Stabilization/destabilization degree

Chemical modification of proteins has been extensively studied over the years to identify which amino acids are involved in catalysis. Much less work has been carried out on its influence on enzyme stability. Chemical modification of proteins may yield stabilization, destabilization or no effect at all. Martinek and Berezin (1978) reported the dependence of the thermostability of chymotrypsin on the degree of alkylation of its amino groups up to 30% alkylation the stability rose slightly at 90% substitution stability increased markedly, with a maximum (110-fold) at 95% stability fell to nearly initial values when 100% amino groups were modified. (With these modifications, the optimum pH of the errzyme can change and one must therefore be cautious in comparing two different... 

The Peierls distortion is not the only possible way to achieve the most stable state for a system. Whether it occurs is a question not only of the band structure itself, but also of the degree of occupation of the bands. For an unoccupied band or for a band occupied only at values around k = 0, it is of no importance how the energy levels are distributed at k = n/a. In a solid, a stabilizing distortion in one direction can cause a destabilization in another direction and may therefore not take place. The stabilizing effect of the Peierls distortion is small for the heavy elements (from the fifth period onward) and can be overcome by other effects. Therefore, undistorted chains and networks are observed mainly among compounds of the heavy elements. 

This approach did not provide a complete explanation for the observed degree of stereoselectivity. On the whole, the endo approach of olefin to nitronate is stabilized by secondary orbital interactions but, at the same time, is destabilized due to steric hindrance. 

Fig. 6 shows the orbital interactions for the union A + (B + C). The diagram allows us to decide unequivocally, and without the need of a calculation, the relative degree of stabilization and destabilization of the two geometries. The following conclusions are readily apparent from considering Fig. 6 and Eq. (1 ) ... 

Branched iV-chlorohydroxamic esters exhibit much lower carbonyl frequencies in their IR spectra. Series of Ai-(phenylethyloxy)amides (Table 2, entries 1-7) and Af-butoxy-amides (Table 2, entries 12-16) show a clear movement to lower carbonyl stretch frequencies with branching alpha to the carbonyl, in accord with greater inductive stabilization of the polar resonance form III of the carbonyl (Figure la). Neopentyl (entry 17) is a special case. While the group should contribute much more inductive stabilization than ethyl, its carbonyl stretch frequency is higher. Similar changes have been noted in the IR spectra of branched ketones and have been ascribed to a degree of steric hindrance to solvation and therefore destabilization of the polar resonance form Dl". ... 

Figure 16.6. The structures of 111 and 117 reveal substantial two-electron, three-center interaction, accompanied by stabilization, whereas destabilization in 102 is minimized by distortion towards a larger C3—N/C5—N distance (BLYP/cc-pVTZ). Bond lengths in picometers angles in degrees.

The physical stability of the casein micelle system is closely related to the degree of lactose crystallization from the unfrozen phase of the frozen concentrate. Crystallization of lactose from the unfrozen solution temporarily raises its freezing point, causing additional water to freeze, thus increasing the concentration and promoting destabilization of casein micelles. 

The values presented in Table 2 depict the variation of C = C re-bond strength with increasing fluorination and are consistent with the differences in reactivity associated with differing degrees of fluorination.3 Fluorination also destabilizes allenes and acetylenes.20 22 Similarly, per-fluoroalkyl groups destabilize C = C bonds. Note, however, that perfluoroalkyl groups can lend kinetic stabilization to strained molecules.3... 

The limited stabilization of a radical by partial fluorination is reflected in the corresponding bond-dissociation energies (sec Table 10) 87 however, increasing degrees of fluorination. such as the introduction of a trifluoromethyl group into a molecule, destabilize the radical (vide supra) 02 104 Even a single fluorine in the /J-position is destabilizing. 

Before determining the degree of stability of an emulsion and the reason lor this stability, the mechanisms of this destabilization should be considered. When an emulsion starts to separate, an oil layer appears on top. and an aqueous layer appears on the bottom. This separation is the final slate of the destabilization of the emulsion the initial two processes are called flocculation and coalescence. In flocculation, two droplets become attached to each other but are still separated by a thin film of the liquid. When more droplets are added, an aggregate is funned, in which the individual droplets cluster but retain the thin liquid films between them. The emulsifier molecules remain at the surface of the individual droplets during this process. 

For each we can write two equivalent planar VB structures, and the qualitative VB method would suggest that both compounds, like benzene, have substantial electron-delocalization energies. However, the planar structures would have abnormal C—C=C angles, and consequently at least some degree of destabilization associated with these bond angles (Section 12-7). Nonetheless, estimation of the strain energies show that while they are substantial, they are not prohibitive. Should then these molecules be stabilized by resonance in the same sense as benzene is postulated to be ... 

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