Unsubstitute (Example: Gly, Ala, Val, Leu, Ile, and Leu)

Heterocyclic (Example: Pro)

Aromatic (Example: Phe, Tyr, and Trp)

Thioether (Example: Met)

Hydroxy (Example: Ser and Thr)

Mercapto (Example: Cys)

Carboxamide (Example: Asn and Gln)