170,958 research outputs found
Chemical consequences of long-range orbital interactions in Perhydronaphtalene-1,4-diol monosulfonate esters
No full textIn this thesis the base-induced reactions of perhydronaphthalene-1,4-diol monosulfonate esters are described. These compounds undergo smoothly, typical carbocationic processes upon treatment with sodium tert -amylate in refluxing benzene. The product outcome, product ratio, and (relative) rate of these reactions is satisfactorily explained when through-bond orbital interactions (TBI) over four abonds are invoked. In order to gather more detailed information about the basic stereochemical and stereoelectronic principles underlying these processes, synthetical organic, computational, and kinetic investigations were undertaken.Most experimental studies on TBI have focussed on its spectroscopic manifestations and are reviewed repeatedly. On the other hand, there are relatively few reviews on the chemical consequences of TBI over three or more σ-bonds. A number of illustrative examples of chemical reactions in which long-range orbital interactions are believed to play an essential role are discussed in chapter 1. In the same chapter also attention is drawn to the synthetic utility of some of these reactions.In chapter 2 the syntheses of the mesylates 39 , 40 , and 58-67 are described. The compounds 39 , 40 , 58 , and 59 are prepared in order to investigate how the orientation of the sulfonate ester group in combination with the orientation of the tertiary hydroxyl group determines the outcome and rate of their reactions with sodium tert -amylate. The results of these investigations are described in chapter 3. It was found that an equatorial sulfonate ester group favors homofragmentation leading to the cyclopropane derivative 105 . In case of an axial sulfonate ester group β-elimination, which strongly depends on the stereochemistry of the tertiary deprotonated hydroxyl group, is the main reaction path. In the chapter 3 the synthesis of the O-silylated mesylates 106 and 107 is also described. These compounds show no reaction at all upon treatment with strong base. On the other hand, fast reactions are observed when 106 and 107 are treated with TBAF. Generation of an alcoholate is crucial for the observed reactions. Homofragmentation and an internal return reaction with inversion of configuration of the mesylate group in the axial mesylates 39 and 58 is explained by assuming a 1,3-bridged intermediate carbocation.The mesylates 60-65 are prepared (Chapter 2) to determine the influence of the geometry of the relaying σ-bonds on the reactions with sodium tert -amylate. In chapter 4 the results of these studies are described in detail. An alcoholate function intramolecularly induces heterolysis of the sulfonate ester group in an apolar solvent via orbital interactions through three intervening C-C single bonds. It is shown that the reactivity of the compounds 60-65 is only affected by the relative position of the hydroxyl function to the sulfonate ester group and not by the orientation of the hydroxyl group. The two chief pathways by which these compounds react are rearrangement ( 60 , 62 , and 63 ) and homofragmentation ( 64 and 65 ). Stereoelectronic effects play a dominant role here, except in compound 61 where steric factors primarily determine the reactivity and product outcome (ether formation). Homofragmentation is much faster than rearrangement and is only possible when a 1,3-bridged through-space interaction accompanies TBI. The extent of TBI as well as the product composition is strongly determined by the σ-relay of the four σ-bonds between the electron donor (alcoholate) and the electron acceptor (sulfonate ester bond). These results are consistent with the " trans rule", which is in line with predictions from theoretical models regarding TBI.The product outcome, product ratio, and relative rate of the base-induced reactions of perhydronaphthalene-1,4-diol monosulfonate esters described in chapter 4 are satisfactorily explained with the concept of TBI. However, the conclusions are all based on empirical results. In chapter 5 the results of semi-empirical calculations, using the MNDO method, performed on model systems are presented. In this way a more detailed understanding of the stereoelectronic features underlying the homofragmentation and rearrangement reaction is obtained. The trends in the results of the MNDO simulations are the same as those found in the reactions of the compounds 60-65 . Whether rearrangement or homofragmentation takes place depends on the geometry of the σ-relay and the inductivity of the system. Cyclopropanoid bridged structures seem to be involved in the rearrangement process as well as in the homofragmentation process.In order to explore the effects of the order of substitution of the carbon atom that borders the carbon atom to which the mesylate group is attached the compounds 40 , 66 , and 67 were synthesized. This subject is discussed in chapter 6. The product formation is strongly dependent on the steric consequences of alkyl substituents at βcarbon atoms. Homofragmentation is highly favored when the repulsive steric interactions do not prevent a homohyperconjugatively stabilized transition state. This is only possible in an ideal "W" arrangement of theσ-relay ( 40 ). Due to the repulsive 1,3- peri -effect in 66 , and a combination of the 1,3- peri -effect and the 1,3- diaxial -effect in 67 the σ-relay diverges from the ideal "all trans " geometry as a result of which other reaction pathways (elimination, 1,3-H, and 1,2-Me shifts) are favored over homofragmentation. Introduction of inductively electron-donating substituents leads to an increase in reaction rate, despite the (slight) deviation of the "W" arrangement. It is concluded that although bridged ions are important intermediates in the observed reaction paths, they are not decisive for the reactivity of these compounds.The O-silylated mesylates 106 and 107 react fast upon treatment with TBAF in refluxing benzene (Chapter 3). At room temperature only desilylation takes place. To investigate the influence of a remote nucleofugal mesylate group on the rate of desilylation, apart from 106 and 107 , also the O-silylated compounds 127-131 are synthesized and treated with TBAF as is described in chapter 7. The rates of desilylation are determined conveniently by HPLC monitoring of the disappearance of the starting material. The desilylation rate of compounds with a mesylate group is much higher than the desilylation rate of corresponding compounds with a hydroxyl group instead ( 130 and 131 ). Furthermore, compounds having a "W" arrangement ( 107 and 129 ) of the relaying σ-bonds react considerably faster than their "sickle relay" analogs ( 106 , 127 , and 128 ). The results presented in this chapter show nicely that longrange electronic effects of distant substituents can exert a substantial influence on the reactivity of certain functional groups in general.In conclusion, the concept of TBI offers a good explanation for the reactivity of the compounds studied throughout this thesis. The stereochemical and stereoelectronic requirements for the base-induced reactions of perhydronaphthalene-1,4-diol monosulfonate esters are now well established. The general utilitly of the concept of σ-delocalization and TBI in everyday chemistry is demonstrate
Potential of photodynamic therapy in the management of infectious oral diseases
No full textIn recent years there are more and more multi-drug resistant infections also at the oral level and this has led researchers to find alternative solutions to conventional pharmacology that have no impact on systemic health. Among these is photodynamic therapy, which has demonstrated efficacy both in vitro and in vivo, both in bacterial, viral (reducing recrudescence) and fungal infections, in particular Candida spp multidrug resistant
Rapid development of cleaning behavior by Torresian crows Corvus orru on non-native banteng Bos javanicus in nothern Australia
No full text© Journal of Avian BiologyIn this paper we report the observation of a rapidly developed vertebrate symbiosis involving ectoparasite cleaning by a native corvid of northern Australia, the Torresian crow Corvus orru, on a recently introduced bovid ungulate, the banteng Bos javanicus. Facultative symbioses benefiting both participants (mutualisms) between birds and mammals appear to be rare, despite the apparent advantages obtained by the participants (ecto-parasite removal from the host and food provision to the cleaner). On three separate dates we observed a total of four C. orru individuals eliciting facilitation behaviours by a total of ten female banteng to assist in the removal of ectoparasites. Our observations document the first-known incidence of facultative cleaning behaviour by a native bird species on a non-native, wild vertebrate that has developed in approximately 150 years since the banteng's introduction to Australia.Corey J. A. Bradshaw and Warren W. Whitehttp://www3.interscience.wiley.com/journal/118604328/abstrac
Combustion synthesis and spark plasma sintering of apatite-tricalcium phosphate nanocomposites
No full textA processing route consisting of Spark Plasma Sintering (SPS) of precursor powders prepared by Solution Combustion Synthesis (SCS) is proposed for the first time for the fabrication of bulk nanostructured biphasic calcium phosphates. The apatite phase content in the product obtained by SCS was maximized using a fuel to oxidizer ratio of 1.1. After a post-synthesis air-annealing step conducted a 700 °C/3 h, powders consisted of 83 wt.% of carbonated apatite, with average crystallite size less than 70 nm, and β- and α-TCP (tricalcium phosphate), as secondary phases. Detailed structural analyses evidenced that the original nanostructure was retained after sintering at 900 °C, with the obtainment of nearly 91% dense, apatite-rich, biphasic bioceramics, with grains size of about 100 nm. The developed nanostructured biphasic material is expected to possess a higher resorption rate than standard microcrystalline hydroxyapatite, which makes it preferable for bone tissue regeneration
Purification and characterization of a small (7.3 kDa) putative lipid transfer protein from maize seeds.
No full textThe present study reports, for the first time in literature, the purification and
biochemical characterization of a small basic protein from maize seeds similar to
plant lipid transfer proteins-2, named mLTP2. The mLTP2 consists of 70 amino acid
residues and has an M(r) of 7303.83, determined by electrospray ionization mass
spectrometry. The primary structure of mLTP2 was determined by automated Edman
degradation of the intact protein and peptides obtained from digestions with
trypsin and by C-terminal sequencing using carboxypeptidase Y. The mLTP2 exhibits
high sequence similarity (51-44% identical positions) with other plant LTP2s
previously described
Designing of sequencing assay assisted by capillary electrophoresis based on DNA folding analysis. An application to the VCAM1 gene
No full textIn this work, we describe a fast standardized molecular method for DNA sequencing assisted by capillary electrophoresis with a particular emphasis on bioinformatic approaches to avoid sequencing errors due to complex DNA regions. In this case, the method was applied on the human vascular adhesion molecule 1 (VCAM1) gene. VCAM1 sequence, in fact, shows many thermodynamically critical parameters such as very low GC content (30-40%), many nucleotide stack areas, i.e. hairpins, self-complementary regions. With a traditional primer design approach it was difficult to design correct PCR oligonucleotides, thus sometimes, the chromatogram showed an illegible profile. By a strategy involving various bioinformatic tools (Mfold, Oligo, Highter), we investigated the role of the DNA-folding analysis in the assistance of primer design for the DNA sequencing of fragments with high -ΔG stem-loop regions. This new approach allowed us to sequence nine different VCAM1 regions each containing the respective exon. Our results, based on different DNA samples recruited from oral brushes taken from ten different subjects, identified four different SNPs (c.662-7C/T, c.1793-79A>G, c.2079C/T, c.2208A>G) with high reproducibilit
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