Common Amino Acids The 20 amino acids that are the building blocks of most proteins can be classified as: (a)nonpolar (hydrophobic), (b)polar,neutral, (c)acidic,or (d)basic. Common Amino Acids The 20 amino acids that are the building blocks of most proteins can be classified as: (a) nonpolar (hydrophobic), (b) polar, neutral, (c) acidic, or (d) basic