My reason for favouring the ‘enol’ over the ‘keto' configuration within the conformation of the major helix is simply down to the fact that it places the oxygen attached to C4 of Thymine within the centre of the primary helix and amongst the NH2’s that make up the secondary helix within the primary. This strikes me as potentially strange in a strictly engineering and architectural sense. That said, it may well be that by use of a keto rather than an enol Thymine molecule, thereby placing an oxygen within the secondary inner helix, some function in relation to amino acid synthesis is enabled or disabled.