Dimer symmetric

Oligomerization and Polymerization Reactions. One special feature of isocyanates is their propensity to dimerize and trimerize. Aromatic isocyanates, especially, are known to undergo these reactions in the absence of a catalyst. The dimerization product bears a strong dependency on both the reactivity and stmcture of the starting isocyanate. For example, aryl isocyanates dimerize, in the presence of phosphoms-based catalysts, by a crosswise addition to the C=N bond of the NCO group to yield a symmetrical dimer (15). 

DimeriZa.tlon. A special case of the [2 + 2] cyclo additions is the dimerization of ketenes. Of the six possible isomeric stmctures, only the 1,3-cyclobutanediones and the 2-oxetanones (P-lactones) are usually formed. Ketene itself gives predominandy (80—90%) the lactone dimer, 4-methylene-2-oxetanone (3), called diketene [674-82-8], approximately 5% is converted to the symmetrical dimer, 1,3-cyclobutanedione [15506-53-3] (4) which undergoes enol-acetylation to so-called triketene [38425-52-4] (5) (44). 

The two peptides form a symmetrical dimer stabilized by four hydrogen bonds (red dashes) and hydrophobic contacts. The two monomers form a four-stranded, anti-parallel pleated sheet. 

Figure 17.12 Ribbon diagram of EMPl bound to the extracellular domain of the erythropoietin receptor (EBP). Binding of EMPl causes dimerization of erythropoietin receptor. The x-ray crystal structure of the EMPl-EBP complex shows a nearly symmetrical dimer complex in which both peptide monomers interact with both copies of EBP. Recognition between the EMPl peptides and EBP utilizes more than 60% of the EMPl surface and four of six loops in the erythropoietin-binding pocket of EBP.

Kolbe electrolysis is a powerful method of generating radicals for synthetic applications. These radicals can combine to symmetrical dimers (chap 4), to unsymmetrical coupling products (chap 5), or can be added to double bonds (chap 6) (Eq. 1, path a). The reaction is performed in the laboratory and in the technical scale. Depending on the reaction conditions (electrode material, pH of the electrolyte, current density, additives) and structural parameters of the carboxylates, the intermediate radical can be further oxidized to a carbocation (Eq. 1, path b). The cation can rearrange, undergo fragmentation and subsequently solvolyse or eliminate to products. This path is frequently called non-Kolbe electrolysis. In this way radical and carbenium-ion derived products can be obtained from a wide variety of carboxylic acids. 

Two equal carboxylates can be coupled to symmetrical dimers (Eq. 4). In spite of the high anode potential, that is necessary for Kolbe electrolysis, a fair number of... 

Despite of the disadvantage, that at least one symmetrical dimer is formed as a major side product, mixed Kolbe electrolysis has turned out to be a powerful synthetic method. It enables the efficient synthesis of rare fatty acids, pheromones, chiral building blocks or non proteinogenic amino acids. The starting compounds are either accessible from the large pool of fatty acids or can be easily prepared via the potent methodologies for the construction of carboxylic acids. 

Lithium dialkylcopper reagents can be oxidized to symmetrical dimers by O2 at -78°C in THF. The reaction is successful for R = primary and secondary alkyl, vinylic, or aryl. Other oxidizing agents (e.g., nitrobenzene) can be used instead of O2. Vinylic copper reagents dimerize on treatment with oxygen, or simply on standing at 0°C for several days or at 25°C for several hours, to yield LS-dienes." ... 

While crystal structures of rubredoxins have been known since 1970 (for a full review on rubredoxins in the crystalline state, see Ref. (15)), only recently have both crystal and solution structures of Dx been reported (16, 17) (Fig. 3). The protein can be described as a 2-fold symmetric dimer, firmly hydrogen-bonded and folded as an incomplete /3-barrel with the two iron centers placed on opposite poles of the molecule, 16 A apart. Superimposition of Dx and Rd structures reveal that while some structural features are shared between these two proteins, significant differences in the metal environment and water structure exist. They can account for the spectroscopic differences described earlier. 

Proteins with the helix-turn-helix or leucine zipper motifs form symmetric dimers, and their respective DNA binding sites are symmetric palindromes. In proteins with the zinc finger motif, the binding site is repeated two to nine times. These features allow for cooperative interactions between binding sites and enhance the degree and affinity of binding. 

The symmetrical dimer [(Me3P)3CoH]2(//-N2) is formed by protonation of the dinitrogen cobaltate precursor and its crystal structure is reported.131 The complex reversibly binds N2, forming the monomer CoH(N2)(PMe3)3. 

The Yao group has made use of a Ic type intramolecular Heck reaction to prepare the C2-symmetric dimeric indole core of chloptosin <06OL4919>. A solvent-free variation of the Bischler indole synthesis, electrophilic cyclization of a-arylamino imine tautomers prepared from aniline derived a-arylamino ketones, has been used by Menendez and co-workers for the preparation of 2-arylindoles <06SL91>. 

This difference in the yields of the symmetrical dimers in the liquid [75%] and the gas (25%) phases (75—50 = 25%) is because of the geminate recombination in the cage of the liquid. Similar results were reported for ethane and methyl acetate isolated by thermolysis of a mixture of protio- and perdeuterioacetyl peroxide [85,86]. 

Hastrup, H., Karlin, A., and Javitch, J. A. (2001) Symmetrical dimer of the human dopamine transporter revealed by cross-linking Cys-306 at the extracellular end of the sixth transmembrane segment. Proc. Natl. Acad. Sci. USA 98,10055-10060. 

The structural analysis of seco-zincporphyrazine 160 reveals the formation of C, symmetric dimer pairs in the solid state wherein one of the amido oxygen atoms of one molecule occupies the apical site of the square... 

In crystal structures in which there is appreciable intermolecular overlap of the C-2=C-3 double bonds, irradiation of the solid leads only to (2 + 2) cycloaddition. This is true of reactants 113, which yield in all cases file centro-symmetric dimers 114, even when the reactant conformation is suited also to intramolecular hydrogen abstraction. 

TVnodic oxidations of heterocycies can produce symmetrical dimers via carbon-carbon coupling. 

In 1987, Eurukawa et al. reported the isolation of oxydimurrayafoline (195) from the root bark of M. euchrestifolia. This alkaloid represented the first example of a dimeric carbazole alkaloid with an ether linkage (69). The UV spectrum (/Imax 242, 253, 294, 324, and 337nm) and the base peak at m/z 211 in the mass spectrum of oxydimurrayafoline (195) indicated a symmetrical dimeric carbazole with two murrayafoline A units. The H-NMR spectrum resembled that of murrayafoline A (7), except for the presence of a singlet at 8 4.76 instead of the singlet for the aromatic methyl group in murrayafoline A at 5 2.42. The singlet at 8 4.76 suggested a benzylic oxymethylene moiety. The spectral data, supported by NOE experiments, led to structure 195 for oxydimurrayafoline (Scheme 2.46). 

Scheme 2.11) indicating a similar carbazole unit. The EI-MS spectrum showed a molecular ion peak at m/z 392, and a peak at m/z 196, suggesting a symmetrical dimeric carbazole with two, 2-hydroxy-3-methylcarbazole units. The H-NMR spectrum was also similar to that of 2-hydroxy-3-methylcarbazole (52), except for the lack of the signal for H-1 (H-1 ), indicating a C-1/C-T-linkage between the two 2-hydroxy-3-methylcarbazole units. Based on the spectroscopic data and additional support by NOE experiments, structure 213 was assigned to bis-2-hydroxy-3-methylcarbazole (82). 

Figure 1. Calculated side-views of the equilibrium configurations of (a) Sa step (b) non-rebonded Sb step edge with symmetric dimers (c) non-rebonded dimers with p(2x2) reconstructions and (d) rebonded Sb step edge with p(2x2) reconstruction. 

We begin our discussion with the diffusion of a Si adatom over a flat terrace. This problem has previously been addressed with ab initio calculations for the case of symmetric dimers. The main result is that diffusion is highly anisotropic on the surface, with fast diffusion taking place over the top of the dimers with a saddle point energy of about 0.60 eV. Slow adatom diffusion is predicted to take place across the dimer rows with a barrier of 1.0 eV. Experiments based on a number counting of the island density are in agreement with these results. ... 

Further studies by the same researchers suggested that these compounds exert their effect on tumour cells by growth inhibition and that the endoperoxide bridge is essential for activity. They also investigated the influence that the stereochemistry of the ether bridge had on cytotoxicity towards EN2 tumour cells and found that the non-symmetrical dimer was more cytotoxic than the symmetrical one. 

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