If the pH of the surrounding solution environment is below the pKa of a functional group the predominate form will be the acidic form of the group. The Henderson-Hasselbalch equation serves as good conceptual shorthand for this relationship.

The acidic form of an amine group is the ammonium cation, so if the pH is below the pKa of the amine, it will be in the ammonium form.

Lysine and arginine are the amino acids with amine side chains. Because the typical pKa of a lysine side chain is 10.0 and that of an arginine side chain is 12.0, if a protein has a high percentage of lysine or arginine, at neutral pH, the protein will be positively charged. This is a favorite concept for the MCAT.