Amino Acids - structures & Pka values

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Last updated 9:05 PM on 9/13/25

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20 Terms

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Alanine (Ala/A)

Nonpolar
pKa1 (carboxylic acid group): 2.34
pKa2 (amino group): 9.69
pKaR (side chain): not applicable, has a simple aliphatic side chain.

<ul><li><p>Nonpolar</p></li><li><p>pKa1 (carboxylic acid group): 2.34</p></li><li><p>pKa2 (amino group): 9.69</p></li><li><p>pKaR (side chain): not applicable, has a simple aliphatic side chain.</p></li></ul><p></p>

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Arginine (Arg/R)

+
pKa1 (carboxylic acid group): 2.17
pKa2 (amino group): 9.04
pKaR (guanidinium group): 12.48

<ul><li><p>+</p></li><li><p>pKa1 (carboxylic acid group): 2.17</p></li><li><p>pKa2 (amino group): 9.04</p></li><li><p>pKaR (guanidinium group): 12.48</p></li></ul><p></p>

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Asparagine (Asn/N)

Polar uncharged
pKa1 (carboxylic acid group): 2.02
pKa2 (amino group): 8.80
pKaR (side chain): not applicable, has a simple amide side chain.

<ul><li><p>Polar uncharged</p></li><li><p>pKa1 (carboxylic acid group): 2.02</p></li><li><p>pKa2 (amino group): 8.80</p></li><li><p>pKaR (side chain): not applicable, has a simple amide side chain.</p></li></ul><p></p>

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Aspartic Acid (Asp/D)

-
pKa1 (carboxylic acid group): 2.09
pKa2 (amino group): 9.82
pKaR (side chain (β-COOH)): 3.86

<ul><li><p>-</p></li><li><p>pKa1 (carboxylic acid group): 2.09</p></li><li><p>pKa2 (amino group): 9.82</p></li><li><p>pKaR (side chain (β-COOH)): 3.86</p></li></ul><p></p>

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Cysteine (Cys/C)

Polar uncharged
pKa1 (carboxylic acid group): 1.71
pKa2 (amino group): 10.78
pKaR (side chain, thiol): 8.33

<ul><li><p>Polar uncharged</p></li><li><p>pKa1 (carboxylic acid group): 1.71</p></li><li><p>pKa2 (amino group): 10.78</p></li><li><p>pKaR (side chain, thiol): 8.33</p></li></ul><p></p>

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Glutamic Acid (Glu/E)

-
pKa1 (carboxylic acid group): 2.19
pKa2 (amino group): 9.67
pKaR (side chain(γ-COOH)): 4.25

<ul><li><p>-</p></li><li><p>pKa1 (carboxylic acid group): 2.19</p></li><li><p>pKa2 (amino group): 9.67</p></li><li><p>pKaR (side chain(γ-COOH)): 4.25</p></li></ul><p></p>

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Glutamine (Gln/Q)

Polar uncharged
pKa1 (carboxylic acid group): 2.17
pKa2 (amino group): 9.13
pKaR (side chain, amide): none

<ul><li><p>Polar uncharged</p></li><li><p>pKa1 (carboxylic acid group): 2.17</p></li><li><p>pKa2 (amino group): 9.13</p></li><li><p>pKaR (side chain, amide): none</p></li></ul><p></p>

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Glycine (Gly/G)

Nonpolar
pKa1 (carboxylic acid group): 2.36
pKa2 (amino group): 9.78
pKaR (side chain(Hydrogen)): none

<ul><li><p>Nonpolar</p></li><li><p>pKa1 (carboxylic acid group): 2.36</p></li><li><p>pKa2 (amino group): 9.78</p></li><li><p>pKaR (side chain(Hydrogen)): none</p></li></ul><p></p>

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Histidine (His/H)

+
pKa1 (carboxylic acid group): 1.82
pKa2 (amino group): 9.17
pKaR (side chain, imidazole): 6.00

<ul><li><p>+</p></li><li><p>pKa1 (carboxylic acid group): 1.82</p></li><li><p>pKa2 (amino group): 9.17</p></li><li><p>pKaR (side chain, imidazole): 6.00</p></li></ul><p></p>

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Isoleucine (Ile/i)

Nonpolar
pKa1 (carboxylic acid group): 2.36
pKa2 (amino group): 9.76
pKaR (side chain, sec-butyl): none

<ul><li><p>Nonpolar</p></li><li><p>pKa1 (carboxylic acid group): 2.36</p></li><li><p>pKa2 (amino group): 9.76</p></li><li><p>pKaR (side chain, sec-butyl): none</p></li></ul><p></p>

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Leucine (Leu/L)

Nonpolar
pKa1 (carboxylic acid group): 2.36
pKa2 (amino group): 9.60
pKaR (side chain, isobutyl): none

<ul><li><p>Nonpolar</p></li><li><p>pKa1 (carboxylic acid group): 2.36</p></li><li><p>pKa2 (amino group): 9.60</p></li><li><p>pKaR (side chain, isobutyl): none</p></li></ul><p></p>

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Lysine (Lys/K)

+
pKa1 (carboxylic acid group): 2.18

pKa2 (amino group): 8.95
pKaR (side chain, ϵϵ-amino group): 10.79

<ul><li><p>+</p></li><li><p>pKa1 (carboxylic acid group): 2.18</p></li></ul><ul><li><p>pKa2 (amino group): 8.95</p></li><li><p>pKaR (side chain, ϵ<em>ϵ</em>-amino group): 10.79</p></li></ul><p></p>

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Methionine (Met/M)

Nonpolar
pKa1 (carboxylic acid group): 2.28

pKa2 (amino group): 9.21
pKaR (side chain): N/A, as its thioether side chain is non-ionizable within physiological pH ranges.

<ul><li><p>Nonpolar<br>pKa1 (carboxylic acid group): 2.28</p></li></ul><ul><li><p>pKa2 (amino group): 9.21</p></li><li><p>pKaR (side chain): N/A, as its thioether side chain is non-ionizable within physiological pH ranges.</p></li></ul><p></p>

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Phenylalanine (Phe/F)

Aromatic
pKa1 (carboxylic acid group): 1.83

pKa2 (amino group): 9.13
pKaR (side chain, benzyl group): N/A, as its side chain is non-ionizable within physiological pH ranges.

<ul><li><p>Aromatic</p></li><li><p>pKa1 (carboxylic acid group): 1.83</p></li></ul><ul><li><p>pKa2 (amino group): 9.13</p></li><li><p>pKaR (side chain, benzyl group): N/A, as its side chain is non-ionizable within physiological pH ranges.</p></li></ul><p></p>

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Proline (Pro/P)

Nonpolar
pKa1 (carboxylic acid group): 1.99

pKa2 (amino group): 10.60
pKaR (side chain): N/A, as its cyclic structure incorporates the amino group and lacks an additional ionizable side chain.

<ul><li><p>Nonpolar</p></li><li><p>pKa1 (carboxylic acid group): 1.99</p></li></ul><ul><li><p>pKa2 (amino group): 10.60</p></li><li><p>pKaR (side chain): N/A, as its cyclic structure incorporates the amino group and lacks an additional ionizable side chain.</p></li></ul><p></p>

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Serine (Ser/S)

Polar uncharged
pKa1 (carboxylic acid group): 2.21
pKa2 (amino group): 9.15
pKaR (side chain, hydroxyl group): N/A, as the hydroxyl group typically does not ionize within physiological pH ranges. It is considered a polar, uncharged side chain.

<ul><li><p>Polar uncharged</p></li><li><p>pKa1 (carboxylic acid group): 2.21</p></li><li><p>pKa2 (amino group): 9.15</p></li><li><p>pKaR (side chain, hydroxyl group): N/A, as the hydroxyl group typically does not ionize within physiological pH ranges. It is considered a polar, uncharged side chain.</p></li></ul><p></p>

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Threonine (Thr/T)

Polar uncharged
pKa1 (carboxylic acid group): 2.1

pKa2 (amino group): 9.1
pKaR (side chain, hydroxyl group): N/A, as its hydroxyl group typically does not ionize within physiological pH ranges. It is considered a polar, uncharged side chain.

<ul><li><p>Polar uncharged</p></li><li><p>pKa1 (carboxylic acid group): 2.1</p></li></ul><ul><li><p>pKa2 (amino group): 9.1</p></li><li><p>pKaR (side chain, hydroxyl group): N/A, as its hydroxyl group typically does not ionize within physiological pH ranges. It is considered a polar, uncharged side chain.</p></li></ul><p></p>

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Tryptophan (Trp/W)

Aromatic
pKa1 (carboxylic acid group): 2.38

pKa2 (amino group): 9.39
pKaR (side chain, indole N-H): not applicable, as its indole side chain is generally non-ionizable within physiological pH ranges.