arginine is basic amino acid because it's side chain group contains a full positive charge at the physiological pH. Histidine is also considered basic but it can have a positive or a neutral charge on its side chain group at the physiological pH
There are three amino acids that have basic side chains at neutral pH. These are arginine (Arg), lysine (Lys), and histidine (His). Their side chains contain nitrogen and resemble ammonia, which is a base. Their pKa's are high enough that they tend to bind protons, gaining a positive charge in the process.
