| The side chains of lysine and arginine are fully protonated at neutral pH and, therefore, positively charged. Lysine contains a primary amino group (NH2) attached to the terminal ε-carbon of the side chain. The ε-amino group of lysine has a pKa ≈11. Arginine is the most basic amino acid (pKa ≈13), and its guanidine group exists as a protonated guanidinium ion at pH 7.0.
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| Histidine (pKa ≈6) has an imidazole ring as the side chain and functions as a general acid-base catalyst in many enzymes. The protonated form of imidazole is called an imidazolium ion.
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