Final answer:
The sequence of polar and nonpolar side chains affects protein folding mainly because nonpolar side chains repel water and tend to face the interior of the protein.
Step-by-step explanation:
Proteins fold into their three-dimensional structures due to a variety of interactions between amino acid side chains. The sequence of polar and nonpolar side chains plays a strong role in protein folding because nonpolar side chains are hydrophobic and tend to repel water. As a result, nonpolar side chains typically face the interior of the protein, away from water, while polar side chains face the exterior and can form interactions with other polar side chains through hydrogen bonding or ionic bonding.