Amino Acids: Structure, Polarity, Charge, & Properties

Front 1

GLY

Back 1

Glycine, G
Nonpolar, aliphatic

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ALA

Back 2

Alanine, A
Nonpolar, aliphatic

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VAL

Back 3

Valine, V
Nonpolar, aliphatic

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LEU

Back 4

Leucine, L
Nonpolar, aliphatic

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MET

Back 5

Methionine, M
Nonpolar, aliphatic

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ILE

Back 6

Isoleucine, I
Nonpolar, aliphatic

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PHE

Back 7

Phenylalanine, F
Aromatic

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TYR

Back 8

Tyrosine, Y
Aromatic, polar side group

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TRP

Back 9

Tryptophan, W
Aromatic

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SER

Back 10

Serine, S
Polar, uncharged

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THR

Back 11

Threonine, T
Polar, uncharged

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CYS

Back 12

Cysteine, C
Polar, uncharged

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PRO

Back 13

Proline, P
Polar, uncharged

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ASN

Back 14

Asparagine, N
Polar, uncharged

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GLN

Back 15

Glutamine, Q
Polar, uncharged

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ASP

Back 16

Aspartic Acid, D
Polar, negatively charged

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GLU

Back 17

Glutamic Acid, E
Polar, negatively charged

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LYS

Back 18

Lysine, K
Polar, positively charged

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ARG

Back 19

Arginine, R
Polar, positively charged

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HIS

Back 20

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

Front 22

Eg: Stereochemistry of Alanine (Fischer Proj)

Back 22

Front 23

Eg: Stereochemistry of Alanine (Perspective View)

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Front 24

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.

Front 26

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.

Front 27

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

Back 28

Front 29

Acidity: α-COOH groups

Back 29

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.

Front 30

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.

Front 31

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

Front 33

Isoelectric point

Back 33

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

Front 34

Isoelectric point: For amino acid without ionizable side chains, the isoelectric point (equivalence point, pI) is

Back 34

Front 35

Each individual amino acid has a characteristic pI

Back 35

Front 36

pKa of amino acids I.

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Front 37

pKa of amino acids II

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Front 38

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).

Back 38

Front 39

Amino acids with two chiral centers

Back 39

Front 40

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

Back 41

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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Front 43

Synthesis of amino acids II

Back 43

Front 44

Reactions of amino acids

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Front 45

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