"The molecular structure of polyadenylic acid"

Date:
1957-1960
Reference:
PP/CRI/H/2/19/1
Catalogue details

Licence: In copyright

Credit: "The molecular structure of polyadenylic acid". Source: Wellcome Collection.

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    elusion that there were none which were plausible. This point will be dis cussed in greater detail when we consider the model advanced by Morgan and Bear (1958). Investigation of multi-stranded models with the bases on the inside showed that the ribose phosphate backbone is not long enough to cover the distance required for a translation of 3.8 A and a rotation of 90° which was indicated by the strong meridional reflexion on the 4th layer line. However, two- stranded models can be built with a two-fold rotation axis running down the helix axis. The effect of this symmetry axis is to reduce the unit rotation to 45®, and within this framework these are two structures in which the adenines of opposite chains are joined together in pairs by hydrogen bonds. In the first of these structures the 6-amino group is hydrogen bonded to Nj of the opposite purine ring. Models of this type were rejected by us because they require a large radial separation between phosphates of opposite chains. This would lead to strong reflexions near the meridian on the first layer line, contrary to what is observed. They were examined in more detail by Morgan and Bear (1958), and rejected for similar reasons. The second of these structures has the adenines paired in the same way as in the crystal structure of adenine hydrochloride (Broomhead, 1948; Cochran, 1951), namely with hydrogen bonds between the 6-amino nitrogen