Opening image: Bacteriorhodopsin.
All the amino acids are found in α-helices, but glycine and proline are uncommon, as they destabilize the α-helix. Glycine is exempt from many steric constraints because it lacks a β carbon. Because glycine residues have more conformational freedom than other residues, glycine favors the unfolded conformation over the helix conformation. Proline, on the other hand, is too rigid. Steric crowding between the 5-membered ring of proline residue in the middle of α-helix and the preceeding residue causes a kink the helix. However, it is worth noting that about half of the kinked α-helices do not have prolines.
For two decades after the crystallographic structure of myoglobin was solved at atomic resolution, proline residues were never seen in the middle of an α-helix. Now that there are over 30,000 protein structures in the Protein Data Bank, it is clear that proline residues are present in α-helices, where they often play important roles in the structure and function of the protein.