Common structural elements in proteins such as α-helices or β-sheets are characterized by uniformly repeating, energetically favorable main chain conformations which additionally exhibit a completely saturated hydrogen-bonding network of the main chain NH and CO groups. Although polyproline or polyglycine type II helices (PPII or PGII ) are frequently found in proteins, they are not considered as equivalent secondary structure elements. Which of the following statements are correct about PP and PGs. A. PGII -like helices form hexagonal bundles which appear to fulfill the criterion of a (largely) saturated hydrogen-bonding network of the main-chain groups B. main chain NH and CO groups of the central PGII -helix are saturated by either intra- or intermolecular hydrogen-bonds, resulting in a self-contained hydrogen-bonding network C. The formation of the right-handed PPI helix is possible only with proline residues because of the required cis conformation. D. PPII helices seem to be stabilized by main chain-water hydrogen bonds (in the absence of main chain- main chain H-bonds), and tend to have a regular pattern of hydrogen bonds with water. After this, it is not surprising, that PPII helices are found mostly on the protein surface. -triyambak.org