Titration of an amino acid

 1. Dissociation of the carboxyl group: The titration curve of ana mino acid can be analyzed in the same way as described fora cetic acid. Consider alanine, for example, which contains botha n α-carboxyl and an α-amino group. At a low (acidic) pH, both oft hese groups are proton ated (shown in Figure 1.10). As the pH oft he solution is raised, the – COOH group of Form I can dissociateb y donating a proton to the medium. The release of a protonr esults in the formation of the carboxylate group, – COO–Thiss tructure is shown as Form II, which is the dipolar form of them olecule (see Figure 1.10). This form, also called a zwitterion, ist he isoelectric form of alanine, that is, it has an overall (net) Charge of zero.

2. Application of the Henderson-Hasselbalch equation: The dissoci-

ation constant of the carboxyl group of an amino acid is called K1,

rather than Ka, because the molecule contains a second titratable

group. The Henderson-Hasselbalch equation can be used to

analyze the dissociation of the carboxyl group of alanine in the

same way as described for acetic acid:

where I is the fully protonated form of alanine, and II is the iso-

electric form of alanine (see Figure 1.10). This equation can be

re arranged and converted to its logarithmic form to yield:

3. Dissociation of the amino group: The second titratable group of

alanine is the amino (– NH3

+) group shown in Figure 1.10. This is

a much weaker acid than the – COOH group and, therefore, has a

much smaller dissociation constant, K2. [Note: Its pKa is therefore

larger.] Release of a proton from the protonated amino group of

Form II results in the fully deprotonated form of alanine, Form III

(see Figure 1.10).

4. pKs of alanine: The sequential dissociation of protons from the

carboxyl and amino groups of alanine is summarized in Figure

1.10. Each titratable group has a pKa that is numer ically equal to

the pH at which exactly one half of the protons have been

removed from that group. The pKa for the most acidic group

(–COOH) is pK1, whereas the pKa for the next most acidic group

(– NH3

+) is pK2