Why are lysine and arginine positively charged at physiological pH?

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Why our bodies need proteins — Amino acid The structure and function of amino acids.

Lysine and arginine are classified as basic amino acids because their side chains are capable of accepting a proton.1

At physiological pH (approximately 7.0–7.4), the side chains of these amino acids remain in a protonated state, which results in them carrying an overall positive charge of +1. This property is a consequence of the high pKa values associated with their basic side groups—for instance, the guanidino group in arginine has a pKa of 12.5. Because the environmental pH is significantly lower than the pKa of these side chains, they remain protonated and positively charged in biological systems.1

These charged side chains are hydrophilic and often participate in the formation of ionic bonds within proteins.1