Structure steric effects upon

Because protein-ba sed foams depend upon the intrinsic molecular properties (extent and nature of protein-protein interactions) of the protein, foaming properties (formation and stabilization) can vary immensely between different proteins. The intrinsic properties of the protein together with extrinsic factors (temperature, pH, salts, and viscosity of the continuous phase) determine the physical stability of the film. Films with enhanced mechanical strength (greater protein-protein interactions), and better rheological and viscoelastic properties (flexible residual tertiary structure) are more stable (12,15), and this is reflected in more stable foams/emulsions (14,33). Such films have better viscoelastic properties (dilatational modulus) ( ) and can adapt to physical perturbations without rupture. This is illustrated by -lactoglobulin which forms strong viscous films while casein films show limited viscosity due to diminished protein-protein (electrostatic) interactions and lack of bulky structure (steric effects) which apparently improves interactions at the interface (7,13 19). 

Contrary to earlier expectations (see Dorfman, 1965), Hentz and Kenney-Wallace (1972, 1974) failed to find any correlation between s and Emax. Actually, there is a better correlation of matrix polarity with the spectral shift from e(to e upon solvation and the time required to reach the equilibrium spectrum (Kevan, 1974). Furthermore, Hentz and Kenney-Wallace point out that emax is smaller f°r alcohols with branched alkyl groups, the spectrum being sensitive to the number, structure, and position of these groups relative to OH. Clearly, a steric effect is called for, and the authors claim that a successful theory must not rely too heavily on continuum interaction as appeared in the earlier theories ofjortner (1959,1964). Instead, the dominant interaction must be of short range, and probably the spectrum is determined by optimum configuration of dipoles within the first solvation shell. 

The preferred facial selectivity from the less exposed side (b-side) obtained in the irradiation of 301 under kinetically controlled conditions (entry 4) cannot be explained only on the basis of steric effect however, it is consistent with the direction of pyramidalization in structure 299. The increase in the facial selectivity from the same side, upon reducing the steric effect in compound 301d, emphasizes that steric effects cannot be neglected in rationalizing the facial selectivity in these or related systems (Scheme 65). 

These results show that the supramolecular structures of thin films of block copolymers can be manipulated by varying the rod-to-coil ratios. Variables such as the polydispersity, the nature of the structures, and their crystallinity can be controlled in this manner. The factors that govern the formation of ordered structures from these copolymers are, however, complex. Important factors include entropy effects associated with the flexible coil segments, crystallization of the rods, and steric considerations. Upon crystallization of the rods, the entropies of the coil blocks may be increasingly compromised as a result of increasing steric repulsion. This may effect the sizes of the aggregates that are formed. The organization of ordered structures can furthermore be controlled by non-specific interactions such... 

The origin(s) for the preference of stereostructure A in the acrylic acid ester addition is not known with certainty. A steric effect may explain the observation. The bulky acceptor substituent of the dienophile might be less hindered—and this is quite counterintuitive—in the enrfo-orientation in the transition state shown in Figure 15.31 than in the alternative exo-position. One might use the structure B to suggest that the substituent of the dienophile in A does not try to avoid the C atoms C2 and C3 as much as it tries to stay away from the H atoms cis-H1 and cis-H4. The increase of e/w/o-selectivity upon addition of a Lewis acid could then be explained by the premise that the complexing Lewis acid renders the ester group more bulky. This increased steric demand enhances its desire to avoid the steric hindrance in its ew-posi-tion. 

One exception to the amorphous structure has been reported [30]. Crystalline polybenzyl was obtained from the low temperature (- 125° C) polymerization of benzyl chloride. However, the reaction was difficult to reproduce [31,32]. Consequently this procedure is not an effective method for the synthesis of linear polybenzyls. The usual amorphous, highly branched structure is formed as a result of a lack of positional selectivity and multiple substitution of the arene rings. Similar polymeric structures are obtained upon the polymerization of other nonsubstituted benzyl halides and benzyl alcohol [29]. The highly branched structure is a consequence of the involvement of benzyl carbenium ions in the Friedel-Crafts reaction. Benzyl substituents activate the monosubstituted phenyl groups toward further benzylation reaction. However, monomers containing alkyl substituents that sterically hinder substitution at the ortho position have been polymerized to linear polybenzyls. For example, the following... 

Zeolites are microporous frameworks, and all of the ET chemistry that we have discussed is with molecules smaller than 13 A. The unique features of zeolites are their ion-exchanging ability, a stable structure upon dehydration and a pore/chan-nel structure that allows for a well-defined arrangement of molecules in space and the fact that redox-active atoms can be substituted on the framework. In most cases, the zeolite is an active host, influencing ET reactions via electrostatic fields or steric effects, a feature that is not found with the mesoporous and sol gel materials. Packing of molecules/ions in the intrazeolitic space with very high densities is also possible and was found to be important in charge propagation and electrochemistry. 

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