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X-Ray Studies of Nucleic Acids. 1947.

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Title:	X-Ray Studies of Nucleic Acids [11 of 13]
Creator:	Astbury, William
Contributor:	Symposia of the Society for Experimental Biology, No. 1
Publisher:
Date:	1947-00-00
Subject:	Nucleic acids -- Structure X-rays -- Diffraction
Description:	Symposia of the Society for Experimental Biology, No. 1
Type:	Text
Format:	text/plain
Language:	en
Identifier:	astbury-pg11
Source:	Master scanned with Epson GT-10000+ flatbed scanner at 600 dpi.
Rights:	http://osulibrary.orst.edu/specialcollections/coll/pauling/dna/copyright.html
Full Text:	74 X-RAY STUDIES OF NUCLEIC ACIDS to be a pseudomorph after all, giving only, an unsharp powder photograph from which it was difficult to deduce anything new to add to the clues we were looking for. A specimen of barium thymate, sent by Prof. Gulland, did however provide a photograph that appears to throw a certain amount of new light on the problem: it is shown in Pl. 2, fig. 6. There are two rings, one at 11-5 A. and the other at 6-8 A., and it is noteworthy that the latter spacing is twice the internucleotide spacing arrived at from the study of Na thymonucleate. Since thymic acid is produced by removing the purine bases froin thymonucleic acid by partial hydrolysis, this observation is readily explained if purine and pyrimidine bases occur alternately along the thvmonucleic acid coluinn. Quite recently we have taken up these studies again with a resumed attack on the structure of the nucleic acid from tobacco mosaic virus, and this time with the product of high molecular weight prepared as described by Cohen & Stanley (1942). Only a tiny amount of the sodium salt has been available as yet-it was kindly prepared by Dr Bailey-but following our usual technique it has been possible to make small areas of thin film by drying the viscous aqueous solution on a glass plate. With many elongated molecules, e.g. gelatin, myosin, and Na thymonucleate, this procedure leaves them lying roughly parallel to the plane of the film, and an X-ray photograph taken with the beam parallel to this plane gives then an approximate fibre pattern. Considering the high viscosity of the new preparation we fully expected to obtain a similar result again, but un- fortunately it did not happen: we obtained only a powder photograph. This powder photograph is illustrated in the right half of Pl. 2, fig. 7, the left half representing a corresponding powder photograph of Na thymonucleate. The two ring patterns are not identical-one would not expect that-but it will be seen that the resemblance is so close that in their broader features, at least, the two structures must be very much alike. With the experience gained from these preliminary trials we hope soon to continue the investiga- tion on a new purer preparation, but already I think it may be accepted that there can hardly be any outstanding difference in the way the two types of nucleic acid are built up. I am indebted to Dr Davies for examining this finding with the aid of atomic models. In the first place, it is clear that a similar superposition of nucleotides can be accomplished using either a ribose or a desoxyribose sugar, because the additional hydroxyl group is found on the periphery of the column. The more important question concerns the nature of the linkage between successive nucleotides, and on this point the evidence of the model is rather arresting. It has been suggested (see Greenstein, 1944, pp. 216-17), for instance, that in the ribose type of nucleic acid the linkage