Trimeric nucleus

As shown in the two examples described here, formation of the benzene nucleus by trimerization of alkynes is usually catalyzed by a Co-complex. However, Und-heim and coworkers [276] have recently shown that a Ru "-complex can also be used. Reaction of the triyne 6/4-9, which was prepared from SchollkopPs bislactim ether 6/4-8 [277] with Grubbs I catalyst 6/3-13, led to 6/4-10 in an excellent yield of 90%. Hydrolysis of 6/4-10 gave the desired as-indacene-bridged bis(a-amino acid) derivative 6/4-11 (Scheme 6/4.3). 

Binding of substrates to exportins is regulated in a converse manner to importins. Exportins bind their cargoes preferentially in the nucleus, forming a trimeric cargo exportin Ran GTP complex [142]. This trimeric complex is then transferred to the cytoplasm where Ran GTP is converted to Ran GDP. This results in Ran s dissociation from the complex, allowing the exportin to release its substrate, re-enter the nucleus, and to start the next export cycle. 

An extract of T. parthenium is described in a patent [20] which, besides the known sesquiterpene lactones parthenolide (1) and chrysartemin A (4, see above), also contained previously unreported partholide (26) and chry-santhemolide (27), both of unknown stereochemistry. Further, dimers and trimers of the sesquiterpene nucleus such as chrysanthemonin (28) were said to be present in this extract which had not been reported before. 

Triosmium dodecacarbonyl reacts with the simple ligand vp in an atypical manner 37) the trimeric osmium nucleus is retained and the olefin attacked to give a complex of formula Os3(CO)8(vp), which has two hydrides attached to osmium atoms. These are possibly a and one / -hydrogen of the olefin, the remainder being r-bonded to the trimeric osmium core. 

Figure 5.13. Assembly of actin filaments. The diagram shows the steps in the transition of actin monomer, G-actin, to actin filaments, F-actin. Monomers are activated by binding calcium and then exchanging calcium for magnesium, leading to nucleus formation. The nucleus consists of a trimer of G-actin with one pointed end and one barbed end. The addition of activated G-actin monomer to the nucleus causes elongation of the barbed end faster than the pointed end.

Homogeneous Nucleation Homogeneous nucleation is based on accretion of molecules in the liquid phase. Single species (molecules or ions) come together and form dimers. Dimers become trimers by addition of a molecule, and this accumulation process continues until eventually a stable nucleus forms depending on temperature and supers aturation. 

The mode of action of Smad 4 clearly differs from that of the other members of the Smad family. Smad 4 binds to phosphorylated R-Smads and forms trimeric complexes composed of two R-Smad molecules and one Smad 4 molecule. These complexes translocate to the nucleus, where they bind to related DNA elements and activate the transcription of target genes. The mechanism of transcription regulation by Smads is complex and includes both positive and negative influences. Generally, Smad-dependent regulation of transcription requires the interaction with other transcription factors, such as members of the FoxH 1 family of forkhead transcription factors, the Vitamin D receptor and the c-Jun transcription factor, among others (review Attisano et al., 2001). Futhermore, Smads can interact with coactivators and corepressors of transcription and thereby recruit, e. g., histone acetylase activity or histone deacetylase activity to chromatin. 

The trimeric, probably cyclic, ion appears to provide the nucleus of several basic tin(li) salts obtained from aqueous solutions at fairly low pH. Thus the nitrate appears to be Sn3(0H)4(N03)2 and the sulfate, Sn3(OH)2OS04.60 All Sn11 solutions are readily oxidized by oxygen and, unless stringently protected from air, normally contain some SnIv. The chloride solutions are often used as mild reducing agents ... 

Many examples of the usual route to /S-lactams via imines and acid chlorides, ketens, or keten equivalents have appeared.These include syntheses of 3-[bis(alkylthio)methylenamino]-2-azetidinones, ° of spiro-azetidinones and bis-azetidinones, and the use of l-methyl-2-halogenopyridinium salts to activate carboxylic acids towards coupling with imines. A non-hazardous alternative route to 3-amido-2-azetidinones, avoiding the use of azidoacetyl chloride, involves the reaction of Schiff bases with salts of [(a-methyl-/3-methoxycar-bonyl)vinylamino]acetic acid, e.g. (20), in the presence of ethyl chloroformate, as shown in Scheme 5. Formaldimines can be generated from their trimers by treatment with Lewis acids immediately prior to reaction with acid chlorides, so allowing the formation of the nocardicin nucleus which is unsubstituted at C-4. ... 

The charge state of the metals inside the fuUerenes has been studied by electron spin resonance (ESR). The lanthanum atom inside La Cg2 was first determined to be in the +3 state, with scandium and yttrium inside Sc Cg2 and Y Cg2 proving to be in the same state [ 11 b, c, 20]. However, from more recent ESR experiments, there is a standing question on the amount of charge transfer in these compounds, i.e. whether these structures can be best described as either Ln +Cg or Ln +Cg [4b]. Most interesting is the fact that these structures can be formally compared to superatoms in that the positive metal acts like a nucleus and the carbon shell as the electron shell of an atom [4b]. In this regard, dimers and trimers of Y Cg2 have been observed to form spontaneously on a Cu( 111) surface by STM in a way perhaps similar to lithium forming Li2 and Lij cluster molecules [17]. 

Nuclear applications of nanocapsules are related to the emitting physical properties of the encapsulated material. Emitted radiation can be electromagnetic of high energy (y), electrons or positrons (/3), alpha particles (" He nucleus), or fission products [67]. These emitters can be in themselves radioactive or can be activated by a nuclear reaction, usually a neutron capture. The particular advantage of carbon nanocapsules in nuclear applications is related to the protective characteristics that the carbon capsule confers to the interior product. Experiments on irradiation of fullerenes have shown that knocked carbon atoms from one cage are foimd in another fuUerene and even form dimers and trimers by a recoil-implantation mechanism [68]. The observed major damage of capsules in nanoencapsulated molybdenum irradiated in a nuclear reactor was produced by... 

Akitt and Duncan have provided further Al NMR evidence for the structure of the tetramer in solution. The spectrum consisted of a very broad peak due to tetrahedral aluminium and a sharper peak due to the central octahedral aluminium in the required 3 1 ratio of intensities. Owing to the quadrupole moment of the Al nucleus, the line width of the NMR signal depends on the symmetry of the electric field and gives independent evidence of the coordination of the aluminium. Furthermore, the structures of a number of other aluminium alkoxides were elucidated in 1984 by Al NMR spectroscopy, which supported the earlier proposed structures of a dimer (2-VII) and linear trimer (2-XI). It may be mentioned that a tetrameric structure for aluminium isopropoxide [Al(OPr )3]4 ° and a dimeric structure for aluminium f-butoxide [Al(OBu )3]2 have been confirmed by X-ray crystallography. 

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