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Solutions for Amino Acids, Peptides, and Proteins
–1
7 At pH 9.7, alanine (isoelectric point (pI) 6.0) has a charge of –1 and will migrate to the anode. 
Lysine (pI 9.7) is at its isoelectric point and will not move. Aspartic acid (pI 2.8) has a charge of –2 and 
will also migrate to the anode, faster than alanine.
–2
cathode anode
lysine aspartic acid
0
alanine
H3N C
CH2S
COO
H
H3N C
CH2
COO
N
H
H
C COO
CH2
H3N
N
NH
H
H
(The SH is more acidic 
than the NH3
+ group.)
8 At pH 6.0, tryptophan (pI 5.9) has a charge of zero and will not migrate. Cysteine (pI 5.0) has a 
partial negative charge and will move toward the anode. Histidine (pI 7.6) has a partial positive charge and 
will move toward the cathode.
cysteinehistidine tryptophan
cathode
histidine
(partially 
protonated
imidazole 
ring) tryptophan
cysteine (partially
deprotonated sulfur)
anode
C COOH
CH3
NH3
C
CH2CH(CH3)2
COOH
C COOH
CH2OH
C
CH2CH2CONH2
COOH
NH3
NH3
NH3
H2N CH
CH3
COOH
H2N CH
CH2OH
COOH
CH COOH
CH2CH(CH3)2
H2N
CH COOH
CH2CH2CONH2
H2N
9
(a)
H2, Pd
H2, Pd
(b)
(c)
H2, Pd
H2, Pd
(d)
O
O
O
O
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