The secondary protein structure, including alpha-helix and beta-sheet conformations, is stabilized primarily by hydrogen bonding between the backbone amide and carbonyl groups of the polypeptide chain. These interactions occur between the amino acid residues that are close in sequence but not necessarily close in space.In an alpha-helix conformation, the hydrogen bonding occurs between the carbonyl oxygen C=O of one amino acid residue and the amide hydrogen N-H of another residue located four positions ahead in the polypeptide chain. This pattern of hydrogen bonding creates a right-handed helical structure, with 3.6 amino acid residues per turn.In a beta-sheet conformation, the hydrogen bonding occurs between the carbonyl oxygen C=O of one amino acid residue and the amide hydrogen N-H of another residue in an adjacent polypeptide chain or strand. The strands can be arranged in a parallel or antiparallel fashion. In parallel beta-sheets, the N-terminus to C-terminus direction of the adjacent strands runs in the same direction, while in antiparallel beta-sheets, the direction is opposite. The hydrogen bonding pattern in beta-sheets creates a pleated, sheet-like structure.In addition to hydrogen bonding, other forces such as van der Waals interactions, electrostatic interactions, and hydrophobic effects can also contribute to the stability of the secondary protein structure, but hydrogen bonding is the primary stabilizing force.