|"...a very small change indeed"
In 1956, Vernon M. Ingram, a protein chemist at the Cavendish Laboratory in Cambridge, developed a technique
called fingerprinting, which maps on paper the various amino acids in a protein sample.
Fingerprinting, a two-step process, utilizes paper electrophoresis and paper chromatography.
It produces splotches on paper at various locations; each mark corresponds to a peptide
residue (two or more linked amino acids). By comparing the fingerprints of sickle
cell and normal hemoglobin, Ingram found that one spot differed between the two fingerprints
and concluded that normal and sickle cell hemoglobin have a small difference in their
amino acid sequences.
Less than one year later, Ingram conclusively showed that normal and sickle cell hemoglobin
differ by one amino acid. Accordingly, he stated that normal hemoglobin has a glutamic
acid in the fourth peptide chain whereas sickle cell hemoglobin has a valine at the
same location. The substitution of glutamic acid with valine accounts for the difference
in charges on the two types of hemoglobin, and for the observed electrophoretic difference.
Ingram acknowledged the importance of the work accomplished by Pauling and his colleagues
in 1949: "We owe to Pauling and his collaborators the realization that sickle cell
anaemia is an example of an inherited 'molecular disease' and that it is due to an
alteration in the structure of a large protein molecule, an alteration leading to
a protein which is by all criteria still a haemoglobin. It is now clear that, per
half-molecule of haemoglobin, this change consists in a replacement of only one of
nearly 300 amino-acids, namely, glutamic acid, by another valine – a very small change