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Table 2 The amino acids and the terminal groups that influence the pI of a protein with their charge polarity are shown. Four pKr values are tabulated for each of them, as defined by Lehninger [21], Solomon [22], Sillero [22], Rodwell [22]. In this work, Lehninger's values were used.

From: A Perl procedure for protein identification by Peptide Mass Fingerprinting

Amino acid Polarity pKr Lehninger pKr Solomon pKr Sillero pKr Rodwell
Glutamic acid (E) - 4.25 4.30 4.50 4.25
Aspartic acid (D) - 3.65 3.90 4.00 3.68
Cysteine (C) - 8.18 8.30 9.00 8.33
Tyrosine (Y) - 10.07 10.10 10.00 10.07
Histidine (H) + 6.00 6.00 6.40 6.00
Lysine (K) + 10.53 10.50 10.40 11.50
Arginine (R) + 12.48 12.50 12.00 11.50
Carboxyl terminal (COOH) - 3.10 2.40 3.20 3.10
Amino terminal (NH2) + 8.00 9.60 8.20 8.00