li, A. D. Q.; Wang, W.; Wang, L. Q. Chem. Eur. J. 2003, 9, 4594. (c) Neuteboom, E. E.; Meskers, S. C. J.; Meijer, E. W.; Janssen, R. A. J. Macromol. Chem. Phys. 2004, 205, 217. (d) Balbo Block, M. A.; Hecht, S. Macromolecules 2004, 37, 4761.
(5) (a) Ghosh, S.; Ramakrishnan, S. Angew. Chem. Int. Ed. 2004, 43, 3264. (b) Ghosh, S.; Ramakrishnan, S. Angew. Chem. Int. Ed. 2005, 44, 5441.
(6) Lokey, R. S.; Iverson, B. L. Nature 1995, 375, 303.
(7) Zhao, X.; Jia, M. X. Jiang, X. K.; Wu, L. Z.; Li, Z. T.; Chen. G. J. J. Org. Chem. 2004, 69, 270.
(8) De, S.; Koley, D.; Ramakrishnan, S. Macromolecules 2010, 43, 3183.
(9) Williams, S. R.; Long, T. E. Prog. Polym. Sci. 2009, 34, 762.
(10) De, S.; Ramakrishnan, S. Macromolecules 2009, 42, 8599.
(11) De, S.; Ramakrishnan, S. Chem. Asian J. 2011, 6, 149.
(12) De, S.; Aswal, V. K.; Ramakrishnan, S. Langmuir 2010, 26, 17882.
(For structural formula pl see the abstract file
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