Goals for polypeptides:

• Obtain a low-energy conformation for individual amino acid residues
• Provide hydrogen bonding opportunities for polar groups, including buried ones
• Form a compact, well-packed structure that minimizes contact between non-polar amino acid sidechains and water

• The polypeptide backbone adopts sterically favorable combinations of phi and psi angles.

• The alpha-helices and beta-sheets that form when the same phi/psi combination are repeated are particularly suited to providing hydrogen bonding opportunities for the backbone amide groups.

• Turns and loops are used to join alpha-helices and beta-sheets and to allow a polypeptide to fold into a compact structure.