Historical article: Titration studies and the structure of DNA.

During the period of 1946-1956 ideas on the structure of the nucleic acids, especially DNA, were profoundly influenced by physico-chemical studies and, in particular, by electrometric titration studies pioneered by J. Masson Gulland and D.O. Jordan. It was eventually conclusively inferred when such studies had been sufficiently refined that: in DNA, the only hydrogen bonding that occurred was between the 1:6 amino groups of adenine and cytosine, and the -NH-CO "enolic" groups of thymine and guanine; the polynucleotide chains of DNA were not branched (or interrupted); and the internally hydrogen-bonded double-helical structure was stable both in aqueous solution and in the fibrous state.