O of the backbone. O groups must be oriented in one directions, and all of the NH groups in the other. The R-groups are held on the outside of the helix. These long helical structures are interrupted by having the amino acid proline in the peptide chain. The nitrogen in proline has no H bonded to it, so it can't serve as a H-bond donor.
Proline tends to terminate -helices.
Similarly, side chains with similar charges could repel each other, preventing the formation of a stable -helix. O and NH groups on the backbone, but the groups bonded together are not close to each other in the chain sequence . They may even be on different chains. This is called anti-parallel. If neither of these 2 o structures exists, the secondary structure is called random coil .