α-helix and β-pleated sheet are stable, commonly encountered peptide conformations. Generic proteins naturally assume these conformations in aqueous solution for thermodynamic reasons. These forms maximize the hydrogen bonding that can occur between the N-H group of one peptide and the carbonyl group of another, thus minimizing protein energy. Because a great deal of side-chain repulsion will preclude the β-pleated sheet conformation, β-pleated sheets occur especially in sequences composed of amino acids with small side chains.