50 Years of DNA continued a little further

Rex Palmer’s criticism of Mark Spackman’s commentary on the origins of the DNA structure can be extended to correct a regrettable omission in Watson’s The Double Helix and Watson and Crick’s 1953 Nature paper. As Palmer says “the whole thing is based largely on crystallographic results..... from other groups!”. One of the experimentally derived factors mentioned by Palmer used in the DNA model building was “The possible base pairing H-bonding patterns and their geometry. This could be derived from a wealth of crystallographic data, and was revealed to Watson and Crick by Jerry Donohue, a world renowned expert on H-bonding in 1953.”

Robert Olby’s 1974 The Path to the Double Helix shows that Watson was working on base pairing before Donohue was brought into the discussions. He was studying June Broomhead’s papers and thesis of 1948-1951 on the crystal structures of adenine HCl and guanine HCl. Happily for him the thesis was available close at hand in the Cavendish Laboratory. These papers were part of a series of five in Acta Crystallographica which Clews, Cochran and Broomhead published in those years on the structures of pyrimidines and purines. This work had been undertaken in close collaboration with the chemical work on nucleotides by Professor A.R. Todd and his colleagues.

Watson p184 writes “..... I was drawing the fused rings of adenine on paper. Suddenly I realized the profound implications of a DNA structure in which the adenine residue formed hydrogen bonds similar to those found in crystals of pure adenine. If DNA was like this, each adenine residue would form two hydrogen bonds to an adenine residue related to it by a 180-degree rotation.” Watson had very cleverly abstracted the pair bonding from Broomhead’s diagrams of the layers in her structures of adenine and guanine. But Broomhead is not named. Nor does Watson mention his good fortune in having available the Cavendish work on pyrimidines and purines.

At this stage Donohue, with his H-bonding knowledge, entered the action to point out that Watson should be using keto rather than enol forms. Shortly Watson saw that like-with-like pairings “led nowhere”. Then brilliantly he spotted that adenine-thymine pairs would be identical in shape to guanine-cytosine pairs. Hence this type of double helix would immediately explain the Chargaff ratio rules. Crick then joined in to check and thrash out more features of their double helix model – and the rest is history. But it is sad that the famous duo omitted to acknowledge their debt to Broomhead and her colleagues.