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81.
Medium-bridged twisted lactams, in which a non-planar amide bond is achieved by incorporating the nitrogen atom at the bridgehead position in a medium-sized heterocycle, offer an attractive setting in which to study the properties of distorted amide linkages. This Emerging Area article will describe progress in the preparation and study of these compounds. This work shows that compounds containing an even moderately distorted amide bond display useful and unusual chemical properties while retaining a measure of stability that enables their study. 相似文献
82.
Guangrong Meng Michal Szostak 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2015,127(48):14726-14730
Metal‐catalyzed reactions of amides proceeding via metal insertion into the N CO bond are severely underdeveloped due to resonance stabilization of the amide bond. Herein we report the first Heck reaction of amides proceeding via highly chemoselective N CO cleavage catalyzed by Pd0 utilizing amide bond ground‐state destabilization. Conceptually, this transformation provides access to a myriad of metal‐catalyzed transformations of amides via metal insertion/decarbonylation. 相似文献
83.
84.
Komsta Ł Czarnik-Matusewicz H Szostak R Gumieniczek A Pietraś R Skibiński R Inglot T 《Journal of AOAC International》2011,94(3):743-749
This paper presents and discusses the building of discriminant models from attenuated total reflectance (ATR)-FTIR and Raman spectra that were constructed to detect the presence of acetaminophen in over-the-counter pharmaceutical formulations. The datasets, containing 11 spectra of pure substances and 21 spectra of various formulations, were processed by partial least squares (PLS) discriminant analysis. The models found in the present study coped greatly with the discrimination, and their quality parameters were acceptable. A root mean square error of cross-validation was in the 0.14-0.35 range, while a root mean square error of prediction was in the 0.20-0.56 range. It was found that standard normal variate preprocessing had a negligible influence on the quality of ATR-FTIR; in the Raman case, it lowered the prediction error by 2. The influence of variable selection with the uninformative variable elimination by PLS method was studied, and no further model improvement was found. 相似文献
85.
L. N. Puntus V. F. Zolin V. A. Kudryashova V. I. Tsaryuk J. Legendziewicz P. Gawryszewska R. Szostak 《Physics of the Solid State》2002,44(8):1440-1444
A ligand-cation charge transfer band was observed in the region of 360–400 nm in the Eu3+ luminescence excitation spectrum of europium 3,4-pyridine-dicarboxylates, but was absent in the spectra of europium 2,6-and 2,3-pyridine-dicarboxylates. This band is due to noticeable energy transfer through a charge-transfer state formed owing to the high polarizability of the ligand in the former compound. Energy transfer through the excited ligand-lanthanide charge-transfer state can explain the well-known effect of luminescence quenching by redox-sensitive lanthanide ions. 相似文献
86.
G. v. Oppen W. -D. Perschmann D. Szostak 《Zeitschrift für Physik A Hadrons and Nuclei》1978,286(3):243-247
He-atoms have been excited using 35 keV-He+-ion-He-atom-collisions. Investigating the electric and magnetic depolarization of fluorescence light, cascade-free lifetime measurements on the 1s 3d 1 D and3 D-levels of He I have been performed. From our measurements we deduced:τ(31 D)=(15.3±0.3)ns,τ(33 D)=(14.2±0.6)ns. 相似文献
87.
Dehydroalanine is a nonproteinogenic amino acid, but it is a component of a wide variety of natural products with therapeutic activities. Indeed, this alpha,beta-unsaturated residue is a highly versatile building block due to its rigidifying effect on peptide backbones and its electrophilicity which allows site-specific thiol ligations of peptides with small molecules or proteins. To harness such versatility in genetically encoded, combinatorial peptide libraries, we report a simple and robust method for the ribosomal synthesis of dehydroalanine-containing peptides. Selenalysine, a selenium-containing lysine analogue, was recruited as a masked dehydroalanine equivalent. This residue is efficiently incorporated by a reconstituted Escherichia coli translation system at high fidelity and efficiency despite the presence of low levels of lysine. Mild oxidative conditions were used to convert selenalysine into dehydroalanine post-translationally. Using this method, we demonstrate the preparation of polyunsaturated and highly decorated peptides. This report is an important step toward the preparation and selection of large libraries of protein-reactive compounds with potential use as novel drugs or as analytical tools. 相似文献
88.
Guangrong Meng Lazaros Kakalis Steven P. Nolan Michal Szostak 《Tetrahedron letters》2019,60(4):378-381
The σ-donor properties of NHC ligands (NHC?=?N-heterocyclic carbene) are crucial in controlling their interaction with transition metals, and as a consequence, to determine the selectivity and reactivity of NHCs in transition-metal-catalysis. Herein, we report a simple NMR method for estimating the σ-donor properties of NHC ligands based on a straightforward 1H NMR measurement of ligand precursors. We present evaluation of σ-donating properties for a range of NHC ligands varied by structure and electronics that are relevant to transition-metal-catalysis. We expect that the simple measurement of σ-donating properties of NHCs, together with the known methods for evaluating sterics and π-backbonding, will enhance the understanding of the properties of NHCs in transition-metal-catalysis. 相似文献
89.
We report the in vitro selection of an RNA-based ATP aptamer with the ability to discriminate between adenosine ligands based on their 5' phosphorylation state. Previous selection of ATP aptamers yielded molecules that do not significantly discriminate between ligands at the 5' position. By applying a selective pressure that demands recognition of the 5' triphosphate, we obtained an aptamer that binds to ATP with a Kd of approximately 5 muM, and to AMP with a Kd of approximately 5.5 mM, a difference of 1100-fold. This aptamer demonstrates the ability of small RNAs to interact with negatively charged moieties. 相似文献
90.
A. S. Aynacioglu G. v. Oppen W. -D. Perschmann D. Szostak 《Zeitschrift für Physik A Hadrons and Nuclei》1981,303(2):97-102
He atoms have been excited by Ne+ ion impact and the depolarization of the fluorescence lines at 668 nm and 492 nm by magnetic and electric fields has been studied. The Ne+ ion energy could be chosen such that pure cascade level crossing signals were observed. From the widths of magnetic depolarization signals the radiative lifetimes τ(1s4f 1 F)=74(2) ns and τ(1s5f 1 F)=133(5) ns have been determined. By investigating the electric field splitting of the magnetic depolarization signals the tensor polarizabilities ¦α ten(1s4f 1 F)¦=0.58(1) kHz/(V/cm)2 and ¦α ten(1s5f 1 F)¦=4.2(1) kHz/(V/cm)2 have been deduced. From the latter value a mean frequencyv(1s5g?1s5f)=14.4 GHz of the transitions between the levels of the 1s5f configuration and those of the 1s5g configuration has been derived.α ten(1s4f 1 F) depends sensitively on the singlet-triplet mixing in the 1s4f configuration and thus a mixing coefficient could be deduced for this configuration. 相似文献