The asymmetry of L-amino acids determines the asymmetrical features of α-helices and β-sheets. These in turn determine two principal aspects of the three-dimensional structure of proteins: the preferred ways in which α-helices and β-sheets pack together, and certain topological features of the paths followed by polypeptide chains through structures. Though the asymmetrical nature of amino acids plays the central role in determining the asymmetrical aspects of protein structures, it has little or no influence on the next level of biological structures-assemblies of protein molecules.