That is, at a pH of 6.9-7.4, the a-carboxyl group (pK
is dissociated to yield a negatively charged carboxylate
ion (-COO ), while the «-amino group (pK' 9.7) is pro-
tonated to yield an ammonium group (-NHJ). The pK
value of the «-carboxyl group is considerably lower than
that of a comparable aliphatic acid, e.g., acetic acid (pK'
4.6). This stronger acidity is due to electron withdrawal
by the positively charged ammonium ion and the con-
sequent increased tendency of a carboxyl hydrogen to
dissociate as an H+. The a-ammonium group is corre-
Electrolyte and Acid-Base Properties
spondingly a weaker acid than an aliphatic ammonium
ion, e. g., ethylamine (pK
9.0) because the inductive
effect of the negatively charged carboxylate anion tends to
prevent dissociation of H+. The titration profile of glycine
hydrochloride (Figure 2-7) is nearly identical to the pro-
files of all other monoaminomonocarboxylic amino acids
with nonionizable R-groups (Ala, Val, Leu, lie, Phe, Ser,
Thr, Gin, Asn, Met, and Pro).
The titration of glycine has the following major fea-
tures. The titration is initiated with glycine hydrochloride,
F IG U R E
Titration profile of lysine.