Amino acids are the building blocks of peptides and proteins, and while they all have common elements of an amine group, a carboxyl group and a side chain, the various functional groups that comprise the side chain give each amino acid distinct physical properties that influence protein formation and function. Understanding these physical properties, including charge, solubility and pKa, aid in designing peptide sequences that are optimized for high synthesis yield and purity. To simplify the handling of peptide sequences, each amino acid has a designated single-letter code. The table below includes the single-letter codes and physical properties of the 20 standard amino acids.

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Table of amino acid physical properties

Amino Acid Code Hydropathy Charge pKa, NH2 pKa, COOH pK(R) Solubility
Arginine R hydrophilic + 9.09 2.18 13.2 71.8
Asparagine N hydrophilic N 8.8 2.02   2.4
Aspartate D hydrophilic - 9.6 1.88 3.65 0.42
Glutamate E hydrophilic - 9.67 2.19 4.25 0.72
Glutamine Q hydrophilic N 9.13 2.17   2.6
Lysine K hydrophilic + 10.28 8.9 2.2  
Serine S hydrophilic N 9.15 2.21   36.2
Threonine T hydrophilic N 9.12 2.15   freely
Cysteine C moderate N 10.78 1.71 8.33 freely
Histidine H moderate + 8.97 1.78 6 4.19
Methionine M moderate N 9.21 2.28   5.14
Alanine A hydrophobic N 9.87 2.35   15.8
Valine V hydrophobic N 9.72 2.29   5.6
Glycine G hydrophobic N 9.6 2.34   22.5
Isoleucine I hydrophobic N 9.76 2.32   3.36
Leucine L hydrophobic N 9.6 2.36   2.37
Phenylalanine F hydrophobic N 9.24 2.58   2.7
Proline P hydrophobic N 10.6 1.99   1.54
Tryptophan W hydrophobic N 9.39 2.38   1.06
Tyrosine Y hydrophobic N 9.11 2.2 10.1 0.038

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Recommended reading

  1. Lloyd-Williams P. et al. (1997) Chemical approaches to the synthesis of peptides and proteins. Boca Raton: CRC Press. 278
  2. Merrifield R. B. (1963) Solid phase peptide synthesis. I. The synthesis of a tetrapeptide. Journal of the American Chemical Society. 85, 2149-54.
  3. Carpino L. A. (1957) Oxidative reactions of hydrazines. Iv. Elimination of nitrogen from 1, 1-disubstituted-2-arenesulfonhydrazides1-4. Journal of the American Chemical Society. 79, 4427-31.
  4. McKay F. C. and Albertson N. F. (1957) New amine-masking groups for peptide synthesis. Journal of the American Chemical Society. 79, 4686-90.
  5. Anderson G. W. and McGregor A. C. (1957) T-butyloxycarbonylamino acids and their use in peptide synthesis. Journal of the American Chemical Society. 79, 6180-3.
  6. Carpino L. A. and Han G. Y. (1972) 9-fluorenylmethoxycarbonyl amino-protecting group. The Journal of Organic Chemistry. 37, 3404-9.