All the amino acid in human proteins are L-amino acids. Glycine does not have a chiral carbon. So, there is no D or L-form for glycine. Because of this flexibility, Glycine can form left-handed helix like collagen helix. (Note: Collagen helix is NOT an alpha helix). A "chiral" molecule is one that is not superimposable with its mirror image. The only amino acid that does not have four different groups attached to the alpha carbon is glycine. So, it lacks chirality. The image shows Chirality: