Franklin and Gosling (1953)

Date:
1953
Reference:
PP/CRI/I/3/5
Catalogue details

Licence: In copyright

Credit: Franklin and Gosling (1953). Source: Wellcome Collection.

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    Reprinted from Acta Crystallographica, Vol. 6, Part 8-9, September 1953 ATJ^gi t '•»Mir® Acta Gryst. (1953). 6, 673 The Structure of Sodium Thymonucleate Fibres. I. The Influence of Water Content By Rosalind E. Franklin* and R. G. Gosling Wheatstone Physics Laboratory, King's College, London W.O. 2, England (Received 6 March 1953) A qualitative survey has been made of the types of X-ray diagram given by highly orientated specimens of sodium thymonucleate at different humidities. From the nature of the structural changes that occur when the humidity is varied, certain general conclusions have been drawn concerning both the way in which the sodium thymonucleate molecules are bound to one another and the part played by water in the structure. Introduction The fundamental substance of cell nuclei, nucleo- protein, consists of a fairly loose conjugation of desoxyribose nucleic acid (DNA) and simple proteins. The nucleic acid component can be separated from the protein and precipitated in the form of its sodium salt by a mild process (Signer & Schwander, 1949). A knowledge of the structure of DNA would therefore not only be interesting on its own account, but might well prove to be a valuable aid in the study of the nucleoproteins. DNA is known to exist in the form of long-chain molecules of very high molecular weight; when suf ficient precautions to avoid degradation are taken, values up to 8 million are obtained (Signer & Schwan- der, 1949; Katz, 1952). The chains are believed to be un-branched or nearly so. They are built up of four different nucleotides, each nucleotide being the phos phoric ester of a nucleoside, and each nucleoside the desoxypentose derivative of an amino-purine or amino-pyrimidine. The phosphate group is linked to two nucleotides through the C3 and C5 positions on the sugar ring (Davidson, 1950). It was for some time believed that DNA contained the four bases, adenine and guanine (purines) and cytosine and thymine (pyrimidines) in equal propor tions, and the suggestion was made that the fundamen tal unit was a tetranucleotide. However, careful analy ses by Chargaff et al. (1949) have shown this to be untrue and their results, together with the more recent studies of Sinsheimer & Koerner (1952) and of Mark- ham & Smith (1952) of the fragments obtained by enzymatic digestion of DNA, suggest that the sequence of nucleotides in the chain may be complicated. Sodium thymonucleate, the sodium salt of the nu cleic acid extracted from calf thymus (referred to below as NaDNA), is a fibrous solid which is capable of taking up large quantities of water to form a gel. * Present address : Department of Physics, Birkbeck College, London W.C. 1, England. By means of suitable mechanical treatment and drying, this gel can be used to obtain highly orientated spec imens of NaDNA and these give characteristic X-ray fibre-diagrams (Astbury, 1947; Wilkins et al., 1951). The X-ray powder photographs of non-orientated aqueous systems of NaDNA have been studied by Riley & Oster (1951). None of the X-ray diagrams obtained from NaDNA shows discrete reflexions at angles corresponding to spacings smaller than 2-5 Â. Even a complete quan titative study will not, therefore, be capable of yielding the atomic positions in the structure. But, since the chemical composition and structure of the constituent sugar and base components are well-established, this need not necessarily prove an insuperable obstacle to structure determination. It was felt that, in any case, a comparative study of the different diagrams obtained under different conditions, combined with a detailed quantitative investigation of those diagrams which show the highest degree of crystalline order, should yield a certain amount of new information. In particular, the diagrams show that the structure is highly sensitive to the humidity of the surrounding atmosphere. A general qualitative survey of this effect is described in the present paper. We believe that it throws some light on the way in which the long-chain molecules are bound to one another in the structure and this, in turn, may possibly have some bearing on the way in which nucleic acid and protein are united in nucleoprotein. A quantitative study of the fibre diagram given by NaDNA in its most highly ordered state will be described in later papers. Preparation of fibres The material used throughout this investigation was the highly purified sodium salt of calf thymus des oxyribose nucleic acid, which was kindly supplied to us by Prof. Signer. Fibres were prepared by the method of Wilkins;