Apolar dimers

So far, such measurements have only been performed on cyanobiphenyl molecules ( -CB and n-OCB). These molecules exhibit polar ordering (with the cyano groups pointing towards the surface) on hydrophilic substrates such as water, glass, and certain polymer films [49-51], but not on hydrophobic layers [52]. On the latter, the molecules orient with their aliphatic chains in contact with the substrate, and arrange following their natural tendency to form apolar dimers (two molecules oriented head to head). In all cases where a surface dipolar ordering exists, it is lost after the first molecular layer [49]. 

Several /i-solenoid domains appear to promote the oligomerization of multidomain proteins. There are at least three types of /i-solenoid association. First, oligomers (dimers or trimers) are formed by lateral interaction of the solenoids. For example, the C-terminal domain of the bacterial cell division inhibitor MinC is a short right-handed T-type solenoid with an apolar lateral face that mediates homodimerization (Cordell et al., 2001). Trimers of several bacterial transferases are formed by lateral, in-register, interaction of left-handed T-type /1-solenoids (Fig. 5). Second, dimers may form via interactions of the open terminal coils of /1-solenoids as in the dimeric structure of iron transporter stabilizer SufD (Badger et al., 2005). Finally, dimerization may be mediated by swapping of /1-strands of the terminal coils, as in the CAP (Dodatko et al., 2004) (Fig. S). 

Dimer presence in apolar solvents is indicated as being responsible for some particular kinetic features when protic amines are nucleophilic reagents87,88. Aliphatic amines are slightly more associated than aromatic amines89-91. 

The van der Waals model of monomeric insulin (1) once again shows the wedge-shaped tertiary structure formed by the two chains together. In the second model (3, bottom), the side chains of polar amino acids are shown in blue, while apolar residues are yellow or pink. This model emphasizes the importance of the hydrophobic effect for protein folding (see p. 74). In insulin as well, most hydrophobic side chains are located on the inside of the molecule, while the hydrophilic residues are located on the surface. Apparently in contradiction to this rule, several apolar side chains (pink) are found on the surface. However, all of these residues are involved in hydrophobic interactions that stabilize the dimeric and hexameric forms of insulin. 

While arylnitrile oxides dimerize in protic solvents and in pure pyridine (cf. 4.04.8.1.3.), they form bis(adducts) (191) and (192) via zwitterions (189) with pyridine in apolar solvents (Scheme 83) <89JHC757,90Gi>. Significantly, the cycloaddition of the nitrile oxide to pyridine to give (190) is not a concerted process. Heterocycles (191) undergo base catalyzed ring cleavage (Scheme 84). 

On the basis of fluorescence measurements, the driving force responsible for the photoinduced conformational change was attributed to interactions between the photochromic side chains, which differ depending on whether they are in the zwit-terionic merocyanine form or the apolar spiro form. In the dark, the merocyanine units have a strong tendency to give dimeric species as a result the macromolecules are forced to adopt a disordered structure. When the side chains are photoisome-rized to the spiro form, such dimers are destroyed, and the macromolecules assume the helical structure.1631... 

H NM R spectra in apolar solvents are less informative. Broad unresolved signals indicate the presence of undefined species. Obviously, the multicydic structure does not allow the formation of regular dimers, which would lead to an unfavorable overlap of the loops, but the interaction via the urea functions creates irregular aggregates. This inability to form hydrogen-bonded homodimers can be exploited in further reactions, e.g., for the construction of multiple catenanes (see Section 5.5). 

Therefore, we connected the tri-urea 27 with three tetra-ureas 28, via covalent linkers, to a building block A. In apolar solvents molecules A alone would form crosslinked polymers via dimeric substructures formed by the tri- and tetra-ureas. Together with the building block B, a tetra-tosylurea substituted by four phthalimido alkylether residues, this polyassociation is prevented by the formation of heterodimers between tetra-aryl- and tetra-tosylureas. A solution of A and B in the ratio 2 6 therefore contains exclusively the dendritic assembly shown in Figure 5.21, as proved by light scattering and DOSY NMR spectroscopy [69]. 

Calix[4]arenes 133, substituted at their wide rim by four urea-functions, are self-complementary. In aprotic, apolar solvents such as benzene or CDC13, they exist as dimers held together by hydrogen bonds between the urea functions of opposite molecules as indicated in Figure 28. A suitably sized guest (often the solvent) is included in the cavity thus created.269... 

This interpretation was confirmed to some extent by experimental results, since it had been found that nematic mixtures composed of apolar nematic compounds and polar nematic compounds (nitriles) consist of a number of unassociated polar and non-polar monomers as well as associated polar dimers. This not only gives rise to a high value for the observed dielectric anisotropy of the mixture due to the non-associated polar compounds, but also... 

Apolar nematic compounds usually possess a low k2,-ilk ratio e.g. 1.0 < 33/ < 1.5), partially due to the absence of molecular dimers. However, they are still essential components of nematic mixtures for STN-LCDs, since they are used to lower the viscosity and melting point of a nematic mixture of polar components as well as improve the multiplexability of the mixture due in part to the reduction in the proportion of molecular dimers of associated polar molecules. Therefore, the synthesis of the first polar alkenyl liquid crystals with high k jku ratios led to the synthesis of a series of apolar alkenyl-substituted compounds with the carbon-carbon double bond in various positions in the terminal chain. Some typical compounds (140-149) are shown in Table... 

Summary Vinylsilanes are known to react with lithium metal either to 1,2-dilithioethanes by reduction or to 1,4-dilithiobutanes by reductive dimerization. The reaction of substituted vinylsilanes with lithium metal is employed in the approach to vicinal and geminal dilithiated vinylsilanes by two consecutive additions of lithium metal and subsequent eliminations of lithium hydride. A mechanistic investigation in the reactivity of a- and (3-substituted vinylsilanes towards lithium metal discloses several new reaction pathways, whereby the choice of solvent plays an important role in apolar solvents like toluene vinyllithium compounds are obtained. Compound 14, R = Ph, which is not stable under the reaction conditions, finally affords the 1,4-dilithium compound 27. Compound 18, R = SiMes, on the other hand either adds to the starting vinylsilane (forming the monolithium compound 39) or shows an unusual dimerization to 47, which is studied in detail. 

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