The a-helix. 3.2 Secondary structure (continued) We can describe the arrangement of atoms around the peptide link (the conformation) by giving the degree and direction in which the Ca-CO and N-Ca bonds are rotated. When a number of successive peptide links have identical rotations the polypeptide chain takes up a particular secondary structure. There are several types of secondary structure, but we will concentrate on just two: the a-helix and the b-pleated sheet. In both cases you will see how the regular conformation allows the structure to be stabilised by forming many relatively strong hydrogen bonds. The a-helix The a-helix is like a narrow-bore tube.