Amino acids are classified according to the charge of their residue:
Hydrophobic amino acids
Gly, Ala, Val, Leu, Ile, Met, Phe, Trp, Pro
Neutral amino acids (non-charged but polar)
No ionisable groups in the residue
Ser, Thr, Cys, Asn, Gln, Tyr
Acidic amino acids (negatively charged at a physiological pH)
–COOH group in the residue, ionised into –COO- at a cellular pH
Asp, Glu