The Amino Acids

Paul Swartz

 

http://web.indstate.edu/thcme/mwking/amino-acids.html  - pK1, pK2, pKR

http://psyche.uthct.edu/shaun/SBlack/aagrease.html - MW and Hydrophobicity

 

Amino Acid

Abbr./

Symb.

Formula

pK1
(COOH)

pK2
(NH2)

pK R Group

MW†

Hydro-phobicity‡

Aliphatic Sidechain Amino Acids

Glycine

Gly - G

C2NO2H5

2.4

9.8

---------

75.07

0.501

Alanine

Ala - A

C3NO2H7

2.4

9.9

---------

89.09

0.616

Valine

Val - V

C5NO2H11

2.2

9.7

---------

117.15

0.825

Leucine

Leu - L

C6NO2H13

2.3

9.7

 ---------

131.18

0.943

Isoleucine

Ile - I

C6NO2H13

2.3

9.8

 ---------

131.18

0.943

Alcoholic Sidechain Amino Acids

Serine

Ser - S

C3NO3H7

2.2

9.2

~13

105.09

0.359

Threonine

Thr - T

C4NO3H9

2.1

9.1

~13

119.12

0.450

Thiolate Sidechain Amino Acids

Cysteine

Cys - C

C3NO2SH7

1.9

10.8

8.3

121.16

0.680

Methionine

Met-M

C5NO2SH11

2.1

9.3

 ---------

149.21

0.738

Acidic and Acidamide Sidechain Amino Acids

Aspartic Acid

Asp - D

C4NO4H7

2.0

9.9

3.9

133.10

0.028

Asparagine

Asn - N

C4N2O3H8

2.1

8.8

 ---------

132.12

0.236

Glutamic Acid

Glu - E

C5NO4H9

2.1

9.5

4.1

147.13

0.043

Glutamine

Gln - Q

C5N2O3H10

2.2

9.1

 ---------

146.15

0.251

Basic Amino Acids

Arginine

Arg - R

C6N4O2H14

1.8

9.0

12.5

174.20

0.000

Lysine

Lys - K

C6N2O2H14

2.2

9.2

10.8

146.19

0.283

Histidine

His - H

C6N3O2H9

1.8

9.2

6.0

155.16

0.165

Aromatic Sidechain Amino Acids

Phenylalanine

Phe - F

C9NO2H11

2.2

9.2

 ---------

165.19

1.00

Tyrosine

Tyr - Y

C9NO3H11

2.2

9.1

10.1

181.19

0.880

Tryptophan

Trp-W

C11N2O2H11

2.4

9.4

 ---------

204.23

0.878

Imino Acid

Proline

Pro - P

C5NO2H10

2.0

10.6

 ---------

115.13

0.711

†The molecular weights given are those of the neutral, free amino acids; residue weights can be obtained by subtraction of one equivalent of water (18 g/mol).

‡The hydrophobicities given are the "Scaled" values from computational log(P) determinations by the "Small Fragment Approach" (see, "Development of Hydrophobicity Parameters to Analyze Proteins Which Bear Post- or Cotranslational Modifications" Black, S.D. and Mould, D.R. (1991) Anal. Biochem. 193, 72-82). The equation used to scale raw log(P) values to the scaled values given is as follows: Scaled Parameters = (Raw Parameters + 2.061)/4.484 .

Hydrophobicity Trend: 

 

 Phe > Leu = Ile > Tyr = Trp > Val > Met > Pro > Cys > Ala > Gly > Thr > Ser > Lys > Gln > Asn > His > Glu > Asp > Arg

 

 

 

Acid-Base Properties of the Amino Acids

(From: http://web.indstate.edu/thcme/mwking/amino-acids.html) 

Amino acids are ionizable and titrable with the following reversible reactions defining the electronic state of the amino acid at a particular pH.

 

The preceeding reactions are for the weakly acidic and basic groups that are found in biologically relevant amino acids and free amino acids are zwitterionic at neutral pH.  The isoelectic pH is the pH at which the net charge on the amino acid is zero – or in the zwitterion form.  The following figure shows the titration of a typical amino acid indicating the multiple pKs and the ionic state of the amino acid.