These structures arise due
to the regular folding of the backbone of the polypeptide
chain due to hydrogen bonding between 
 and
–NH– groups of the peptide bond a-Helix is one of the most common ways in which
a polypeptide chain forms all possible hydrogen bonds
by twisting into a right handed screw (helix) with the
–NH group of each amino acid residue hydrogen bonded to the
C   O of an adjacent turn of the helix as shown in Fig 10 1