Ampholytes Zwitterions

A large part of the dissolved amino acid exists as the ampholyte (zwitterion). The isoelectric point (p J) is the pH at which the net electric charge of a dissolved amino acid molecule is zero, p J is expressed as... 

Ampholytes (zwitterions) include an extensive range of substances that could be used as self buffers. Table 2.6 gives a list of isoionic ampholytes proposed for use as buffers in protein fractionation in a natural pH gradient (Svensson, 1962). Solutions of these substances in water have pH values close to the listed value of pi and when p7 - pA i is less than 1.5 they can be considered to be self buffers. 

Niflumic acid, which has two pKa values, was studied both pH-metrically and spectroscopically using the shake-flask method [224]. The monoprotonated species can exist in two forms (1) zwitterion, XH 1 and (2) ordinary (uncharged) ampholyte, XH°. The ratio between the two forms (tautomeric ratio) was measured spectroscopically to be 17.4. On assuming that a negligible amount of zwitterion XH partitions into octanol, the calculated micro-log/1 for XH° was 5.1, quite a bit higher than the macro-log/1 3.9 determined pH-metrically in 0.15 M NaCl. It is noteworthy that the distribution coefficient D is the same regardless of whether the species are described with microconstants or macroconstants [275]. 

Tab. 2.6. Measured versus literature values for pKas of ampholytic and zwitterionic compounds.

P. I. Nagy and K. Takacs-Novak, Theoretical and experimental studies of the zwitterions neutral form equilibrium of ampholytes in pure solvents and mixtures, J. Am. Chem. 

Figure 4. Scheme of lipophilicity profile of zwitterionic compounds. The line drawn represents the case where the neutral tautomer predominates or the zwitterion is rather hydrophobic, resulting in a bell-shaped profile. The dashed line represents the case where the zwitterion predominates and intramolecular interactions are not possible, resulting in a U-shaped profile. Adapted with permission from [133] Pagliara, A. et al. (1997). Lipophilicity profiles of ampholytes , Chem. Rev., 97, 3385-3400 copyright (1997) American Chemical Society... 

Particularly interesting examples are also the lipophilicity profiles of ampholytes. Depending on the ratio between the neutral tautomer and the zwitterionic tautomer, the log Dow versus pH profile may be bell-shaped or U-shaped [133] (Figure 4). For zwitterions, the shape of the lipophilicity profile depends upon the structure and conformation of the molecule. If the charged groups are situated in proximity and can interact with each other, the zwitterion might be more hydrophobic than the anionic and the cationic species, resulting in a bell-shaped lipophilicity profile. If, however, intramolecular interactions are not possible for steric reasons, the lipophilicity profile is U-shaped [133],... 

Amphiphile-oil-water system, temperature of, 16 424-426 Amphiphiles, 16 420 Amphiphile strength, 6 424 Amphiphilic chemicals, 17 56 Amphiphilic copolymers, 20 482 behavior of, 20 483 well-defined, 20 485-490 Amphiphilic molecules, 15 99-101 Amphiphilic plasticizers, 14 480 Amphiphilic polymer blend, 23 720 Amphiphilic polymers statistical, 20 484-490 stimuli-responsive, 20 482-483 Ampholytes, 9 746-747 Amphoteric cyclocopolymers, water-soluble, 23 721 Amphoteric starches, 4 722 Amphoteric (zwitterionic) surfactants, 24 148... 

FIGURE 12 Application of capillary isoelectric focusing (clEF) for the determination of apparent p/ values of rMAb samples. Capillary Bio-Rad Bio-CAP XL capillary (50 pm x 24 cm) ampholyte 80% clEF Bio-Lyte Ampholyte 3-10 (2% solution with 0.5% TEMED, 0.2% HPMC) anolyte 20 mM phosphoric acid catholyte 40 mM sodium hydroxide focusing l5kV (625V/cm) for 5 min mobilization 20 kV (833V/cm) for 25 min with zwitterions (cathodic mobilizer from Bio-Rad) capillary temperature 25°C. (Reprinted from reference 40, with permission.)... 

Note An ampholytic polymer in which ionic groups of opposite sign are incorporated into the same pendant groups is called, depending on the structure of the pendant groups, a zwitterionic polymer, polymeric inner salt, or poly betaine. 

Ampholytes or Zwitterions. An ampholyte is a molecule that can be either positively or negatively charged, depending on the pH. These... 

The pH gradient in cIEF is produced by the use of reagents, known as carrier ampholytes, that are zwitterionic and are chosen so that the... 

A chemical structure having both anionic and cationic charges on the same molecule is called a zwitterion. At a specific pH, the degree of the ionization of the zwitterion to an anionic electrolyte or to a cationic electrolyte is the same. The pH is called the isoelectric point (IEP). At the IEP, the same amount of anionic and cationic electrolytes exist. The zwitterion has the lowest solubility, denoted as SG. One can write the ionization of an ampholyte simply as ... 

Ampholyte — A substance that can react both as an acid and as abase is called an ampholyte, or amphoteric compound. Usually this property refers to the - Bronsted acid-base theory. An example is HCOj which can act as a proton acceptor and as a proton donator. An ampholyte can be a zwitterion, as in case of amino acids in the range between pH = pJCai and pH = pfCa2, they exist as [+(H3N)HRC-COO-]. 

The ideal sample run on the Rotofor cell would contain only the protein mixture, water, and ampholytes or buffers. However, pi precipitation may require that 3 M urea be included for solubility. When higher urea concentrations are needed, the Rotofor cell is run at 12°C. Detergents (1-2% w/v) may also be added to samples. Zwitterionic detergents, such as CHAPS, CHAPSO, and nonionic octyl-glucoside are satisfactory. 

Depending on their chemical structure, surfactants capable of forming micelles are usually classified into cationic e.g. ammonium salts), anionic e.g. sulfates, carboxylates), ampholytic e.g. zwitterionic salts), and non-ionic surfactants (usually containing polyoxyethene chains) cf. Table 2-10 in Section 2.5. 

Ampholytes (amphoteric electrolytes) can function as either weak acids or weak bases in aqueous solution and have plC values corresponding to the ionisation of each group. They may be conveniently divided into two categories - ordinary ampholytes and zwitterionic ampholytes - depending on the relative acidity of the two ionisable groups. 

Figure 3.8 Distribution of ionic species for the zwitterionic ampholyte labetalol.

The zwitterionic or ampholytic surfactants shown in Box 6.1 possess both positively and negatively charged groups and can exist as either an anionic or a cationic surfactant depending on the pH of the solution. A typical example is N-dodecyl-N,N-dimethylbetaine (Ci2H25N"(CH3)2CH2COO). 

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