Trimer formation

The use of polar solvents, such as /V, /V- dim ethyl fo rm am i de [68-12-2] is noted to result in extensive trimer formation. However, if the isocyanate is trapped using compounds such as alcohols, carboxyUc acids, and amines which contain active hydrogen, high yields are obtained (93). 

Rabindran, S.K., Haroun R.I., Clos, J., Wisniewski, J., Wu, C. (1993). Regulation of heat shock factor trimer formation Role of a conserved leucine zipper. Science 259,230-234. 

Equation 2.9. Trimer formation from butanal 2.6.2.4 Formation of Conjugated Carbonyls... 

TNF- a was first purified from conditioned medium from HL-60 cells. It has a relative molecular mass of 17 kDa when analysed by SDS-PAGE, but 45 kDa when analysed by gel filtration. Thus, the molecule exists as a non-glycosylated trimer with a pi of 5.3. Each monomer comprises 157 amino acids and contains two cysteine residues that form a disulphide bridge. Trimer formation appears to be due to noncovalent interactions between the monomers. Human TNF-a is synthesised as a 233-amino-acid protein that is proteolytically cleaved during processing. Whilst the 17-kDa form is readily secreted (and hence can function as an extracellular mediator), a 26-kDa transmembrane form has also been identified. This form of TNF-a may thus function in cytotoxicity resulting from cell-cell contact. 

As described in Section II,B, an H2A H2B-H4 trimer may be obtained by UV cross-linking of chromatin. Therefore, H2B apparently has two distinct binding sites for H2A and H4 (Martinson et al., 1979a). The lack of trimer formation by addition of a third histone to a preformed pair consequently must indicate that a trimeric unit is not stable. The dimer is the basic structural unit in the assembly of histones (Sperling and Bustin, 1976). This idea is supported by the fact that in a mixture of histones only even complexes have been isolated,... 

The pyrolysis gas chromatogram of ABS at 550°C changes considerably when the pyrolysis products are passed over zeolite catalysts. The specific activity towards certain reactions, e.g., cycliza-tion, aromatization, or chain cleavage is somewhat dependent on the nature of the individual zeolite. In general, enhanced benzene, toluene, ethylbenzene at the cost of dimer, trimer formation is observed. Nitrogen containing compounds do not appear in the pyrolysis oil after catalytic conversion. However, the product gas is rich in nitriles (132). 

Protonation of pyrrole, furan and thiophene derivatives generates reactive electrophilic intermediates which participate in polymerization, rearrangement and ring-opening reactions. Pyrrole itself gives a mixture of polymers (pyrrole red) on treatment with mineral acid and a trimer (146) under carefully controlled conditions. Trimer formation involves attack on the neutral pyrrole molecule by the less thermodynamically favored, but more reactive, (3-protonated pyrrole (145). The trimer (147) formed on treatment of thiophene with phosphoric acid also involves the generation of an a-protonated species. 

Y. Wang, B.D. Esry, Efimov trimer formation via ultracold four-body recombination, Phys. Rev. Lett 102 (2009) 133201. 

Kametani et al. have studied the biotransformations of TV-methyl-homococlaurine (130) and homoorientaline (138) by peroxidase enzyme preparations (100, 111, 112). The peroxidase activity of a potato peel homogenate-hydrogen peroxide preparation resulted in dimer and trimer formation when benzylisoquinolines were used as substrate (Section IV, C), and... 

Polymerisation of n-butyl isocyanate was accompanied with cyclic trimer formation, which has been explained in terms of the backbiting reaction [268],... 

Phthalocyanines and porphyrins are often used as model compounds for the manipulation (guest molecule) of surface-supported supramolecular two-dimensional arrays both in UHV and in solution. Spillmann et al. reported that zinc(II) 5,15-bis(3-cyanophcnyl)-10,20-bis 3,5-di(/c/7-bu(yl )phcnylj porphyrin, 2, molecules predominantly formed a hexagonal pattern by trimer formation, when 0.5-0.7 MLs of 2 was deposited on Ag(l 11) especially, 2 forms a characteristic nanostructure on Ag(l 11) at low coverage [163], As reported by Yokoyama et al., it is suggested that the molecular assembly is driven by both van der Waals and dipole-dipole interactions involving the polar 3-cyanophenyl residues [79], After further deposition of C60... 

Structures of crystals prepared from solutions containing cis-[(NH3)2Pt(H20)2]2+ revealed not only a dimer [24][25], but also a trihy-droxo-bridged trimer [26-28]. We formulate trimer formation as follows. 

From an analysis of titration curves it was suggested that pKal = 6.1, log Kd = 3.7 (m ), and log Kt = 6.5 (m 2) [18], From these constants, it may be predicted that in a range of pH 6-9, a 0.37m solution containing originally [(en)Pd(H20)2]2+ would consist predominantly of trimers and dimers in a 2.9 1 trimer/dimer mole ratio on a Pd11 basis. More than ten years later, and by authors apparently innocent of a prediction from the titration analysis, in a 0.37m solution the trimer/dimer mole ratio was found to be 2.8 1 by 15N-NMR [29]. Thus, there is striking agreement between the conclusions by two different methods determined wholly independently. (In contrast, some authors have evidently based their analysis on only one concentration and rejected trimer formation as unimportant [30].) It may be shown that the trimer/dimer mole ratio of 2.8 1 implies that Kt = 8.6 Kd5, and we incorporate this relationship in all further analyses. 

B203 and P205 modified silica as well as bentonite show similar selectivities as pentasil zeolites, but they deactivate much faster. The reason for that is the formation of trimers by aldolcondensation and aromatisation in the case of the nonshape selective materials. In contrast, the use of MFI-zeolites avoids the trimer formation due to the steric constraints and consecutive reactions resulting in coke. The aim of producing a fine chemical with 100% yield is achieved by the use of a Cs doped boron pentasil zeolite having an extremely weak acidity or basicity. 

Various aliphatic aldehydes, alkynes and dialkyl amines have been coupled using silver(I) iodide as catalyst in [C4Ciim][BF4] with moderate to excellent yield, as illustrated in Scheme 9.8.[34] The nature of the aldehyde has a much greater influence on the outcome of the reaction in the ionic liquid than in water. With aliphatic aldehydes, the reaction proceeded cleanly with hardly any trimer formation whereas aromatic aldehydes afforded complex product mixtures. 

Baes et al. (19) have investigated this trimer formation in n-hexane through isopiestic measurements and have found little indication of the trimer species. In subsequent investigations Baes (20) and Baes and Baker (21) suggested HDEHP partial trimerization as an explanation for... 

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