Protein structure
Secondary structure is the most simple structure made by proteins' polypeptide chains. There are two main types of secondary structure: alpha helix and beta strand (beta sheet). Alpha helices are coils that turn in a clockwise rotation. Beta strands are chains of amino acids that lie beside each other to make a beta sheet. These two types of secondary structure were suggested in 1951 by Linus Pauling and coworkers. Unlike the polypeptide chain, which is bonded through covalent peptide bonds, secondary structure is formed by hydrogen bonds.