... together. They tried adding magnesium and calcium ions to the sugar phosphate backbone. Three chains twisted about each other in a way that gave a crystallographic repeat a certain distance along the helical axis. However, the initial data from Franklin had suggested that the phosphate groups were near the outside and not in the middle. She objected to the idea of divalent ions holding together phosphate groups because according to her calculations they would be surrounded by tight shells of water molecules. A new model was then suggested by Linus Pauling, who had previously discovered the ?-helical structure of proteins. Pauling's model consisted of 3 intertwined helical polynucleotide chains. This model was compatible with the main features of the x-ray diagram and could account for some of the chemical properties of DNA. It was suggested that each chain formed by phosphate diester groups and linking D-ribose and D-deoxyribose residues had 24 nucleotide ...

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