Final answer:
Polypeptide chains are cross-linked together through inter-chain hydrogen bonds and salt bridges.
Step-by-step explanation:
Polypeptide chains are cross-linked together through different types of side bonds. One type of side bond is the inter-chain hydrogen bond, which forms between the chains and involves each peptide group participating in hydrogen bonding. Another type of side bond is the salt bridge, which links together the alpha (α) chains and the beta (ß) chains.