The Hp-π donors and acceptors of natural amino acids. In the 20 natural amino acids 4 of them possess aromatic side chains (Phe, Tyr, Trp, and His), which are the possible Hp-π interaction acceptors. On the other hand, 10 amino acids (Ser, Thr, Asn, Gln, Cys, Tyr, Trp, His, Lys, and Arg) possess various Hp-π donors. The atomic group –OH is the Hp-π donor in Ser, Thr, and Tyr. The atomic group > NH or –NH2 is the Hp-π donor of Asn, Gln, Lys, Arg, Trp, and His. Amino acid Cys has the donor –SH. Three amino acids (Tyr, Trp, and His) play the roles of both Hp-π donor and acceptor. In proteins total 11 amino acids may be involved in the Hp-π interactions. The Hp-π donors are indicated using blue cycles, and the Hp-π acceptors are indicated using red cycles.