Glutamic acid titration curve

From these decompositions, amino acid titration curves can be predicted. The evolution of pH is correctly predicted by the model for amino acids such as glycine, aspartic and L-glutamic acids (Figure 3.3). Most predictions for the other amino acids are correct, however, for some, small differences between measurements and predictions have been observed, because they are due to the pKa values that are not generally measured at zero ionic strength and infinite dilution. 

With multiple ionizable groups, such as in amino acids and proteins, each group titrates separately according to its pKa. The titration curves shown in Fig. 23-5 are for the amino acids glycine, histidine, and glutamate. 

Brignon et al. (1969) demonstrated that the maximum acid-binding capacity of /3-lactoglobulin D is the same as that of the other variants. The curves are identical at pH 4.0. At pH 6.5, one less proton is dissociated in the titration of the D variant than with the B variant, as would be predicted from the substitution of a glutamine residue for a glutamic acid residue in B. The anomalous carboxyl group observed in the other variants is also detected in the D variant. 

Using the pKa values from problem 3, construct the theoretical titration curve showing the equivalents of H+ or OH reacting with 1 mol of glycine as a function of pH. Note that the shape of this curve is independent of the pfCa. Sketch similar curves for glutamic acid (pK./s equal 2.19,4.25, and 9.67), histidine (pfCa s equal 1.82,6.00, and 9.17) and lysine (pfCa s equal 2.18,8.95, and 10.53). 

Titration curves of glutamic acid, lysine, and histidine. In each case, the pK of the R group is designated pKR. 

Titration curve of /3-lactoglobulin. At very low values of pH (<2) all ionizable groups are protonated. At a pH of about 7.2 (indicated by horizontal bar) 51 groups (mostly the glutamic and aspartic amino acids and some of the histidines) have lost their protons. At pH 12 most of the remaining ionizable groups (mostly lysine and arginine amino acids and some histidines) have lost their protons as well. 

Given the pKa values in the text, predict how the titration curves for glutamic acid and glutamine differ. 

FIGURE 2.18 Titration curve of glutamic acid ( pKal = 2.2, pKa2 = 4.3, and pK = 9.4). Thus, in the titration of a diprotic weak acid H2A, Equation (2.150) reduces to ... 

The titration curves of these amino acids have an extra inflection, as shown for glutamic acid in Fig. 3-3. 

A. Wada, Helix-coil transformation and titration curve of poIy-L-glutamic acid. Molec. Phys. 3 409-416 (1960). 

Additionally, Ti data for smaller oligomers of lysine have also been measured (Saito and Smith, 1974). Similarly, titration curves have been applied to the study of helix-coil transitions in aqueous solutions of poly-L-glutamic acid (Lyerla et al., 1973). 

Figure 4. Titration curves of glutamic acid, glycine, histidine, and lysine. The three amino acids whose titration curves sharply cut the zero-charge-horizontal-line also have a net charge apart from zero in the vicinity of the pH where they cut the zero line, i.e., the pi. That means that they have buffering capacity and conductance in the neighborhood of the isoelectric point and therefore are useful as carrier ampholytes. Glycine, on the other hand, with its extended horizontal part of the curve is not suitable. (Svensson, 2.)...

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)... 

Given p/Cg values for ionizable groups from Table 18.2, sketch curves for the titration of (a) glutamic acid with NaOH and (b) histidine with NaOH. 

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