Fingerprint analysis

   The protein is digested into peptide fragments by proteolytic enzymes.

   The  mixture  is  then  subjected to paper electrophoresis followed by
   paper chromatography at 90° to the electrophoresis.

   The   resulting  two-dimensional  "fingerprint"  reveals  that  normal
   hemoglobin  HbA  and  sickle-cell  HbS  hemoglobin  differ by a single
   peptide fragment.

   Further   analysis  shows  a  single  amino  acid  change:  valine  is
   substituted for glutamic acid at the 6th position of the beta chain.