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Proteins are enormously complicated molecules, and until Sanger's work on insulin, no one had ever been able to determine the structure of even the simplest of them. Chemists have known for many years that protein molecules are made of amino acids (nitrogen-containing organic acids) strung together in long chains or cables. By various kinds of rough treatment, the chemists could separate and count the amino acid building blocks. But this did not reveal their structural plan.
Dr. Sanger tried treating the insulin molecule gently, succeeded in breaking it into large chunks. He separated the fragments and labeled the amino acids on their ends by making them combine with a material called DNP (for dinitropheny). When he broke the fragments into smaller fragments, the amino acids that had been in the end positions were stained yellow with DNP. There are 51 amino acid units in insulin, a comparatively simple protein. But Sanger's patience and skill eventually found the place of each in the long chain. Then he reassembled the fragments and learned how the chain is folded over and locked together. At last he had the first full picture of one of the giant molecules that are the stuff of life.