二級結構
Secondary structure of proteins consists of interactions between the atoms of the backbone of a polypeptide chain, excluding the R-groups.  One common type of secondary structures is the alpha helix, which is a right-handed helix that results from hydrogen bonds between the "nth" amino acid residue and the "n+4th" amino acid residue.  The other common type of secondary structure is the beta sheet, which displays a right-handed twist, can be parallel or anti-parallel depending on the direction of the direction of the bonded polypeptides, and consists of hydrogen bonds between the carbonyl and amino groups of the backbone of two polypeptide chains.