Dimerization by formation

C-type kinesins have a class-specific neck at the N-terminal side of their motor domain. In the case of DmNcd, the neck forms a continuous a-helix with the less conserved stalk. Constructs of the motor domain with a sufficiently long part of the neck dimerize by formation of a coiled-coil. 

Insulin monomers undergo noneovalent dimerization by formation of antiparallel /1-pleated sheet associations between monomers involving the C-terminal portion of the B chain. As discussed earlier, lispro insulin, in which the B28 and B29 is reversed from the normal prolyl and lysyl sequence, does not dimerize. Three insulin dimers subsequently self-associate to form hexamers in the presence of Zn +. The Zn + hexameric array of insulin probably gives the /3-cell granule its unusual morphologic characteristics. 

In the crystalline state, the synthetic a-carboxy-(0-amino ligand forms a 2 2 dimer by formation of hydrogen bonds with the CIO anions (see Fig. 48). This arrangement is somewhat similar to that found in the 2 2 lasalocid complex with Na" " if one neglects the presence of the perchlorate anions. 

The photosensitized dimerization of isoprene in the presence of henzil has been investigated. Mixtures of substituted cyclobutanes, cyclohexenes, and cyclooctadienes were formed and identified (53). The reaction is beheved to proceed by formation of a reactive triplet intermediate. The energy for this triplet state presumably is obtained by interaction with the photoexcited henzil species. Under other conditions, photolysis results in the formation of a methylcydobutene (54,55). 

Dimerization is also prevented by the presence of a formal positive charge on a j8-substituent, thus tryptamine cannot be dimerized. The formation of the trimer can be rationalized by analogy with the reactions of gramine and its derivatives. Thus, Thesing and Mayer found that methylphenylskatylamine (49) reacts with... 

Treatment of diallenyl sulfone 354 with n-butyllithium resulted in a cyclodimerization to afford 2,6-dithiaadamantane derivative 356. This dimerization is considered to be initiated by formation of the a-sulfonyl carbanion 355 and to proceed through a carbanion walk or carbanion tour process426. 

However, the significant key difference for rhodium arises from the chemistry of the Rh(ll) dimer, [Rh(Por)]2, which exhibits a relatively low Rh—Rh bond strength. It undergoes homolytic dissociation and exists in equilibrium with the monomer, Rh(Por)- (Eq, (15)). The rhodium dimer can also exist in equilibrium with the hydride Rh(Por)H (Eq. (16)), and thus the hydride complex can exhibit the chemistry of the dimer, driven by formation of the Rh(Por)- monomer formed as in Eqs. (15) and (16). 

The cationic complex [CpFe(CO)2(THF)]BF4 (23) can also catalyze the proton reduction from trichloroacetic acid by formation of Fe-hydride species and may be considered as a bioinspired model of hydrogenases Fe-H Complexes in Catalysis ) [44]. This catalyst shows a low overvoltage (350 mV) for H2 evolution, but it is inactivated by dimerization to [CpFe(CO)2l2-... 

The [2 + 2] photodimerization of a, j8-unsaturated sulfones is correctly viewed as a photoreaction of alkenes, rather than the sulfone group, and this aspect has been reviewed recently by Reid, as part of a wider survey of the photoreaction of O- and S-heterocycles. The topic continues to attract considerable interest and a few recent examples, as well as some synthetic applications, will be discussed here. Much of the photodimerization work has been carried out on the benzo[fc]thiophene (thianaphthene) 1,1-dioxide system. For example. Porter and coworkers have shown that both 3-carboxybenzo[i]thiophene 1,1-dioxide (65) and its methyl ester give only the head-to-head (hth), anti dimer (66) on irradiation in ethanol. In a rather unusual finding for such systems, the same dimer was obtained on thermal dimerization of 65. Similar findings for a much wider variety of 3-substituted benzo[fi]thiophene 1,1-dioxides have been reported more recently by Geneste and coworkers . In the 2-substituted analogs, the hth dimer is accompanied by some of the head-to-tail (htt), anti dimer. The formation of the major dimer appears to proceed by way of an excited triplet and the regiochemistry observed is in accord with frontier MO theory. 

Now let us consider some other types of reactions that might be encountered in a biological system. Consider, for example, the formation of a dimer, by the combination of two monomer molecules ... 

In order to determine the multiplicity of the reactive species, the photodimerization was carried out in the presence of the triplet quenchers oxygen and ferrocene. The results of these experiments are shown in Table 10.4.<41) It is obvious that the presence of oxygen exerts a large quenching effect on the production of the tram dimer and a smaller but significant effect on the formation of the cis dimer (the formation of tram dimer is decreased by oxygen by a factor of 25, while the cis dimer is decreased by a factor of 1.2). As with oxygen, the production of the tram dimer was quenched in the... 

If, on the other hand, unsymmetrically substituted carbonyl compounds such as monosubstituted benzophenones (X = OCH3, CH3, Cl), tert-butyl methyl ketone, acetophenone, acetaldehyde, or benzaldehyde are used for trapping 39a, diastere-omeric mixtures are formed in each case they could all be resolved except for the products obtained with p-methoxybenzophenone and acetophenone 33>. An X-ray structure analysis has been performed for the E-isomer 57g 36) which, in conjunction with H-NMR studies, permitted structural assignment in cases 56 and 57e, g and h35>. Additional chemical evidence for the structure of the six-membered heterocycles is provided by the thermolysis of 56 a considered in another context (see Sect. 3.1). In general the reaction 39a- 56 or 57 is accompanied by formation of phosphene dimers, presumably via [4 + 4]- and via [4 + 2]-cycloaddition 35). 

A set of oxygen donor atoms, providing both a and tt donation to a metal center, is not appropriate to stabilize any low oxidation state of a metal.19 This is, however, a synthetic advantage since very reactive, unstable, low-valent metalla-calix[4]arenes can be generated in situ and intercepted by an appropriate substrate. In the absence of a suitable substrate, the reactive fragment, however, can collapse to form metal-metal bonded dimers. The formation of metal-metal bonds has been, however, so far observed in the case of Group V and VI metals only. The most complete sequence so far reported has been for tungsten, molybdenum, and niobium. 

Cyclic dienes which are locked in the cisoid conformation, e.g. (82), are found to react very much faster than acyclic dienes in which the required conformation has to be attained by rotation about the single bond (the transoid conformation is normally the more stable of the two). Thus cyclopentadiene (82) is sufficiently reactive to add to itself to form a tricyclic dimer, whose formation—like most Diels-Alder reactions—is reversible. 

Problems related to the use of a guest dye can be reduced if the polymer contains a fluorescent chemical group. Gohil and Salem [70] took advantage of such intrinsic fluorescence to characterize the in-plane distribution of orientation in biaxially drawn PET films. In these experiments, the chain-intrinsic fluorescent label is due to the formation of dimers by two terephthalic moieties, exclusively within the noncrystalline regions. A comparison between sequential and simultaneous drawing along the MD and TD directions was undertaken for a fixed MD draw ratio of 3.5 and various TD draw ratios. The orientational order was characterized by two "orientation ratios" Rmd and RTD such that... 

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