... model: Pauling formulated a structure of 3 intertwined helical polynucleotide chains. Each chain is formed by phosphate di-ester groups, linking D-ribose or D-deoxyribose residues with 3', 5' linkages. They have approximately 24 nucleotide residues in 7 turns of the helix, which has the sense of a right handed skew. The phosphate groups are closely packed about the axis of the molecule with the pentose residues surrounding them and the purine and pyrimadine groups projecting radially. Watson's response to Pauling's Ideas (1953): Watson realised that the phosphate groups in Linus' model were not ionised as each group contained a bound hydrogen atom and so had no net charge. Therefore the molecule was not an acid at all. Watson also realised that the molecule must contain hydrogen atoms for hydrogen bonding to occur. Without this bonding the molecule would fall apart. Pauling's Later Comments: Pauling realised that a 3-chain helical structure was an extraordinarily tight one with little ...

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