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1.
Leon P. Jenner Mickael V. Cherrier Patricia Amara Luis M. Rubio Yvain Nicolet 《Chemical science》2021,12(14):5269
The nitrogenase MoFe protein contains two different FeS centers, the P-cluster and the iron–molybdenum cofactor (FeMo-co). The former is a [Fe8S7] center responsible for conveying electrons to the latter, a [MoFe7S9C-(R)-homocitrate] species, where N2 reduction takes place. NifB is arguably the key enzyme in FeMo-co assembly as it catalyzes the fusion of two [Fe4S4] clusters and the insertion of carbide and sulfide ions to build NifB-co, a [Fe8S9C] precursor to FeMo-co. Recently, two crystal structures of NifB proteins were reported, one containing two out of three [Fe4S4] clusters coordinated by the protein which is likely to correspond to an early stage of the reaction mechanism. The other one was fully complemented with the three [Fe4S4] clusters (RS, K1 and K2), but was obtained at lower resolution and a satisfactory model was not obtained. Here we report improved processing of this crystallographic data. At odds with what was previously reported, this structure contains a unique [Fe8S8] cluster, likely to be a NifB-co precursor resulting from the fusion of K1- and K2-clusters. Strikingly, this new [Fe8S8] cluster has both a structure and coordination sphere geometry reminiscent of the fully reduced P-cluster (PN-state) with an additional μ2-bridging sulfide ion pointing toward the RS cluster. Comparison of available NifB structures further unveils the plasticity of this protein and suggests how ligand reorganization would accommodate cluster loading and fusion in the time-course of NifB-co synthesis.The K-cluster of NifB as a key intermediate in the synthesis of the nitrogenase active site supports [Fe4S4] cluster fusion occurs before carbide and sulfide insertion and displays ligand spatial arrangement reminiscent to that of the P-cluster. 相似文献
2.
Under high pressure conditions, cycloheptatriene reacts with methyl propiolate to afford mono-, bis- and trisadducts all retaining, the norcaradiene structure. The four new compounds are formed via [4+21] and [2+2+2] cycloadditions. The latter are examples for the high pressure extension of the scope of homo-Diels-Alder reactions. 相似文献
3.
Philippe Andrianary Gérard Jenner Alain Kiennemann 《Journal of organometallic chemistry》1983,252(2):209-220
The CO/H2 homologation of methanol to acetaldehyde and subsequently to its dimethyl acetal in the presence of cobalt acetate promoted by iodine was examined under various conditions. Temperature and pressure were found as critical parameters. High pressures (140 MPa) and low temperatures (160–170°C) give optimal yields and selectivity to acetaldehyde. According to pressure, temperature, contact time, gas ratio and ligand/catalyst ratio, the reaction is oriented towards acetaldehyde, its dimethyl acetal or methyl acetate. 相似文献
4.
Abstract Lactams add to Danishefsky's diene 1 yielding N-alkenyl substituted lactams 3-5. The reaction occurs only under pressure (100-300MPa) and, preferentially, in the presence of a mild Lewis acid catalyst such as bismuth chloride or ytterbium triflate. The reactions are fully regioselective and afford linear trans multifunctional 1:1 adducts. 相似文献
5.
Abstract The effect of lithium perchlorate-diethyl ether solutions (LPDE) is compared to the pressure effect in several addition reactions. The results cannot be accounted for either internal pressure, or electrostriction, but point to a Lewis-acid catalytic effect, in agreement with recent propositions. It is also concluded that the high pressure methodology is generally superior to the use of LPDE to improve yields. 相似文献
6.
The study of the pressure dependence of the rate constant in the (4+2) cyclodimerisation of 2,3-dimethyl 1,3-butadiene reveals a possible competition between concerted and stepwise mechanisms. 相似文献
7.
Christian Hove Janus Lysholm Faaborg Michael Bohl Jenner Saska Lindfors 《Analog Integrated Circuits and Signal Processing》2004,38(1):35-42
This paper describes the design and implementation of a transmit/receive switch for 2.4 GHz ISM band applications. The T/R switch is implemented in a 0.35 m bulk CMOS process and it occupies 150 m · 170 m of die area. A parasitic MOSFET model including bulk resistance is used to optimize the physical dimensions of the transistors with regard to insertion loss and isolation. The measured insertion loss is 1.3 dB without port matching. Simulations using measured s-parameters indicate that an insertion loss of 0.8 dB can be obtained with a conjugate match. The measured isolation is 42 dB and the maximum transmit power is 16 dBm. 相似文献
8.
9.
A comprehensive note on: An inventory model under two levels of trade credit and limited storage space derived without derivatives 总被引:1,自引:0,他引:1
In 2006, Huang proposed an inventory model with two warehouses when the supplier offers the retailer a permissible delay of M periods, and the retailer also provides its customers a permissible delay of N periods. He then solved it without derivatives. In this note, we extend his model to complement the shortcomings of his model. In contrast to the complicated and tedious quadratic–algebraic method suggested by Huang, we propose a simple arithmetic–geometric method to solve the inventory problem. Finally, we run computer programs for several numerical examples to illustrate the proposed model and obtain some managerial implications. 相似文献
10.
Madsen Per Jensen Ole Kiel Amtoft Torben Reynisson Ragnar V. Mikkelsen Jan Hvolgaard Laursen SØren Kolding Troels Emil Larsen Torben Jenner Michael Bohl 《Wireless Personal Communications》2002,23(1):55-66
Unified RF requirements are derived for an UMTS Terrestrial RadioAccess/Frequency Division Duplex (UTRA/FDD) compliant mobile transceiver. Aset of transceiver requirements are proposed with consideration to systemissues including duplex aspects. From these design-compatible requirements areproposed for each functional block in the transceiver. 相似文献