Residual Stability

Fig. 24.—(a) Stereo view of slightly over a turn of the 3-fold double helix of i-carrageenan (23). The two chains are distinguished by open and filled bonds for clarity. The vertical line is the helix axis. Six interchain hydrogen bonds per turn among the galactose residues stabilize the double helix. The sulfate groups lined up near the periphery are crucial for intermolecular interactions. 

From a catalytic point of view, the particle dimension represents for sure an important factor but not the only one. In fact the nature of the support, the possible presence of residual stabilizing agent, metallic dispersion and accessibility of active phase by the reactant are also important. 

Once the sample has been processed in such a way as to maintain residue stability, to prevent cross-contamination, and to ensure homogeneity, strategies to extract the drug from the tissue and to isolate the drug residue from potential interferences must be evaluated. The following sections will review these two concepts separately. 

The hydroxyproline residues stabilize the triple helix by forming hydrogen bonds between the a-chains, while the hydroxyl groups of hydroxylysine are partly glycosylated with a disaccharide (-Glc-Gal). 

FIGURE 3-7 Reversible formation of a disulfide bond by the oxidation of two molecules of cysteine. Disulfide bonds between Cys residues stabilize the structures of many proteins. 

Numerous methods can be used for quantitative determination of stabilizers in extracts from polymers [35]. Great difficulties may arise in analyses of agedpolymers. In practice, the amount of an extractable stabilizer was determined only exeptional-ly. Most experimental data discussing consequences of the physical loss of a stabilizer due to the leaching from polymers have been based on comparison of the residual stability of polymers after extraction with data obtained before extraction. 

While the residue stability constants are purely thermodynamic quantities defined for all residues, the protection factors also contain non-thermodynamic contributions and are defined only for a subset of residues. For example, proline residues lack the amide group and therefore are not included. From a statistical standpoint, the protection factor for any given residue] can be defined as the ratio of the sum of the probabilities of the states in which residue j is closed, to the sum of the probabilities of the states in which residue] is open ... 

Figure 2. The temperature dependence of the natural logarithm of the apparent residue stability constants. For clarity a single line is shown for groups of residues that exhibit similar behavior.

These branched-chain aliphatic amino acids contain bulky nonpolar R-groups and participate in hydrophobic interactions. All three are essential amino acids. A defect in their catabolism leads to maple syrup urine disease (Chapter 17). Isoleucine has asymmetrical centers at both the a- and 8-carbons and four stereoisomers, only one of which occurs in protein. The bulky side chains tend to associate in the interior of water-soluble globular proteins. Thus, the hydrophobic amino acid residues stabilize the three-dimensional structure of the polymer. 

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