Glycine, dissociation constant

Table 9 includes data on the first dissociation constants of seven weak acids it will be recalled that we expect these to fall into class III. The table includes the second dissociation constants of five acids, phosphoric, sulfuric, oxalic, malonic, and carbonic, which fall into class IV, while the amino acids glycine and alanine provide four examples that should fall into class II. 

Using these acid dissociation constants for glycine, calculate the ratios [Z]/[C+] and [Z]/[A-] at pH... 

Because dissociation constants vary with temperature, the zwitterion/uncharged ratio may also vary with temperature. For glycine , the temperature dependencies are dpKJdT = 0.0020 and dpA 2/dT = -0.025. 

Dissociation Constants of Amino-Acids.—A very considerable simplification in the treatment of amino-acids can be achieved by regarding them as dibasic acids. Consider, for example, the hydrochloride of glycine, i.e., C1"" +NH3CH2C02H when this is neutralized by an alkali hydroxide, there are two stages of the reaction, corresponding in principle to the two stages of neutralization of a dibasic acid, thus... 

TABLE LXXIV. DETERMINATION OF DISSOCIATION CONSTANTS OP GLYCINE AT 20 Mixtures of Glycine (c) and Hydrochloric acid (a)... 

Figure 2-6. Titration curves for glycine (A) and histidine (6). The molecular species of glycine present at various pHs are indicated by the molecules above the curve. For histidine, pKa is the dissociation constant of the imidazole (side chain) group.

The fact that Walbltjm found a large temperature coefficient for the Ka of glycine supports the hybrid ion theory. The dissociation constants of carboxylic acids and of ammonia show small temperature coefficients. If Walblum s data are recalculated to yield the true basic dissociation constant, we find only a twofold increase in this constant between 10° and 70°. 

Kishimoto, H., Simon, J.R., Aprison, M.H., 1981. Determination of the equilibrium dissociation constant at a number of glycine binding site in several areas of the rat central nervous system, using a Na-indepen-dent system. J. Neurochem. 37, 1015-1024. 

For a simple zwitterion such as glycine, in which there are two ionizing groups, the isoionic point is simply related to the pK values for the two groups. The ionization of glycine is shown on p. 315, and the corresponding dissociation constants are... 

Rossotti et al [Fa 70] found that the ratio of stability constants obtained by equilibrium measurements in solvent mixtures similar to the above was independent of the composition of the solvent. McBryde et al showed that in most of the systems they examined, the ratio of the first two stepwise complex stability constants did not depend on the solvent. In the case of the stepwise acid dissociation constants this correlation was approximately true only for ethylenediamine for glycine the ratio of the protonation constants increase with increase in the concentration of the organic component. However, if the formation of the glycine complexes is defined as a reaction between the protonated ligand and the metal ion, accompanied by the liberation of a hydrogen ion, the ratio of the equilibrium constants is approximately independent of the composition of the solvent mixture. 

WAUCHOPE, D. 1976. Acid dissociation constants of arsenic acid, methylarsonic acid (MAA), dimethyl-arsinic acid (cacodylic acid), and N-(phosphono-methyl)glycine (glyphosate). J. Agric. Food Chem. 24 717-721. 

The dissociation constants of amino acids can be determined, for example, by titration of the acid. Figure 1.2 shows titration curves for glycine, histidine and aspartic acid. Table 1.2 lists the dissociation constants for some amino acids. In amino acids the acidity of the carboxyl group is higher and the basicity of the amino group lower than in the corresponding carboxylic acids and amines (cf. pK values for propionic acid, 2-propylamine and alanine). As illustrated by the comparison of pK values of 2-aminopropionic acid (alanine) and 3-aminopropionic acid ( 3-alanine), the pK is influenced by the distance between the two functional groups. 

The measure of acidity and basicity of functional groups of amino acids are the dissociation constants. The dissociation constant Xj describes the acidity of the a-carboxyl group of glycine, as shown by the following reaction ... 

Knight and Barrett (7) have confirmed that pepstatin is a good titrant for the determination of the molarity of solutions of cathepsin D and have measured the apparent dissociation constants (Kq) for the binding of pepstatin, pepstatin methyl ester and pep-statinyl C H ] glycine to cathepsin D. At pH 5 and below, Kj) was 5 X 10 M, but binding was strongly pH-dependent. Qualitatively,... 

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