30
chapter 2
Amino Acids
FIGURE 2-1«
Titration profile of aspartic acid.
Cl (H|NCH
2
COOH), which is the fully protonated form
of the amino acid. In this form, the molecule contains
two acidic functional groups; therefore, two equivalents
of base are required to completely titrate
1
mol of glycine
hydrochloride. There are two pK' values: pKj due to re-
action of the carboxyl group and pK
2
due to reaction
of the ammonium group. Addition of 0.5 eq of base to
1 mol of glycine hydrochloride raises the pH 2.34 (pKj),
whereas addition of 1.5 eq further increases the pH to
9.66 (pK2). At low pH values (e.g., 0.4), the molecules are
predominantly cations with one positive charge; at pH val-
ues of 5-7, most molecules have a net charge of zero; at
high pH values (e.g., 11.7), all of the molecules are essen-
tially anions with one negative charge. The midpoint be-
tween the two pK' values [i.e., at pH = (2.34 + 9.66)/2 =
6.0] is the pi. Thus, pi is the arithmetic mean of pKj and
pK
'2
values and the inflection point between the two seg-
ments of the titration profile.
The buffering capacities of weak acids and weak bases
are maximal at their pK values. Thus, monoaminomono-
