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891.
Matthias Driess Christian Mons Roland Boese Dieter Blser 《Angewandte Chemie (International ed. in English)》1998,37(16):2257-2259
Inversion-symmetric pairs, each with two weak P–H⋅⋅⋅P interactions, form molecules of Si(PH2)4 in the crystal (see drawing). The title compounds are formed from ECl4 (E=Si,Ge) on reaction with [LiAl(PH2)4] and could serve as single-source CVD precursors for phosphides. 相似文献
892.
Dietmar Leipert Peter Heiduschka Jürgen Mack Hans-Joachim Egelhaaf Dieter Oelkrug Günther Jung 《Angewandte Chemie (International ed. in English)》1998,37(17):2337-2340
Microstructuring of surfaces: Electrochemical polymerization after removal of a photolabile protecting group (nitrobenzyl group) represents a new method for spatially resolved immobilization of ligands or receptors. Thus, the electropolymerization of 3-hydroxyphenylacetyl peptides such as 1 on electrodes can be controlled by light 相似文献
893.
[(Cp*Fe)3{(η3-As3)Fe}As6] and [(Cp*Fe)3As6][FeCl3(THF)], Two New Ironarsenide Cluster Containing As6 and As3 Fragments The compounds [(Cp*Fe)3{(η3-As3)Fe}As6] ( 1 ) and [(Cp*Fe)3As6][FeCl3(THF)], ( 2 ) can be obtained in good yields through the reaction of As7(SiMe3)3 with FeCl2 and LiCp*. Both compounds have been characterized by mass spectroscopy and crystal structure analysis. The As7 nortricyclane frame of As7(SiMe3)3 is still recognizable in 1 . According to the Wade rules the cation of 2 has 20 valence electrons inside the clustercore. This leads to a structure very similar to closo-B9H92–. 相似文献
894.
895.
Biodegradable polyesters such as biologically produced poly[(R)-3-hydroxybutyric acid], (PHB) other polyhydroxyalkanoic acids and related chemosynthetic polyesters have attracted industrial interest, and bacterial produced PHB is commercially available since 1990. A large variety of polyester degrading microorganisms have been found to be present in environment. The microorganisms decompose the polymers by secretion of extracellular polyester depolymerases and utilize low molecular weight degradation products for growth. Microbial polyester depolymerases have the unique property to be water soluble and to be able to bind specifically to polyester surfaces. The objective of this contribution is a functional analysis of a bacterial PHB depolymerase polyester binding domain. In addition, a detailed summary of the present knowledge on the biochemistry of enzymatic polyester hydrolysis is provided. 相似文献
896.
897.
In view of the prominent role of the 1H‐indol‐3‐yl side chain of tryptophan in peptides and proteins, it is important to have the appropriately protected homologs H‐β2 HTrp OH and H‐β3 HTrp OH (Fig.) available for incorporation in β‐peptides. The β2‐HTrp building block is especially important, because β2‐amino acid residues cause β‐peptide chains to fold to the unusual 12/10 helix or to a hairpin turn. The preparation of Fmoc and Z β2‐HTrp(Boc) OH by Curtius degradation (Scheme 1) of a succinic acid derivative is described (Schemes 2–4). To this end, the (S)‐4‐isopropyl‐3‐[(N‐Boc‐indol‐3‐yl)propionyl]‐1,3‐oxazolidin‐2‐one enolate is alkylated with Br CH2CO2Bn (Scheme 3). Subsequent hydrogenolysis, Curtius degradation, and removal of the Evans auxiliary group gives the desired derivatives of (R)‐H β2‐HTrp OH (Scheme 4). Since the (R)‐form of the auxiliary is also available, access to (S)‐β2‐HTrp‐containing β‐peptides is provided as well. 相似文献
898.
Matthias Albert Dieter Seebach Elke Duchardt Harald Schwalbe 《Helvetica chimica acta》2002,85(2):633-658
Oligomers of 3‐hydroxyalkanoic acids that contain two, three, and six residues with and without O‐terminal (tBu)Ph2Si and C‐terminal PhCH2 protection have been synthesized in such a way that the side chains on the oligoester backbone were those of the proteinogenic amino acids Ala (Me), Val (CHMe2), and Leu (CH2CHMe2). The enantiomerically pure 3‐hydroxyalkanoates were obtained by Noyori hydrogenation of the corresponding 3‐oxo‐alkanoates with [Ru((R)‐binap)Cl2](binap=2,2′bis(diphenylphosphanyl)‐1,1′‐binaphthalene)/H2 (Scheme 1), and the coupling was achieved under the conditions (pyridine/(COCl)2, CH2Cl2, −78°) previously employed for the synthesis of various oligo(3‐hydroxybutanoic acids) (Schemes 2 and 3). The Cotton effects in the CD spectra of the new oligoesters provided no hints about chiral conformation (cf. a helix) in MeOH, MeCN, octan‐1‐ol, or CF3CH2OH solutions (Figs. 1 and 2). Detailed NMR investigations in CDCl3 solution (Figs. 3–6, and Tables 1–5) of the hexa(3‐hydroxyalkanoic acid) with the side chains of Val (HC), Ala (HB), Leu (HH), Val, Ala, Leu (from O‐ to C‐terminus; 3 ) gave, on the NMR time‐scale, no evidence for the presence of any significant amount of a 21‐ or a 31‐helical conformation, comparable to those identified in stretched fibers of poly[(R)‐3‐hydroxybutanoic acid], or in lamellar crystallites and in single crystals of linear and cyclic oligo[(R)‐3‐hydroxybutanoic acids], or in the corresponding β‐peptide(s) (the oligo(3‐aminoalkanoic acid) analogs; 1 – 3 ). Thus, the extremely high flexibility (averaged or ‘random‐coil' conformation) of the polyester chain (CO−O rotational barrier ca. 13 kcal/mol; no hydrogen bonding), as compared to polyamide chains (CO−NH barrier ca. 18 kcal/mol; hydrogen bonding) has been demonstrated once again. The possible importance of this structural flexibility, which goes along with amphiphilic properties, for the role of PHB in biology, in evolution, and in prebiotic chemistry is discussed. Structural similarities of natural potassium‐channeling proteins and complexes of oligo(3‐hydroxybutanoates) with Na+, K+, or Ba2+ are alluded to (Figs. 7–9). 相似文献
899.
Touraj Etezady‐Esfarjani Christian Hilty Kurt Wüthrich Magnus Rueping Jürg Schreiber Dieter Seebach 《Helvetica chimica acta》2002,85(5):1197-1209
The structural properties of an all‐β3‐dodecapeptide with the sequence H‐β‐HLys(Nε‐CO(CH2)3‐S Acm)‐β‐HPhe‐β‐HTyr‐β‐HLeu‐β‐HLys‐β‐HSer‐β‐HLys‐β‐HPhe‐β‐HSer‐β‐HVal‐β‐HLys‐β‐HAla‐OH ( 1 ) have been studied by two‐dimensional homonuclear 1H‐NMR and by CD spectroscopy. In MeOH solution, high‐resolution NMR spectroscopy showed that the β‐dodecapeptide forms an (M)‐314‐helix, and the CD spectrum corresponds to the pattern expected for an (M)‐314‐helical secondary structure. In aqueous solution, however, the peptide adopts a predominantly extended conformation without regular secondary‐structure elements, which is in agreement with the absence of the characteristic trough near 215 nm in the CD spectrum. The NMR and CD measurements with solutions of 1 in MeOH containing 3M urea further indicated that the peptide retains the regular secondary structural elements under these conditions, whereas, after addition of 40% (v/v) H2O to the MeOH solution, the large 1H‐chemical‐shift dispersion indicative of a defined spatial peptide fold was lost. The β3‐dodecapeptide is – so far – the longest β‐peptide shown to adopt a regular (M)‐314‐helix conformation in an organic solvent. The observation that the structure of this long β3‐peptide is not maintained in aqueous solution indicates that the (M)‐314‐fold is primarily stabilized by short‐range interactions. 相似文献
900.