Self-associating molecules

Note the general principle that can be drawn from this As the total concentration of a self-associating molecule increases, the proportion of the associated form increases, A corollary is that dilution favors the monomeric species. This is an example of Ostwald s dilution principle. 

Several aspects of the adsorption of self-associating molecules in microporous structures have been developed by Talu and Meunier [32], Their approach is similar to that of the chemical interpretation of nonideality of vapor and liquid phases. The theory leads to type V isotherm behavior and can explain the transition between types I and V. The data at one temperature are represented by three parameters Henry s law constant, saturation capacity, and reaction constant for cluster formation in the micropores. To describe a set of isotherms obtained over a certain temperature interval, the theory can be used with five temperature-independent parameters to determine the entire phase behavior including the heat of adsorption. Besides obtaining a good agreement with experimental data, they found that the dimerization enthalpy of water in the micropores is lower than that in the vapor phase. 

Table 4. Populations of self-associated molecules (100 P.) and of intramolecular ly stacked species (100 P ) calculated from best NMR thermodynamical parameters of m AptJ, Comparison with CD.

Clathrates solid state host-guest compounds, consisting of a network of self-associating molecules forming cavities or charmels in which guest species can reside. They are usually only stable in the presence of guest species. 

Finally, for benzene both the Hansen and the PSP methods need improvement. The Hansen method has a dispersion value (25.9) and a hydrogen bonding (3.0) part - the latter to represent polar and base effects, but benzene is not a self-associating molecule. The PSP method has a dispersion part (19.95) and a significant polar part (8.25) but no basic contribution, thus assuming that all interactions with water are due to polar effects, which is not correct (benzene is actually a rather non-polar molecule). The result shown in Example 15.3 verifies this problem. 

The conclusion of all these thermodynamic studies is the existence of thiazole-solvent and thiazole-thiazole associations. The most probable mode of association is of the n-rr type from the lone pair of the nitrogen of one molecule to the various other atoms of the other. These associations are confirmed by the results of viscosimetnc studies on thiazole and binary mixtures of thiazole and CCU or QHij. In the case of CCU, there is association of two thiazole molecules with one solvent molecule, whereas cyclohexane seems to destroy some thiazole self-associations (aggregates) existing in the pure liquid (312-314). The same conclusions are drawn from the study of the self-diffusion of thiazole (labeled with C) in thiazole-cyclohexane solutions (114). 

A (macro)emulsion is formed when two immiscible Hquids, usually water and a hydrophobic organic solvent, an oil, are mechanically agitated (5) so that one Hquid forms droplets in the other one. A microemulsion, on the other hand, forms spontaneously because of the self-association of added amphiphilic molecules. During the emulsification agitation both Hquids form droplets, and with no stabilization, two emulsion layers are formed, one with oil droplets in water (o /w) and one of water in oil (w/o). However, if not stabilized the droplets separate into two phases when the agitation ceases. If an emulsifier (a stabilizing compound) is added to the two immiscible Hquids, one of them becomes continuous and the other one remains in droplet form. 

It has been shown that the polarizability of a substance containing no dipoles will indicate the strength o/any dispersive interactions that might take place with another molecule. In comparison, due to self-association or internal compensation that can take place with polar materials, the dipole moment determined from bulk dielectric constant measurements will often not give a true indication of the strength of any polar interaction that might take place with another molecule. An impression of a dipole-dipole interaction is depicted in Figure 11. 

Insulin is composed of two peptide chains covalently linked by disulfide bonds (Figures 5.17 and 6.35). This monomer of insulin is the active form that binds to receptors in target cells. However, in solution, insulin spontaneously forms dimers, which themselves aggregate to form hexamers. The surface of the insulin molecule that self-associates to form hexamers is also the surface that binds to insulin receptors in target cells. Thus, hexamers of insulin are inactive. 

The theory of hydrophobic interaction [70-72] indicates that hydrophobic residues tend to associate with one another so as to minimize the surface area exposed to the aqueous phase and thereby to release a maximum number of structured water molecules. Therefore, the steric fit between the hydrophobic groups may be an important factor for the hydrophobic association. It is reasonable to consider that aromatic hydrophobic groups may undergo tighter hydrophobic self-association because planar aromatic rings would sterically fit with each other to favor the release of structured water. 

Alzheimer s Disease. Figure 1 A(3 monomers can self-associate to form dimers, trimers and higher oligomers. Globular structures of synthetic A(342 are known as A(3-derived diffusible ligands (ADDLs) (3-12-mers of A(3). These structures are similar to the smallest protofibrils and represent the earliest macromolecular assembly of synthetic A(3. The characteristic amyloid fiber exhibits a high beta-sheet content and is derived in vitro by a nucleation-dependent self-association and an associated conformational transition from random to beta-sheet conformation of the A(3 molecule. Intermediate protofibrils in turn self-associate to form mature fibers. 

Obviously, this shift implies the self-association of DMSO. Further frequency shifts to even lower wave numbers (1050-1000 cm " ) are observed in both aprotic polar and protic solvents. In aprotic solvents such as acetonitrile and nitromethane, the association probably takes place between the S—O bond of DMSO and the —C=N or the —NOz group in the molecules by dipole-dipole interaction as shown in Scheme 331,32. Moreover, the stretching frequency for the S—O bond shifts to 1051 cm 1 in CHC13 and to 1010-1000 cm -1 in the presence of phenol in benzene or in aqueous solution33. These large frequency shifts are explained by the formation of hydrogen bonds between the oxygen atom in the S—O bond and the proton in the solvents. Thus, it has been... 

In spite of the overwhelming evidence suggesting that recombinant resilin is amorphous, there are some results that suggest that a level of defined stmcmre cannot be completely ruled out. In particular, the fact that the protein solution coacervates when cooled (Figure 9.7) suggests that there is a degree of self-association between protein molecules. 

Stability of anthocyanins can be attained by self-association, that is, when two or more anthocyanin molecules are associated. This effect was verihed by increasing the concentration of the cyanidin 3,5-diglucoside solution from 10 M to 10 M with a consequent bathochromic shift in maximum wavelength absorption in the visible region. "... 

From the asymmetrical concentration profile with front tailing (see Figure 2.4b), it can correctly be deduced that (1) the adsorbent layer is already overloaded by the analyte (i.e., the analysis is being run in the nonlinear range of the adsorption isotherm) and (2) the lateral interactions (i.e., those of the self-associative type) among the analyte molecules take place. The easiest way to approximate this type of concentration profile is by using the anti-Langmuir isotherm (which has no physicochemical explanation yet models the cases with lateral interactions in a fairly accurate manner). 

The manner in which alcohols self-associate is different. Unlike carboxyhc acids, alcohols form the long chain-like structures of the self-associated -mers. Schematic presentation of the self-associated chain of the phenyl-substituted alcohol molecules is given in Figure 2.13. Moreover, an alternative possibility of the self-association with the aforementioned alcohols is shown in Figure 2.14. 

Mixed lateral interactions can occur in the case of the following pairs of molecules A. .. B, AB. .. A, AB. .. B, and ABj. .. ABj. Their appearance is even more importunate than that of the self-associative lateral interactions. With the self-asso-ciative lateral interactions alone, tailing of chromatographic bands lowers separative performance of a given chromatographic system, whereas with the mixed lateral... 

The structure of the trimer [Au3(NC5H4)3] has been studied and shows that individual molecules self-associate through aurophilic interactions into two distinct structural motifs that involve both extended chains of molecules connected by pairwise and individual Au---Au contacts, and discrete dimers linked by pairwise Au---Au interactions (Figure 25).3131... 

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