Not seen in alpha helix
a-Helix: A peptide chain forms regular helical coils called a-helix. These coils are stabilized by hydrogen bonds between carbonyl O of 1st amino and amide N of 4th amino acid residues. Thus in a-helix intrachain hydrogen bonding is present. The a-helices can be either right handed or left handed. Left-handed a-helix is less stable than right-handed a-helix because of the steric interference between the C = O and the side chains. Only the right-handed a-helix has been found in protein structure. Each amino acid residue advances by 0.15 nm along the helix, and 3.6 amino acid residues are present in one complete turn. The distance between two equivalent points on turn is 0.54 nm and is called a pitch. Small or uncharged amino acid residues such as alanine, leucine, and phenylalanine are often found in a-helix. More polar residues such as arginine, glutamate, and serine may repel and destabilize a-helix. Proline is never found in a aa aa-helix. The proteins of hair, nail, skin contain a group of proteins called keratins rich in a aa aa-helical structureRef: Textbook of medical biochemistry, MN Chatterji, 8th edition, page no: 89
Alanine
Leucine
Proline
Isoleucine
Structure and function of protein