Aspartic acid, titration

Draw an appropriate titration curve for aspartic acid, labeling the axes and indicating the equivalence points and the values. 

Langsetmo K, Fuchs JA, Woodward C (1991) The conserved, buried aspartic acid in oxidized Escherichia coli thioredoxin has a pKa of 7.5. its titration produces a related shift in global stability Biochemistry 30 7603-7609. 

Aspartic acid Glutamic acid HjO Pd"L Pd"L2 potentiometric titration Aspartate chelates are slightly more stable than corresponding glutamate chelates. a... 

The acid-base behavior of amino acids may also be illustrated via titration curves. If one started with aspartic acid hydrochloride, that is, aspartic acid crystallized from solution in hydrochloric acid, one would require 3 mol base to remove completely the protons from 1 mol aspartic acid. The titration curve obtained with structures at each step of the reaction series is shown in Figure 4.1. Note that the isoelectric point is attained after one proton equivalent has been removed from the molecule. At this point, aspartic acid contains one positive and one negative charge it is zwitterionic. 

Figure 4.1 Titration curve of aspartic acid, pKv pK2, and pX3 are pk values of the a-carboxyl, -y-carboxyl and a-amino groups, respectively. The fully protonated form is present at a very low pH (e.g., pH 1) the fully deprotonated form is present at a very high pH (e.g., pH 12). The zwitterionic form (net charge 0) is present at pH = 3, which is also the pi.

How do the true titration curves of aspartic acid and lysine differ from that of glycine Explain your answer. 

The pH of solutions of isoelectric aspartic acid may be calculated from the pKa, and pK, values (the pif values on either side of isoelectric aspartic acid in its titration curve). 

Figure 1-8 Titration curve of aspartic acid. For clarity, the vertical axis is not drawn to scale.

Titration profiles of the basic and acidic amino acids lysine and aspartic acid are shown in Figures 2-9 and 2-10. The R-groups are ionized at physiological pH and have anionic and cationic groups, respectively. The pi value for aspartic acid is the arithmetic mean of pKj and pK2, whereas for lysine and histidine the pi values... 

Valik19 made differential potentiometric titrations of aspartic acid, one series of results being given in curve (c) of Fig. 6. The volume of solution of alkali necessary to titrate tlie second acid dissociation for which the ionization constant is 2.5 X 10"10 should be exactly equal to that for the first unless there is a difference between the potentiometric and stoichiometric end points. Within the rather large limit of error, this was found to be true, but the end point could not be located with accuracy due to the flatness of the curve, as shown above. Differences between the stoichiometric and potentiometric end points are predicted for titrations of weak acids or weak bases. Such a difference increases the weaker the acid or base, but the difficulty of locating the end point also increases. It may be safely concluded that within the accuracy to which the potentiometric end point of a titration can be established it is identical with the stoichiometric end point. 

Kokufuta, E., Suzuki, S. and Harada, K., 1977. Potentiometric titration behavior of poly-aspartic acid prepared by thermal polycondensation. BioSystems, 9 211 -214. 

Mathematical Sketch a titration curve for aspartic acid, and indicate the p values of all titratable groups. Also indicate the pH range in which the conjugate acid-base pair +1 Asp and 0 Asp will act as a buffer. 

The titration cinwes of these amino acids have an extra inflection, e.g., aspartic acid as shown in Fig. 3-52. 

Fig. 3-52 Titration of 10 mM aspartic acid hydrochioride with NaOH. The structures indicate the various stages of protonation of the titratabie moieties, and the number in brackets denotes the net change on the moiecuie.

It has been possible to analyse these titration curves and attribute the various steps to the carboxyl groups of glutamic acid and aspartic acid (to about pH = 5), the imidazcl group of the histidine (pH=5—9)... 

Peptides are known to show some specificity toward metal binding, as was shown by titration experiments nsing H-NMR [7,8], potentiometry [9], luminescence measnrements [10], infrared (IR), circular dichroism (CD), and ultraviolet (UV spectroscopies [11]. Poly-L-aspartic acid binds to metals such as Eu, Ce, La +, Cu ", and Pb ", and acts as a corrosion inhibitor for steel and iron. This property has been ascribed to the carboxylate side chain of aspartic acid [12,13]. 

Hen egg-white lysozyme has been partially N-acetylated in order to facilitate a study of the interactions between the carboxylic acid groups and amino-groups. Structural considerations and the results of pH titration indicated that the carboxylic acid groups of aspartic acid-48, -66, and -87 are not titratable in the acetylated lysozyme, which contains fewer titratable groups in the pH range 7—10 than does the native enzyme. 

Table XXXI. On considering the large number of groups determined, the difference between the number of groups in the native and methylated lysozymes, 3.6 1.0, is in fairly good accord with the observed difference, 3.0 0.2, between the titrations of the native lysozyme in KCl and in GU. The data of Table XXXI also are in good accord with the analytical data of Jollfes and Jollfes (1959), who report 22 aspartic acid and 5 glutamic acid residues. These, together with the terminal carboxyl group, correspond fairly well with the 29 groups found in the methylated derivative, and seem to be consistent with the hypothesis that these extra groups in lysozyme are actually masked carboxylate ions.

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. 

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