Dimerization prevention

Exposure of a cell to ultraviolet light can result in the covalent joining of two adjacent pyrimidines (usually thymines), producing a dimer. These thymine dimers prevent DNA polymerase from replicating the DNA strand beyond the site of dimer formation. Thymine dimers are excised in bacteria as illustrated in Figure 29.27. A similar pathway is present in humans. 

A further difference between rhodium(II) and cobalt(II) complexes becomes apparent from a study of the monomeric complexes. Whereas virtually all cobalt(II) complexes are high spin d1 species, with the exception of [Co(CN)5]3 and related species, the rhodium(II) complexes are all low spin complexes. Thus, because of the lack of spin reorientation required in forming a low spin rhodium(III) complex, they are excellent reducing agents. The stability of the rhodium-rhodium bond in the dimers prevents their facile oxidation. 

Halopyridines are typically poor substrates for Heck reactions due to the possibility of pyridyl-bridged palladium dimers preventing additional reactions. Improved reaction conditions, of catalyst and the use of DMA as cosolvent, have allowed greater functional group tolerance [136], This was reduced to practice in the transformation of 388 into 389. 

If Ru is chosen to be L -I- w/2 and Rl to be in the range [0, L -I- w/2), then the definition of the density of dimers prevents polymerization beyond the dimer level. There is some arbitrariness in the choice of these limits, particularly Ry, and the use of more realistic binding potentials would make this choice even more difficult. 

Exposure of DNA to ultraviolet light induces a [2 + 2] cycloaddition reaction between the double bonds of adjacent thymines. The resulting cyclobutane ring, or thymine dimer, prevents DNA reproduction and can lead to the development of skin cancer. 

The difference between % and at hi concentrations ( 0.5M) namely 26 x 10- has been ascribed to the orbital diamagnetism. At this concentration, one expects primarily dimer clusters as well as some monomer clusters and cavities. The unavailability of any wave functions for the dimers prevents a quantitative interpretation of this difference. However the value wave function for the monomer cluster. 

Myrcene Manufacture. An important commercial source for mycene is its manufacture by pyrolysis of p-piaene at 550—600°C (87). The thermal isomerization produces a mixture of about 75—77 wt % myrcene, 9% limonene, a small amount of T -limonene [499-97-8] and some decomposition products and dimers. The cmde mixture is usually used without purification for the production of the important alcohols nerol and geraniol. Myrcene may be purified by distillation but every precaution must be taken to prevent polymerization. The use of inhibitors and distillation at reduced pressures and moderate temperatures is recommended. Storage or shipment of myrcene in any purity should also include the addition of a polymerization inhibitor. 

The thermally induced Diels-Alder dimerization reaction producing vinylcyclohexene is very difficult to prevent except by lowering the storage... 

The neutral complexes of chromium, molybdenum, tungsten, and vanadium are six-coordinate and the CO molecules are arranged about the metal in an octahedral configuration as shown in stmcture (3). Vanadium carbonyl possesses an unpaired electron and would be expected to form a metal—metal bond. Steric hindrance may prevent dimerization. The other hexacarbonyls are diamagnetic. 

In the presence of aluminum, oxidative degradation or dimerization supply HCl for the formation of aluminum chloride, which catalyzes further dimerization to hexachlorobutene. The latter is decomposed by heat to give more HCl. The result is a self-sustaining pathway to solvent decomposition. Sufficient quantities of aluminum can cause violent decomposition, which can lead to mnaway reactions (1,2). Commercial grades of trichloroethylene are stabilized to prevent these reactions in normal storage and use conditions. 

Except for the solvent process above, the cmde product obtained is a mixture of chloroprene, residual dichlorobutene, dimers, and minor by-products. Depending on the variant employed, this stream can be distiUed either before or after decantation of water to separate chloroprene from the higher boiling impurities. When the concentration of 1-chloro-1,3-butadiene [627-22-5] is in excess of that allowed for polymerisation, more efficient distillation is required siace the isomers differ by only about seven degrees ia boiling poiat. The latter step may be combiaed with repurifying monomer recovered from polymerisation. Reduced pressure is used for final purification of the monomer. All streams except final polymerisation-grade monomer are inhibited to prevent polymerisation. 

Properties. Cyanamide [420-04-2] also called carbamodiimide or carbamic acid nitrile, crystallises from a variety of solvents as somewhat unstable, colorless, orthorhombic, dehquescent crystals (2). Dimerization is prevented by traces of acidic stabilizers such as monosodium phosphate and by storage at low temperature. 

If pure monomer is to be used ia a reactioa, it must be used iaimediately or stored at < — 20° C to preveat dimerization to any appreciable extent. Chemical inhibition does not prevent dimerization low temperature is preferred. If the monomer has to be stored for more than a few hours, it must be protected against oxygen to prevent peroxidation and polymer formation. Cyclopentadiene monomer reacts spontaneously with oxygen of the air to form brown, gummy peroxide-containing products. 

Siace dimer acids, monomer acids, and trimer acids are unsaturated, they are susceptible to oxidative and thermal attack, and under certain conditions they are slightly corrosive to metals. Special precautions are necessary, therefore, to prevent product color development and equipment deterioration. Type 304 stainless steel is recommended for storage tanks for dimer acids. Eor heating coils and for agitators 316 stainless steel is preferred (heating coils with about 4s m (50 ft ) of heat transfer surface ia the form of a 5.1 cm schedule-10 U-bend scroU are recommended for a 37. 9-m (10,000-gal) tank. Dimer acid storage tanks should have an iaert gas blanket. 

Note 2. Under acidic conditions and in the presence of water, the 18,20-hemiacetal formed from the 18-iodo-18,20-ether is transformed into a dimeric anhydro product, resistant to further oxidation. Pyridine is added to prevent this transformation. 

However, one should emphasize that the proposed model of the interactions does not exclude a possibility of the formation of higher-order associates. On average, the concentration of trimers and higher-order mers is about 8% in the case of systems presented in Fig. 20. Thus, the presence of associates larger than dimers does not prevent the applicability of the... 

In the monomeric structure of 10.21 the solvation of both Li" ions by two THF molecules prevents further aggregation. The four S-N bond lengths are equal at ca. 1.60 A indicating that the negative charge is delocalized over the S(N Bu)4 unit. In the dimer 10.22 one [McS(N Bu)3] anion is coordinated to both Li" ions, one of which is bis-solvated by THF, while the other is chelated only to the unsolvated Li" ion. 

In contrast with 69, molecules of benzotelluradiazole 75 in the solid state are associated in dimers rather than polymeric chains. The bulky t-butyl groups of 75 prevent a higher degree of association and provide for good solubility of 75, which readily crystallizes from its pentane solution. 

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