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981.
The unusual stability of a structured but locally flexible protein, human growth hormone (hGH) at pH 2.7, was investigated using the temperature dependence of the nanosecond-picosecond dynamics of the backbone amide groups obtained from (15)N NMR relaxation data. It is found that the flexibility of the backbone of the helices decreases with temperature in the range from 24 °C to ~40 °C, corresponding to an increasing stability. A concomitant increase with temperature of the electrostatic interactions between charged residues forming an interhelical network of salt bridges at the center of the four-helix core suggests that these interactions give rise to the decreasing flexibility and increasing stability of the protein. However, numerous hydrophobic interactions in the interior of the four-helix core may also contribute. Above ~40 °C, where the thermal energy overcomes the electrostatic and hydrophobic interactions, a substantial increase in the flexibility of the helix backbones results in a highly positive contribution from the local conformational heat capacity, C(p,?conf), of the helix backbones to the total heat capacity, C(p), of the protein. This reduces the change in heat capacity upon unfolding, ΔC(p), increases the change in the Gibbs free energy, ΔG(unfold), and stabilizes the protein at high temperatures. A similar decrease in flexibility is found near other salt bridges in hGH and in Calmodulin and may be of general importance for the thermostability of proteins and, in particular, of the salt bridge intensive thermophilic proteins. 相似文献
982.
Peng Xu Da Zhao Florian Berger Aboubakr Hamad Jens Rickmeier Roland Petzold Mykhailo Kondratiuk Kostiantyn Bohdan Tobias Ritter 《Angewandte Chemie (International ed. in English)》2020,59(5):1956-1960
Site‐selective functionalization of C?H bonds in small complex molecules is a long‐standing challenge in organic chemistry. Herein, we report a broadly applicable and site‐selective aromatic C?H dibenzothiophenylation reaction. The conceptual advantage of this transformation is further demonstrated through the two‐step C?H [18F]fluorination of a series of marketed small‐molecule drugs. 相似文献
983.
Mengyi Zhang Jens Becking Marian Cristian Stan Arthur Lenoch Peter Bieker Martin Kolek Martin Winter 《Angewandte Chemie (International ed. in English)》2020,59(39):17145-17153
Li metal batteries (LMBs) containing cross‐linked polymer electrolytes (PEs) are auspicious candidates for next‐generation batteries. However, the wetting behavior of PEs on uneven Li metal surfaces has been neglected in most studies. Herein, it is shown that microscale defect sites with curved edges play an important role in a wettability‐dependent electrodeposition. The wettability and the viscoelastic properties of PEs are correlated, and the impact of wettability on the nucleation and diffusion near the Li|PE interface is distinguished. It is found that the curvature of the edges is a key factor for the investigation of wetting phenomena. The appearance of microscale defects and phase separation are identified as main causes for erratic nucleation. It is emphasized that the implementation of stable and consistent long‐term cycling performance of LMBs using PEs requires a deeper understanding of the “soft‐solid”–solid contact between PEs and inherently rough Li metal surfaces. 相似文献
984.
Marian Olaru Alexandra Mischin Lorraine A. Malaspina Stefan Mebs Jens Beckmann 《Angewandte Chemie (International ed. in English)》2020,59(4):1581-1584
The bis(ferrocenyl)phosphenium ion, [Fc2P]+, reported by Cowley et al. (J. Am. Chem. Soc. 1981 , 103, 714–715), was the only claimed donor‐free divalent phosphenium ion. Our examination of the molecular and electronic structure reveals that [Fc2P]+ possesses significant intramolecular Fe???P contacts, which are predominantly electrostatic and moderate the Lewis acidity. Nonetheless, [Fc2P]+ undergoes complex formation with the Lewis bases PPh3 and IPr to give the donor–acceptor complexes [Fc2P(PPh3)]+ and [Fc2P(IPr)]+ (IPr=1,3‐bis(2,6‐diisopropylphenyl)imidazole‐2‐ylidene). 相似文献
985.
Torsten Ampßler Georg Monsch Jens Popp Tobias Riggenmann Pedro Salvador Daniel Schrder Peter Klüfers 《Angewandte Chemie (International ed. in English)》2020,59(30):12381-12386
Nitrosyl–metal bonding relies on the two interactions between the pair of N–O‐π* and two of the metal's d orbitals. These (back)bonds are largely covalent, which makes their allocation in the course of an oxidation‐state determination ambiguous. However, apart from M‐N‐O‐angle or net‐charge considerations, IUPAC′s “ionic approximation” is a useful tool to reliably classify nitrosyl metal complexes in an orbital‐centered approach. 相似文献
986.
Philipp Stegner Christian Frber Jan Oetzel Ulrich Siemeling Michael Wiesinger Jens Langer Sudip Pan Nicole Holzmann Gernot Frenking Uta Albold Biprajit Sarkar Sjoerd Harder 《Angewandte Chemie (International ed. in English)》2020,59(34):14615-14620
Double deprotonation of the diamine 1,1′‐(tBuCH2NH)‐ferrocene ( 1 ‐H2) by alkaline‐earth (Ae) or EuII metal reagents gave the complexes 1 ‐Ae (Ae=Mg, Ca, Sr, Ba) and 1 ‐Eu. 1 ‐Mg crystallized as a monomer while the heavier complexes crystallized as dimers. The Fe???Mg distance in 1 ‐Mg is too long for a bonding interaction, but short Fe???Ae distances in 1 ‐Ca, 1 ‐Sr, and 1 ‐Ba clearly support intramolecular Fe???Ae bonding. Further evidence for interactions is provided by a tilting of the Cp rings and the related 1H NMR chemical‐shift difference between the Cp α and β protons. While electrochemical studies are complicated by complex decomposition, UV/Vis spectral features of the complexes support Fe→Ae dative bonding. A comprehensive bonding analysis of all 1 ‐Ae complexes shows that the heavier species 1 ‐Ca, 1 ‐Sr, and 1 ‐Ba possess genuine Fe→Ae bonds which involve vacant d‐orbitals of the alkaline‐earth atoms and partially filled d‐orbitals on Fe. In 1 ‐Mg, a weak Fe→Mg donation into vacant p‐orbitals of the Mg atom is observed. 相似文献
987.
Jens Mahnke Delia Bugnariu Frank Ruthe Peter G. Jones 《Phosphorus, sulfur, and silicon and the related elements》2014,189(7-8):963-976
AbstractThe formation and decomposition of P-tellurium-substituted phosphaalkenes was followed by 31P- and 125Te-NMR spectroscopy. Acyclic compounds with C?P-Te moieties are in general thermally labile, but bulky substituents enhance the lifetime of a number of species. The P-chlorophosphaalkene (Me3Si)2C?PCl (1a) reacts with the disilyltelluride (iPrMe2Si)2Te (2) leading to the mixed-substituted telluride (Me3Si)2C?PTeSiMe2iPr 3a which reacts with another equivalent of 1a furnishing the tellurobis(phosphaalkene) [(Me3Si)2C?P]2Te (4a). 4a is a shortlived compound decomposing thermally with precipitation of elemental tellurium, leading to a known diphosphabicyclobutane 5a. In a similar way, the bulkier P-chlorophosphaalkene (iPrMe2Si)2C?PCl (1b) reacts with (iPrMe3Si)2Te furnishing [(iPrMe2Si)2C?P]2Te (4b), which loses tellurium much more slowly than 4a and can be kept in cold solutions for an extended time. Reactions of in situ-prepared lithium aryltellurolates LiTeAr 6 – 9 [Ar?Ph: 6, Ar?2,4,6-Me3Ph (?Mes): 7, Ar?2,4,6-iPr3Ph (?TIP): 8, Ar?2,4,6-tBu3Ph (?Mes*): 9] with 1a provide P-aryltellurophosphaalkenes 10 – 13, which decompose with the loss of diarylditellurides leading to 5a. After a 2 + 4 cycloaddition trapping experiment of 12 with cyclopentadiene, a metastable P-aryltelluro phosphanorbornene 14 was detected by 31P-NMR. Reactions of elemental tellurium with P-phosphanylphosphaalkenes (Me3Si)2C?PPR′R′;′ 15 – 17 (R′, R′′?iPr: 15; R′?iPr, R′′?tBu: 16; R′, R′′?tBu: 17) lead to metastable insertion products (Me3Si)2C?PTePR′R′′ 18 – 20 that decompose with formation of the tellurobisphosphanes (R′R′′P)2Te 21 – 23, and of the bicyclic diphosphane 5a, which isomerises thermally to the diphosphabicyclooctane 24. The P-di-i-propylphosphanyl-phosphanorbornene 25 dismutates under the action of tellurium into the symmetric diphosphanes iPr4P2 and bis-phosphanorbornene 26. The tellurium-free products 24 and 26 were characterized by X-ray crystallography. 相似文献
988.
989.
990.
Ultraviolet laser desorption/ionization mass spectrometry of single‐core and multi‐core polyaromatic hydrocarbons under variable conditions of collisional cooling: insights into the generation of molecular ions,fragments and oligomers 下载免费PDF全文
Francisco Gámez Ana R. Hortal Bruno Martínez‐Haya Jens Soltwisch Klaus Dreisewerd 《Journal of mass spectrometry : JMS》2014,49(11):1127-1138
The ultraviolet laser desorption/ionization of polyaromatic hydrocarbons (PAHs) has been investigated under different background pressures of an inert gas (up to 1.2 mbar of N2) in the ion source of a hybrid, orthogonal‐extracting time‐of‐flight mass spectrometer (oTOF‐MS). The study includes an ensemble of six model PAHs with isolated single polyaromatic cores and four ones with multiple cross‐linked aromatic and polyaromatic cores. In combination with a weak ion extraction field, the variation of the buffer gas pressure allowed to control the degree of collisional cooling of the desorbed PAHs and, thus, to modulate their decomposition into fragments. The dominant fragmentation channels observed are related to dehydrogenation of the PAHs, in most cases through the cleavage of even numbers of C―H bonds. Breakage of C―C bonds leading to the fragmentation of rings, side chains and core linkages is also observed, in particular, at low buffer gas pressures. The precise patterns of the combined fragmentation processes vary significantly between the PAHs. The highest abundances of molecular PAH ions and cleanest mass spectra were consistently obtained at the highest buffer gas pressure of 1.2 mbar. The effective quenching of the fragmentation pathways at this elevated pressure improves the sensitivity and data interpretation for analytical applications, although the fragmentation of side chains and of bonds between (poly)aromatic cores is not completely suppressed in all cases. Moreover, these results suggest that the detected fragments are generated through thermal equilibrium processes rather than as a result of rapid photolysis. This assumption is further corroborated by a laser desorption/ionization post‐source decay analysis using an axial time‐of‐flight MS. In line with these findings, covalent oligomers of the PAHs, which are presumably formed by association of two or more dehydrogenated fragments, are detected with higher abundances at the lower buffer gas pressures. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献