The alpha helix and beta pleated sheet forms are alternative configurations of the N - C(R) - C backbone residues. The alpha helix is a right-handed helix, from which the radical (R) groups are directed outward at right angles. The helix is stabilized by H-bonds between non-adjacent residues. The beta sheet consists of a series of molecules running anti-parallel to each other. The radical (R) groups are directed outward at right-angles, alternatively above and below the plane of the sheet: the sheet is stabilized by H-bonds between carboxy and amino groups of adjacent antiparallel molecules. These regular structures are a consequence of the exclusively L forms of the amino acids.