Aggregation intermolecular

The formation of a compact structure accompanying the complexation of PMMA with PEO and the lower flexibility of the PMMA chain in the complex than that of the PAA chain have been confirmed by viscometry [ 16], membrane contraction [2], and polarized luminescence techniques [3]. In addition, comparison of the dynamic light-scattering behavior of PMAA/PEO and PMAA/PEO" in solution shows that the pyrene label, which acts as a hydrophobic species, allows the labeled PEO to aggregate intermolecularly much faster than unlabeled PEO does [30]. 

An enzyme can deactivate irreversibly for two kinds of reasons (i) conformational processes, such as aggregation (intermolecular), or incorrect structure formation (intramolecular), such as scrambled disulfide bond formation between wrong side chains, and (ii) covalent processes, such as reduction and thus destruction of disulfide bonds, deamidation of asparagine (Asn) or glutamine (Gin) side chains, or hydrolysis of (usually) labile asp-X bonds in the protein sequence. 

Fig. 24.15 Above-. Thymidine calix[4]nucleosides [42, 43]. Possible triplex fomiatirai patterns with hairpin structures (a) natural type, (b) hairpin moiety replaced by calixnucleoside 35, (c) hairpin moiety replaced by calixnucleoside 37 or 38 [42]. Possible intra- and intermolecular structures adopted by calixnucleotides (d) Hairpin structure intramolecular base pairing (e) bulged duplex intermolecular base pairing (f) V-shaped aggregate intermolecular base pairing [43]...

The formation of ECC is not only an extension of a portion of the macromolecule but also a mutual orientational ordering of these portions belonging to different molecules (intermolecular crystallization), as a result of which the structure of ECC is similar to that of a nematic liquid crystal. After the melt is supercooled below the melting temperature, the processes of mutual orientation related to the displacement of molecules virtually cannot occur because the viscosity of the system drastically increases and the chain mobility decreases. Hence, the state of one-dimensional orientational order should be already attained in the melt. During crystallization this ordering ensures the aggregation of extended portions to crystals of the ECC type fixed by intermolecular interactons on cooling. 

The rates of intermolecular Diels-Alder reactions of hydrophobic dienes and dienophiles are significantly increased when the cycloadditions are performed in pure ethylene glycol (EG) [49a]. Some examples are illustrated in Scheme 6.30. This performance is due to the fact that the EG (i) forms extensive hydrogen bonding, (ii) is able to solubilize hydrophobic dienes and dienophiles, and (hi) forms molecular aggregations with the reactants. 

Olivares et al. (2006), studies performed viscometers very dilute gelatin solutions with concentrations between 10-5 and 10-3 g/cm3, where either intermolecular aggregation or intramolecular folding are possible, respectively, and the sol-gel transition is not observed. 

Coacervation occurs in tropoelastin solutions and is a precursor event in the assembly of elastin nanofibrils [42]. This phenomenon is thought to be mainly due to the interaction between hydro-phobic domains of tropoelastin. In scanning electron microscopy (SEM) picmres, nanofibril stmc-tures are visible in coacervate solutions of elastin-based peptides [37,43]. Indeed, Wright et al. [44] describe the self-association characteristics of multidomain proteins containing near-identical peptide repeat motifs. They suggest that this form of self-assembly occurs via specific intermolecular association, based on the repetition of identical or near-identical amino acid sequences. This specificity is consistent with the principle that ordered molecular assembhes are usually more stable than disordered ones, and with the idea that native-like interactions may be generally more favorable than nonnative ones in protein aggregates. 

In the case of amphiphilic molecules, characterized by the coexistence of spatially separated apolar (alkyl chains) and polar moieties, both parts cooperate to drive the intermolecular aggregation. This simple but pivotal peculiarity makes amphiphilic molecules soluble in both polar and apolar solvents and able to realize, in suitable conditions, an impressive variety of molecular aggregates characterized by spatially separated apolar and polar domains, local order at short times and fluidity at long times, and differences in size, shape (linear or branched chains, cyclic or globular aggregates, extended fractal-like molecular networks), and lifetime. 

The conformational dynamics of chain segments near the head groups is more restricted than that of those far from the micellar core [8]. Moreover, to avoid the presence of energetically unfavorable void space in the micellar aggregate and as a consequence of the intermolecular interactions, surfactant molecules tend to assume some preferential conformations and a staggered position with respect to the micellar core [9]. A schematic representation of a reversed micelle is shown in Figure 1. 

The frequent breaking and reforming of the labile intermolecular interactions stabilizing the reversed micelles maintain in thermodynamic equilibrium a more or less wide spectrum of aggregates differing in size and/or shape whose relative populations are controlled by some internal (nature and shape of the polar group and of the apolar molecular moiety of the amphiphile, nature of the apolar solvent) and external parameters (concentration of the amphiphile, temperature, pressure) [11], The tendency of the surfactants to form reversed micelles is, obviously, more pronounced in less polar solvents. 

Molecular solids are aggregates of molecules bound together by intermolecular forces. Substances that are gases under normal conditions form molecular solids when they condense at low temperature. Many larger molecules have sufficient dispersion forces to exist as solids at room temperature. One example is naphthalene (Cio Hg), a white solid that melts at 80 °C. Naphthalene has a planar structure like that of benzene (see Section 10-), with a cloud of ten delocalized n electrons that lie above and below the molecular plane. Naphthalene molecules are held in the solid state by strong dispersion forces among these highly polarizable n electrons. The molecules in... 

The UV-visible spectra of the H- and nifro-azobenzene dendrimers in chloroform solution showed strong absorption bands within the visible region due to the transitions of azobenzene chromophores (Table 2). Because of the stronger delocalization of n-electrons in nitro-azobenzene, the maximum absorption band is at a longer wavelength compared with that for H-azoben-zene. There was little spectral shift of the absorption maximum for dendrimers with different numbers of azobenzene units, indicating that dendrimers did not form any special intermolecular aggregates. 

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