That is, at a pH of 6.9-7.4, the a-carboxyl group (pK

7

2.4)

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

7

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-

section 2.3

Electrolyte and Acid-Base Properties

spondingly a weaker acid than an aliphatic ammonium

ion, e. g., ethylamine (pK

7

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,

29

F IG U R E

2-9

Titration profile of lysine.