Glycine (Gly, G), with just a hydrogen atom as its sidechain, is the simplest of the 20 amino acids. It can be grouped with the hydrophobic amino acids or treated as the sole member of a fourth class because of its special properties. Th ese arise from the absence of a bulky sidechain, which allows glycine to adopt three- dimensional conformations that are energetically unfavorable for other amino acids, so that segments of protein chains that contain glycine can be much more fl exible than those that do not.