 After knowing the nature of peptide bond and the rotations which are allowed in a polypeptide chain Pauling and Coray concluded that the simplest arrangement which a polypeptide chain could assume with its rigid peptide bond is a helical structure which they called alpha helix. So in a alpha helix polypeptide chain is coiled round and imaginary axis so there may be an imaginary axis. So along this axis this helix is this helix is helically coiled. In this helix the R groups they lie outside the helix while the main backbone is contained by the C-N-C-N-C-C bonds. So this is main backbone and R groups which are bulky groups they lie outside the polypeptide chain. And in this helix this is one complete turn and this is repeated unit of this alpha helix. It is about 5.4 angstrom long and it contains about 3.6 amino acids. So there are 3.6 amino acids in a single turn of alpha helix and the length of this single turn is about 5.4 angstroms. So this is alpha helix. The helical twist of the alpha helix found in all protein is right handed. What is meant by right handed? If we say what is the direction of a helix we cannot say either it is clockwise or anticlockwise. These terms are not good for helices. We will say it is right handed coiling or left handed coiling. So this alpha helix in the proteins is right handed coiling. So it means if we say this is the right hand and this thumb is the axis of the helix and as the fingers are coiled around this thumb similarly our right handed helix is coiled in this direction. And if it is left handed coiled that means it is coiled like this. So alpha helix in the protein is right handed. The repeating unit as you saw in the last slide is a single turn of the helix which extends about 5.4 angstroms and includes about 3.6 amino acids along the long axis of the helix. The amino acid residues in an helix have confirmations with psi values ranging between minus 45 to minus 50 and 5 values of about minus 60. So in the helix peptide bond is rigid that doesn't rotate but the other bonds adjacent to the peptide bond they rotate and the values of their angles that is psi and phi this is given here. And helix makes optimal use of internal hydrogen bonds. So in this way by using maximum potential of the hydrogen bonding this structure becomes stable structure. So in a helix almost all the amino acids are hydrogen bonded with some other amino acids. As you can see in this picture the nitrogen contains hydrogen. A hydrogen is attached with the nitrogen similarly oxygen is attached with carbon. So a hydrogen bond is established between amide hydrogen and carbonyl oxygen. So these two amino acids one amino acid this and the other one is this. So these two amino acids are linked together through hydrogen bonding. So all the amino acids present in this helix they all are hydrogen bonded. So this helix uses the full potential of hydrogen bonding of the amino acids and normally one amino acid is making a hydrogen bond with the fourth amino acid next. So first amino acid will make hydrogen bond with the fourth one. Second amino acid will make hydrogen bond with the fifth one and so on. So in this case all the amino acids are hydrogen bonded and this structure becomes very stable structure. So as you know alpha helix is one of the prominent structures found in the protein molecules. So about one fourth of all amino acid residues in a polypeptide chain are found in alpha helices. And in some proteins like keratins the alpha helix is the prominent structure.