SciCombinator

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Concept: Condensation reaction

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Structural evolution from monomer to fibril of amyloid β peptide is related to pathogenic mechanism of Alzheimer disease, and its acceleration is a long-running problem in drug development. This study reveals that ultrasonic cavitation bubbles behave as catalysts for nucleation of the peptide: The nucleation reaction is highly dependent on frequency and pressure of acoustic wave, and we discover an optimum acoustical condition, at which the reaction-rate constant for nucleation is increased by three-orders-of magnitudes. A theoretical model is proposed for explaining highly frequency and pressure dependent nucleation reaction, where monomers are captured on the bubble surface during its growth and highly condensed by subsequent bubble collapse, so that they are transiently exposed to high temperatures. Thus, the dual effects of local condensation and local heating contribute to dramatically enhance the nucleation reaction. Our model consistently reproduces the frequency and pressure dependences, supporting its essential applicability.

Concepts: Alzheimer's disease, Physics, Monomer, Wave, Acoustics, Sound, Condensation reaction, Cavitation

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The amyloid-beta peptide (Aβ) plays a major role in the progression of Alzheimer’s disease. Due to its high toxicity, the 42 amino acid long isoform Aβ42 has become of considerable interest. The Aβ42 monomer is prone to aggregation down to the nanomolar range which makes conventional structural methods such as NMR or X-ray crystallography infeasible. Conformational information, however, will be helpful to understand the different aggregation pathways reported in the literature and will allow to identify potential conditions that favour aggregation-incompetent conformations. In this study, we applied fluorescence correlation spectroscopy (FCS) to investigate the unfolding of Alexa Fluor 488 labelled monomeric Aβ42 using guanidine hydrochloride as a denaturant. We show that our Aβ42 pre-treatment and the low-nanomolar concentrations, typically used for FCS measurements, strongly favour the presence of monomers. Our results reveal that there is an unfolding/folding behaviour of monomeric Aβ42. The existence of a cooperative unfolding curve suggests the presence of structural elements with a Gibbs free energy of unfolding of about 2.8 kcal/mol.

Concepts: Alzheimer's disease, Spectroscopy, Polymer, Peptide, Monomer, Entropy, Gibbs free energy, Condensation reaction

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The first effective organopolymerization of the biorenewable “non-polymerizable” γ-butyrolactone (γ-BL) to a high-molecular-weight metal-free recyclable polyester is reported. The superbase tert-Bu-P4 is found to directly initiate this polymerization through deprotonation of γ-BL to generate reactive enolate species. When combined with a suitable alcohol, the tert-Bu-P4 -based system rapidly converts γ-BL into polyesters with high monomer conversions (up to 90 %), high molecular weights (Mn up to 26.7 kg mol(-1) ), and complete recyclability (quantitative γ-BL recovery).

Concepts: Polymer, Polymer chemistry, Monomer, Recycling, Polyester, Condensation reaction, Polymerization, Polyethylene terephthalate

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Modular non-ribosomal peptide synthetases (NRPSs) are large, multidomain engines of bioactive natural product biosynthesis that function as molecular “assembly lines” where monomer units are selectively bound, modified and linked in a specific order and number dictated by their mega-enzyme templates. Recently a condensation domain in an NRPS was discovered to carry out the synthesis of an integrated β-lactam ring from a substrate seryl residue during antibiotic biosynthesis. We report here a series of experiments supporting a mechanism that involves C-N bond formation by stepwise elimination/addition reactions followed by canonical NRPS-catalyzed amide bond synthesis to achieve β-lactam formation. Partitioning of reactive intermediates formed during the multistep catalytic cycle gave insight into the ability of the NRPS to overcome the reversibility of corresponding reactions in solution and enforce directionality during synthesis.

Concepts: Protein, Amino acid, Enzyme, Function, Chemical reaction, Peptide bond, Monomer, Condensation reaction

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An efficient and alternative synthesis of exo-imidazolidin-2-one dienes is described. A condensation reaction was carried out with bis-imino derivatives, diacetyl, and triphosgene, affording symmetrically N, N-disubstituted dienes. The use of alkyl methyl α-diketones led to the formation of nonsymmetrical dienes, which underwent isomerization to provide more stable inner-outer-ring dienes under Lewis acid conditions. Evaluation was made of the reactivity as well as regio- and stereoselectivity of these dienes in Diels-Alder reactions. They proved to be highly reactive and selective. DFT calculations of the transition states accounted for their behavior.

Concepts: Amino acid, Chemical reaction, Transition state, Gilbert N. Lewis, Acetic acid, Condensation reaction

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We report a bioinspired synthesis of 2,5-dihydropentalene-based chromophores from an aliphatic oligoketone bearing 1,3- and 1,4-diketone subunits. Unlike the natural polyketone sequence, fused five-membered rings were formed via an intramolecular aldol condensation. A subsequent Knoevenagel condensation reaction with malononitrile furnished a multiply cross-conjugated π-system with low-lying LUMO levels. Furthermore, pentalenes obtained from a non-conjugated aliphatic chain exhibited visible absorption and solid-state fluorescence.

Concepts: Aldol reaction, Aldehyde, Condensation reaction, Aldol condensation, Claisen condensation, Knoevenagel condensation, Malononitrile, Condensation reactions

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The synthesis of four C-pyrimidyl nucleosides is described by condensation of small nitrogen molecules (amidines and ureas) onto alkynyl riboside derivatives. These last compounds were obtained by indium mediated stereoselective alkynylation of suitably protected ribose derivatives and the condensation reaction conditions were studied in order to favor the N-attack of the nitrogen molecules leading to the pyrimidine ring formation.

Concepts: DNA, RNA, Chemical reaction, Nitrogen, English-language films, Condensation reaction

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A facile synthesis of 1,5-diketones from 3-acetyl-4-hydroxycoumarin, aldehydes and cyclic-ketones via a one-pot aldol condensation and subsequent Michael addition reaction in the presence of a single catalyst of l-proline under mild reaction conditions has been developed. Novel 1,5-diketones were further cyclized to unexpected 3,4-dihydropyridines rather than generally formed pyridine analogues with ammonium acetate in acetic acid. One pot, high yields (72-92%) for novel 1,5-diketones and (70-90%) for the 3,4-dihydropyridine adducts, easy work-up and purification of products are the key advantages of this method.

Concepts: Ethanol, Chemical synthesis, Acetic acid, Acetaldehyde, Aldehyde, Base, Condensation reaction, Acetates

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ABA triblock polyesters are synthesized using a commercially available chromium salen catalyst, in one pot, from monomer mixtures comprising epoxide, anhydride and lactone. The catalysis is highly selective and applies a single catalyst in two distinct pathways. It occurs first by epoxide/anhydride ring-opening copolymerization and subsequently by lactone ring-opening polymerization. It is used to produce various new ABA polyester polyols; these polyols can undergo post-functionalization and chain extension reactions. The ability to use a commercial catalyst and switchable catalysis with monomer mixtures is expected to facilitate future explorations of new classes of block polymers.

Concepts: Chemical reaction, Polymer, Polymer chemistry, Monomer, Polyethylene, Polyester, Condensation reaction, Polymerization

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The condensation reaction between a γ-methylene-α,β-unsaturated aldehyde and phenylpropargyl aldehyde was revisited and, guided by extensive DFT calculations of NMR shifts, was found to afford a deconjugative aldol condensation product. In this manner, a simple protocol for the preparation of valuable cross-conjugated oxatrienes was uncovered.

Concepts: Carbonyl, Aldol reaction, Aldehyde, Condensation reaction, Aldol condensation, Claisen condensation, Aldehydes, Aldol