CHEM 109C – Zhang
Amino Acids I: Structure and Properties - KEY
CLAS, F10
1. What is the absolute configuration (R or S) at the -carbon in each of the
enantiomers of Alanine? Are they D or L amino acids?
O
H2N
HO
O
H2N
H
O
H3C
OH
CH3
OH
NH2
=
CH3
=
(S)-Ala
(L)-Ala (naturally occurring)
O
H2N
HO
O
H3C
OH
CH3
H
OH
NH2
=
O
CH3
=
(R)-Ala
NH2
(D)-Ala
2. Which isomer (R)-Asp or (S)-Asp is D-Asp?
[Asp = Aspartic Acid, Aspartate]
HO
O
H
O
O
HOOCH 2C
NH2
CH 2COOH =
OH
NH2
(D)-Asp
H2N
or
(R)-Asp
OH
CH 2COOH
3. Name an amino acid besides threonine = Thr that has more than one asymmetric
center.
Isoleucine = Ile
CH3
O
H3C
OH
NH2
Ile
the - and -carbons are stereocenters.
4. Lysine has the following pKa’s -COOH = 2.18, -NH3+ = 8.95, side chain, R =
10.79. Draw principle species at each of the following solution pH’s: 0, 4, 10,
and 13.
Lysine = Lys
O
+
H3N
pKa = 10.79
OH
pKa = 2.18
+
NH3
pKa = 8.95
@ pH = 0 pH < all pKa’s so everything is in acidic form
Page 1 of 3
CHEM 109C – Zhang
Amino Acids I: Structure and Properties - KEY
CLAS, F10
O
+
H3N
OH
+
NH3
@ pH = 4 pH < pKa’s of side chain and -NH3+ so these are in acidic form,
but it is > pKa or -COOH, so that is in basic form
O
+
H3N
-
O
+
NH3
@ pH = 10 < pKa of side chain only, so it is in acid form, others are in basic
form
O
+
H3N
-
O
NH2
@ pH = 13 > all pKa’s, so everything is in basic form
O
H2N
-
O
NH2
5. Calculate the pI of
a. Lys
From the structure in #4 we see that there are two NH3+ groups that
are similar…
pI = 10.79 + 8.95 = 9.87
2
b. Asp
From the structure in #2 we see that there are two COOH groups that
are similar…
pI = 2.09 + 3.86 = 2.98, actually the lowest pI
2
6. Which amino acid has the highest pI value?
Highest pI → largest average between pKa’s → look for amino acid with
largest pKa’s…
Arginine = Arg
pI = 9.04 + 12.48 = 10.76
2
7. Which amino acid has the greatest amount of negative charge @ pH = 6.20?
Greatest amount of negative charge → most -COOH deprotonated →
lowest -COOH pKa, most acidic amino acids
Page 2 of 3
CHEM 109C – Zhang
Amino Acids I: Structure and Properties - KEY
CLAS, F10
O
O
-
O
O
O
O
+
NH3
-
Asp at pH 6.20, pI = 2.98 → lower pI, so more negative
O
-
O
-
+
NH3
Glu at pH 6.20, pI = 3.22
Page 3 of 3