The results revealed that the pKa of A38 is shifted to 5.46 ± 0.05 relative to 3.68 ± 0.06 derived from a reference solution of the nucleotide AMP. To address the role of A38, we used Raman spectroscopy to measure directly the pKa of the N1-imino moiety in the context of hairpin ribozyme crystals representative of a “precatalytic” conformation. Prior evidence has suggested that base A38 of the hairpin ribozyme plays an important role in phosphoryl transfer, possibly functioning as a general acid, or by orienting a specific water molecule for proton transfer. This is the first example in which complete control over the externally initiated P3HT polymerization has been achieved.Ĭatalytic RNA molecules can achieve rate acceleration by shifting base pKa values toward neutrality. Although external initiators have been used previously with a PPh3 ligand, that methodology led to polymers with broad molecular weight distributions. MALDI-TOF mass spectrometry revealed that the polymers had almost complete incorporation of the initiating aryl group, and when the aryl group was o-tolyl, only Tol/H end groups were observed. These complexes served as initiators in the polymerization of 2-bromo-5-chloromagnesio-3-hexylthiophene in THF at room temperature, affording fully regioregular P3HT with controlled molecular weights and narrow molecular weight distributions, as demonstrated by gel-permeation chromatography and 1H NMR spectroscopy. The complexes cis-chloro(phenyl)(dppp)nickel(II) and cis-chloro(o-tolyl)(dppp)nickel(II) were synthesized and characterized by 31P NMR spectroscopy. In this communication, the controlled chain-growth polymerization of regioregular poly(3-hexylthiophene) (P3HT) from an external initiator using 1,3-bis(diphenylphosphino)propane (dppp) as a catalyst ligand is reported. The ability of chemists to design and synthesize π-conjugated organic polymers with precise control remains the key to technological breakthroughs for using polymer materials in electronic and photonic devices. This is also the first example of a nanobiocatalyst for asymmetric oxidation, and the concept could be generally applicable for fabricating active and recyclable nanobiocatalysts.
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Thus, the nanobiocatalyst fabricated here showed the best performance among nanosized biocatalyst particles regarding both the retaining of free enzyme activity and the recycling of catalyst. The thick PGMA shell significantly increased the stability of the nanobiocatalyst: no loss of the sulfoxidation activity was observed after 11 times of recycling and reuse of the catalyst.
The covalently bound CPO did not change the original conformation of the active site and showed the same catalytic activity and enantioselectivity as free CPO for the sulfoxidation of thioanisole to produce (R)-methyl phenyl sulfoxide in >99% ee.
Magnetic nanoparticles (MNPs) with a core diameter of 30 nm comprising several iron oxide crystals, a poly(glycidyl methacrylate) (PGMA) shell with a thickness of 30 nm, and a surface coated with chloroperoxidase (CPO) were facilely fabricated as a nanobiocatalyst for asymmetric sulfoxidation. Calculated activation parameters for the reaction of (dhpe)Ni═CH2 (dhpe = 1,2-bis(dihydridophosphino)ethane) with N3Me compare well with the experimental values. B3LYP/6-311+G(d) calculations support a mechanism for all three reactions that features 1,3-dipolar cycloaddition to give five-membered ring (Huisgen) intermediates, followed by N2 elimination to give the products. The nickel−imido complexes (dtbpe)Ni═NR (4a, R = Mes 4b, R = Ad) react with N3Mes and N3Ad at ambient temperature to give the diazenes RN═NR (6a, R = Mes 6b, R = Ad) in good yield.
Treatment of 1 with N2O at low temperature results in clean conversion to the benzophenone complex (dtbpe)Ni(η2-OCPh2) (5) upon elimination of N2. Kinetic studies for the reaction of 1 with N3Ad yield activation parameters of ΔH‡ = +8(☑) kcal/mol and ΔS‡ = −44(☓) cal/(mol 1-Adamantyl- and mesitylazide react with (dtbpe)Ni═CPh2 (1 dtbpe = 1,2-bis(di-tert-butylphosphino)ethane) at ambient temperature to give the ketimines RN═CPh2 (2a, R = Mes 2b, R = Ad) in high yield.
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