In biochemistry,  protein secondary structure describes common three-dimensional structural motifs found in proteins. Secondary structure is determined by inter-residue interactions mediated by hydrogen bonding occurring between backbone amide nitrogen and carbonyl groups of a polypeptide. The most common secondary structural motifs are the  alpha helices and beta sheets. Tight turns and flexible loops link the regular secondary structural elements.

Amino acids in a polypeptide vary in their tendency to form secondary structure elements. Proline and glycine are sometimes called “helix breakers” because they tend to disrupt the regularity of α-helical backbone conformations; however, both of these amino acids have unusual conformational abilities and are often found in turns. Amino acids that prefer to adopt helical conformations in proteins include methionine, alanine, leucine, glutamic acid, and lysine. In contrast, large aromatic amino acids such as tryptophan, tyrosine,...