Final answer:
Beta-sheets have both polar and nonpolar sides, formed by hydrogen bonds between the backbone of polypeptide chains and the side chains that extend from them. The ionic self-complementary peptides highlight this by forming ß-sheet structures with distinct hydrophobic and hydrophilic surfaces in water.
Step-by-step explanation:
Beta-sheets, or ß-pleated sheets, are indeed characterized by having both polar and nonpolar sides. These structures are formed by the alignment of polypeptide chains in either parallel or antiparallel fashion, where the hydrogen bonds between the carbonyl groups and amine groups on the polypeptide chain's backbone create the characteristic pleat appearance. The R groups, or side chains, extend from the polypeptide backbone above and below the sheet, which leads to the formation of distinct hydrophobic and hydrophilic surfaces on the beta-sheet. Namely, ionic self-complementary peptides such as RADA16-I, RAD16-II, EAK-I, and EAK16-II demonstrate this concept by forming ß-sheet structures in water that possess hydrophobic and hydrophilic sides, allowing them to assemble into nanofiber scaffolds with applications such as high-water-content hydrogels.
The hydrophobic side of a ß-sheet typically consists of nonpolar amino acid residues and often plays a role in the protein's aggregation or interaction with other nonpolar molecules. On the contrary, the hydrophilic side is lined with polar or charged amino acid residues, making it suitable for interaction with the aqueous environment. This segregation of hydrophobic and hydrophilic sides is essential for the protein's function and its three-dimensional structure.