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Amino Acids
C483 Spring 2013
Questions
1. Amino acids with non‐ionizable side chains are zwitterions when they are ________.
A) in any solution D) in alkaline solutions only B) at physiological pH, pH = 7.4 E) All of the above C) in acidic solutions only 2. Tyrosine and tryptophan are less hydrophobic than phenylalanine because A) phenylalanine is a phenol.
B) phenylalanine has no polar group in the side chain.
C) phenylalanine has an indole group. D) tyrosine and tryptophan have smaller R groups.
E) All of the above. 3. ) Basic amino acids are ________ (positive, negative) at pH 7 and acidic R group amino acids are ________ (positive, negative) at pH 7. A) negative; positive C) positive; negative B) negative; negative D) positive; positive 1
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4. A polypeptide chain may have abrupt changes in direction and restriction in geometry because of the presence of
A) arginine.
B) glycine.
C) proline.
D) leucine or isoleucine.
5. The pKa's of arginine's α‐Carboxyl group, α‐Amino group and side chain are 1.8, 9.0 and 12.5, respectively. Calculate the isoelectric point. A) 7.8 B) 7.2 C) 10.8 D) 5.4 6. What is the net charge on the dipeptide Arg‐Pro at pH 9.0? The table below gives the pKa's of the ionizable groups on the free amino acids.
A) ‐1 B) ‐0.5 C) 0
pKa of
pKa of
D) +0.5 E) +1
Free amino acid α‐Carboxyl group α‐amino group
Arginine
Proline
1.8
2.0
9.0
10.6
pKa of side
chain
12.5
‐‐‐
Amino Acid Structure
•
•
•
•
Alpha carbon
Sidechain
Proteins
peptides
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Stereochemisty
• L‐amino acids
– Glycine
– R/S vs D/L
– L‐isoleucine
• racemization
Common Amino Acids
• 20 Common Amino acids
• Groups
– Aliphatic
– Aromatic
– Sulfur
– Hydroxyl
– Positively charged
– Negatively charged
• Hydrophobicity
3
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Aliphatic
• Sidechain
stereochem
• Secondary amine
• “Highly” hydrophobic in blue
Aromatic
4
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Sulfur Containing
• Disulfide bonds
Hydroxyl/Amide Containing
• Stereochem
• H‐bonding
• Highly hydrophilic in red
5
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Charged
Hydrophobicity
• Hydrophobic effect
• Relative scales
6
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Other Amino Acids
• The “other three”
• Modified Amino acids
– Oxidation
– Phosphorylation
– amination
Amino Acid Derivatives
• From which amino acids are these molecules derived? 7
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Ionization of Amino Acids
• Polyprotic acids
• Alanine
– pKa1 = 2.4
– pKa2 = 9.9
• Zwitterion
• More acidic
than typical
carboxylic acid
Titration Curve
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Titration Curve
Which amino acid?
Ionization States Of Amino Acids
G
F
E
D
B
C
A
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Answers
1.
2.
3.
4.
5.
6.
B
B
C
C
C
D
10