Answer:
CH2-Aromatic ring
Explanation:
Amino acids with aromatic R groups are relatively hydrophobic and therefore don't interact with water molecules surrounding the protein. Tryptophan (Trp), phenylalanine (Phe) and tyrosine (Tyr) are amino acids with aromatic R groups. Since these amino acids are relatively nonpolar or hydrophobic, they participate in hydrophobic interactions that are strong when there are several residues with aromatic R groups which are stacked on one another. In this regard, it is also important to highlight that Tyr and Trp are more polar than Phe due to the hydroxyl (OH-) group in Tyr and the nitrogen in the Trp ring.
