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Molecular Structure of DNA

November 1951 — Notes from a colloquium delivered at King’s College London, London UK


The highly crystalline fibre diagram given by DNA fibres is obtained only in a general humidity range, about 70% to 80%. The general characteristics of the diagram suggest that the DNA chains are in a helical form.

. . . This diagram appears to correspond to scattering by individual helical units; i.e., it shows the form factor of the helical units (except for the sharp equatorial spot which is related to an interhelical distance). That is, at high humidity a water sheath disrupts the spatial relationships between neighbouring helices, and only the parallelism of their axes is preserved.

During the change “crystalline–>wet” a considerable increase in length of the fibres occurs. The helix in the wet state is therefore presumably not identical with that of the crystalline state.

. . . The 27A layer-line spacing is very strong, which suggests that it corresponds to one turn of a helix.

. . . The results suggest a helical structure (which must be very closely packed) containing 2, 3 or 4 co‐axial nucleic acid chains per helical unit, and having the phosphate groups near the outside. It is the phosphate groups which would be capable of forming strong interhelical bonds in the presence of considerable quantities of water (leading first to the “wet” structure of independent helices with parallel axes, and ultimately to the solution of DNA in water) and would remain strong in the absence of water, thus explaining the cementing effect of strong drying. The dry structure is distorted and strained due to holes left by the removal or water, but contains intact the skeleton of the crystalline structure. . . .



Source: Interim Annual Report: January 1, 1951-January 1, 1952, by Rosalind Franklin, Wheatstone Physics Laboratory, King’s College London, February 7, 1952.


Also: Rosalind Franklin and DNA, Anne Sayre (New York: W.W. Norton & Company, Inc.) 1975, pp. 125-126.