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X-Ray Studies of Nucleic Acids. 1947.

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Title:	X-Ray Studies of Nucleic Acids [2 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-pg02
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:	X-RAY STUDIES OF NUCLEIC ACIDS 67 as long as they are thick and of weight between 6oo,0oo and i,ooo,ooo. If the units are single columns of nucleotides their weight should be about (300 x 1312),,'7'x 330, i.e. of the order of half a million. Probably some of the units in solution are aggregates of columns, so the agreement is satisfactory enough. If we remember too that both fibres (Schmidt, 1937) and flowing solutions (Signer et. al. 1938) of Na thymonucleate show strong bire- fringence that is negative with respect to the fibre axis or the direction of flow, then, before approaching any X-ray study, we already have a pretty firm general izupression of what the answer is going to he. The molecules of Na thymonucleate and other polynucleotides of similar high density are presumably stiff columns of nucleotides fitting closely on top of one another. X-ray studies more or less clinch this conclusion and give it quantitative expression. There is no need to describe again the experimental side of the Na thymonucleate investigation, but Pl. I, fig. I is now presented as a better oriented fibre photograph than any we have published before. Unfortunately, the degree of orientation attained is still not high enough to allow the unit cell to be determined with certainty, and we prefer to postpone any definite statement in this respect pending the outcome of further experiments in progress. tip' hat is clear, however, is this, that the pattern repeats along the axis of the molecule at a distance corresponding to the thickness of eight nucleotides or a multiple of eight nucleotides-most probably eight or sixteen nucleotides. The least possible value of the fibre period is about 27 A., which would make the effective average thickness of a nucleotide about 3-4 A. The important point here has to do with the tetra- nucleotide constitution of the molecule, and in this connexion I wish to leave no doubt as to just what the X-ray diagram is so far telling us. It says simply that the arrangenient in space is repeated along the fibre axis every eight (or sixteen) nucleotides. It says nothing yet, without very much more investigation, about the nature of the sequence, which may be determined by either chemical or geometrical considerations, or a combination of both. It hardly seems likely, though, that the fact that the intramolecular pattern is found to be based on a multiple of four nucleotides is unrelated to the conclusion that has been drawn from chemical data that the molecule is composed of four different kinds of nucleotides in equal proportions. It seems improbable, too, to judge by the degree of perfection of the Y-ray, fibre diagram, that these four different kinds of nucleotides are distributed simply at random; rather must they follow one another in some definite order-at least, in the more crystalline regions of the structure that give rise to the regular diffraction pattern. It is necessary- to make this proviso, because in high polymeric aggregates there is always the possibility that