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Chapter 3 Amino Acids, Peptides, and Proteins S-27
�H3NOCH2OCOOH �1
�H3NOCH2OCOO� 0 (zwitterion)
H2NOCH2OCOO� �1
The floors are connected by steep stairways, and each stairway has a landing halfway between the
floors. A titration curve traces the path one would follow between the different floors as the pH
changes in response to added OH�. Recall that the pKa of an acid (on a halfway landing) represents
the pH at which half of the acid is deprotonated. The isoelectric point (pI) is the pH at which the aver-
age net charge is zero. Now you are ready to consider statements (a) through (o).
(a) Glycine is present predominantly as the species �H3NOCH2OCOOH.
(b) The average net charge of glycine is � .
(c) Half of the amino groups are ionized.
(d) The pH is equal to the pKa of the carboxyl group.
(e) The pH is equal to the pKa of the protonated amino group.
(f) Glycine has its maximum buffering capacity.
(g) The average net charge of glycine is zero.
(h) The carboxyl group has been completely titrated (first equivalence point).
(i) Glycine is completely titrated (second equivalence point).
(j) The predominant species is �H3NOCH2OCOO�.
(k) The average net charge of glycine is �1.
(l) Glycine is present predominantly as a 50:50 mixture of �H3NOCH2OCOOH and
�H3NOCH2OCOO�.
(m) This is the isoelectric point.
(n) This is the end of the titration.
(o) These are the worst pH regions for buffering power.
Answer
(a) I; maximum protonation occurs at the lowest pH (the highest [H�]).
(b) II; at the first pKa, or pK1 (2.34), half of the protons are removed from the a-carboxyl 
group (i.e., it is half deprotonated), changing its charge from 0 to � . The average net 
charge of glycine is (� ) � 1 � .
(c) IV; the a-amino group is half-deprotonated at its pKa, or pK2 (9.60).
(d) II; from the Henderson-Hasselbalch equation, pH = pKa + log ([A�]/[HA]). If 
[A�]/[HA] � 1, or [A�] � [HA], then pH � pKa. (Recall that log 1 � 0.)
(e) IV; see answers (c) and (d).
(f) II and IV; in the pKa regions, acid donates protons to or base abstracts protons from
glycine, with minimal pH changes.
(g) III; this occurs at the isoelectric point; pI � (pK1 � pK2)/2 � (2.34 � 9.60)/2 � 5.97.
(h) III; the pH at which 1.0 equivalent of OH� has been added, pH 5.97 (3.6 pH units away
from either pKa).
(i) V; pH 11.3 (1.7 pH units above pK2).
(j) III; at pI (5.97) the carboxyl group is fully negatively charged (deprotonated) and the
amino group is fully positively charged (protonated).
(k) V; both groups are fully deprotonated, with a neutral amino group and a negatively
charged carboxyl group (net charge � �1).
(l) II; the carboxyl group is half ionized at pH � pK1.
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