Other Physiochemical Methods

Because of the time and expense involved, biological assays are used primarily for research purposes. The first chemical method for assaying L-ascorbic acid was the titration with 2,6-dichlorophenolindophenol solution (76). This method is not appHcable in the presence of a variety of interfering substances, eg, reduced metal ions, sulfites, tannins, or colored dyes. This 2,6-dichlorophenolindophenol method and other chemical and physiochemical methods are based on the reducing character of L-ascorbic acid (77). Colorimetric reactions with metal ions as weU as other redox systems, eg, potassium hexacyanoferrate(III), methylene blue, chloramine, etc, have been used for the assay, but they are unspecific because of interferences from a large number of reducing substances contained in foods and natural products (78). These methods have been used extensively in fish research (79). A specific photometric method for the assay of vitamin C in biological samples is based on the oxidation of ascorbic acid to dehydroascorbic acid with 2,4-dinitrophenylhydrazine (80). In the microfluorometric method, ascorbic acid is oxidized to dehydroascorbic acid in the presence of charcoal. The oxidized form is reacted with o-phenylenediamine to produce a fluorescent compound that is detected with an excitation maximum of ca 350 nm and an emission maximum of ca 430 nm (81). 

In subsequent studies attempting to find a correlation of physicochemical properties and antimicrobial activity, other parameters have been employed, such as Hammett O values, electronic distribution calculated by molecular orbital methods, spectral characteristics, and hydrophobicity constants. No new insight on the role of physiochemical properties of the sulfonamides has resulted. Acid dissociation appears to play a predominant role, since it affects aqueous solubiUty, partition coefficient and transport across membranes, protein binding, tubular secretion, and reabsorption in the kidneys. An exhaustive discussion of these studies has been provided (10). 

Diversity is typically measured using a distance-based or cell-based method cost is typically given as reactant cost/gm and physiochemical properties such as AMW are typically measured as the difference in the distribution of the property in the library compared to the distribution of the same property in a collection of known drugs. The weights, wl5 w2j w3> are user-defined and are typically set so that diversity is maximized, while the cost and physicochemical properties are minimized. This weighted-sum approach leads to a single solution that represents one particular compromise in the objectives. Several other groups have also adopted this approach [67, 71 73]. 

Two contributions from other disciplines are also desirable. Improved use of pheromones and other semiochemi-cals will depend upon a much better understanding of the biochemical and physiochemical mechanisms within the insect that produces them and upon which they act. Such systems are clearly complex and it seems highly probable that, once they are understood, improved methods of inhibiting them will be found, including the possible uses of individual inhibitory chemicals or classes of them in place of the actual pheromones themselves. 

Any consideration of the immunogenicity of a protein must take into account how its physiochemical properties can dictate the outcome of the immune response. We will consider only some of the intrinsic properties of antigens that are important in this respect other properties will be considered in the section Methods of Immunization. 

Thermoanalytical (TA) methods characterize a system in terms of the temperature dependency of its thermodynamic properties and the physiochem-ical reaction kinetics of TPs and TSs. The techniques reviewed here only include differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and thermomechanical analysis (TMA). Others also are available, which are useful in the processing plant (1, 2). 

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