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45 Cards in this Set
- Front
- Back
GLY
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Glycine, G
Nonpolar, aliphatic |
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ALA
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Alanine, A
Nonpolar, aliphatic |
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VAL
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Valine, V
Nonpolar, aliphatic |
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LEU
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Leucine, L
Nonpolar, aliphatic |
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MET
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Methionine, M
Nonpolar, aliphatic |
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ILE
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Isoleucine, I
Nonpolar, aliphatic |
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PHE
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Phenylalanine, F
Aromatic |
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TYR
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Tyrosine, Y
Aromatic, polar side group |
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TRP
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Tryptophan, W
Aromatic |
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SER
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Serine, S
Polar, uncharged |
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THR
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Threonine, T
Polar, uncharged |
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CYS
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Cysteine, C
Polar, uncharged |
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PRO
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Proline, P
Polar, uncharged |
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ASN
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Asparagine, N
Polar, uncharged |
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GLN
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Glutamine, Q
Polar, uncharged |
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ASP
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Aspartic Acid, D
Polar, negatively charged |
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GLU
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Glutamic Acid, E
Polar, negatively charged |
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LYS
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Lysine, K
Polar, positively charged |
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ARG
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Arginine, R
Polar, positively charged |
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HIS
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Histidine, H
Polar, positively charged |
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Three-dimensional structure of amino acids
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The amino acids are all chiral with the exception of glycine, whose side chain is H. As with lipids, biochemists use the L and D nomenclature
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Eg: Stereochemistry of Alanine (Fischer Proj)
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Eg: Stereochemistry of Alanine (Perspective View)
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α-Amino acids: Properties
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1. Crystalline solids which melt with decomposition at fairly high temperatures.
2. Insoluble in nonpolar solvents and appreciably soluble in water |
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Explanation of properties
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Intramolecular acid–base reaction: the H from the COOH group is transferred to the –NH2 group.
A neutral dipolar ion, an ion that has one (+) charge and one (-) charge, forms. Neutral dipolar ions are known as zwitterions. |
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Zwitterions or dipolar ions
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A zwitterion (dipolar ion) is a compound with no overall electrical charge, but contains separate parts which are positively and negatively charged.
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Zwitterions or dipolar ions (cont)
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Amino acids exist in this form even in the solid state
--> Ionic attractions form between the molecules --> High melting points. |
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Amino acids are amphiprotic
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Acidity: α-COOH groups
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The average pKa of an α-carboxyl group is 2.19, which makes them considerably stronger acids than acetic acid (pKa 4.78).
The greater acidity is accounted for by the electron- withdrawing inductive effect of the adjacent –NH2 group. |
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Basicity: α-NH2 groups
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The average value of pKb for an α-NH3+ group is 9.47, which
makes them less basic than a primary alkyl ammonium ion. |
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Acidity: side chain -COOH
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Due to the electron-withdrawing inductive effect of the α- NH2 group, side chain -COOH groups are also stronger than acetic acid.
– The effect decreases with distance from the α-NH2 group. Compare: α-COOH group of alanine (pKa 2.35) β-COOH group of aspartic acid (pKa 3.86) γ-COOH group of glutamic acid (pKa 4.07) |
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Acid-base properties of amino acids
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In aqueous solution an equilibrium exists between the dipolar ion, the cationic and the anionic forms of the amino acid
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Isoelectric point
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At some intermediate pH called the pI (isoelectric point), the concentration of the dipolar ion is at a maximum and the concentrations of anionic and cationic forms are equal. At this pH the net charge is zero; amino acid is least soluble in water and does not migrate in electric field
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Isoelectric point: For amino acid without ionizable side chains, the isoelectric point (equivalence point, pI) is
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Each individual amino acid has a characteristic pI
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pKa of amino acids I.
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pKa of amino acids II
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D-amino acids
D-amino acids are found in a few small peptides, including some peptides of bacterial cell walls and certain antibiotics (such as penicillin). |
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Amino acids with two chiral centers
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Chemical resolution of amino acids
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• The amino group is acetylated.
• Diastereomers are formed with a base (e.g. brucine). • Diastereomers are separated and the enantiomers are regenerated from them. |
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Enzymatic resolution
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Enzymes selectively catalyze the hydrolysis of an acetyl group from the amino group of an L-amino acid.
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Synthesis of amino acids I
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Synthesis of amino acids II
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Reactions of amino acids
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Reactions of amino group
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1. Saltformation
2. Alkylation 3. Arylation 4. Acylation 5. Reaction w/ nitrous acid 6. Schiff’s base formation 7. Reaction w/ ninhydrin 8. Transamination |