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X-Ray Studies of Nucleic Acids. 1947.

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Title:	X-Ray Studies of Nucleic Acids [5 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-pg05
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:	70 X-RAY STUDIES OF NUCLEIC ACIDS This chain forms a zigzag bridge from the sugar end of one nucleotide to the next that is quite satisfying so long as we consider only the covalent distances between atoms (rc=0-77 A., ro=o-67 A., rp=r-o A.); it is when we attach the O and 01-1 to the phosphorus atom that the apparent anomaly arises. The van der V6'aals separation of oxygen atoms is usually quoted as about z-7 A., but if we adhere to a uniform separation of this order successive nucleotides can hardly be brought` nearer than about 4. A. There is no convincing reason, though, why we should do so in face of the total van der Waals attractions between the large flat ring systems, and it is a fair con- clusion that these attractions are strong enough to distort the oxygen van der ~Vaals fields somewhat, after the manner of the compression of methyl groups that takes place when isobutylene polymerizes (see calculation made by Woods & Astbury in Astbury, 1945 a). It is possible that this supposed distortion of the oxygen fields has the effect of making the polynucleotide column more labile than ,vould otherwise be the case. Right from the beginning of this X-ray study of the nucleic acids we have drawn attention to the fact that the spacing (3-3, A.) of the nucleotides along the thymonueleic acid column is to all intents and purposes the same as the distance from one side chain to the next along an extended poly- peptide, and even at the risk of being accused of dabbling in numerology I should like to say again that I believe this to be no mere coincidence. In a sense the evolution of biological molecules has been one long story of numerology, a sifting and sorting of shapes and sizes and a progressive selection of molecular patterns that has been going on throughout the ages. Biosynthesis is supremely a question of fitting molecules or parts of mole- cules one against another, and one of the great biological developments of our time is the realization that probably the most fundamental interaction of all is that between the proteins and the nucleic acids. I wish to stress again that not only are the basic components of the nucleotides flat or nearly so, but also the chosen sugar is in the flat furanose configuration, and it is this combination that leads to the correspondence between inter- nucleotide spacing and inter-side chain spacing. I suggest that the agree- ment is not an arithmetical accident but a stereochemical correlation of deep significance. The spacing of the nucleotides along the Na thymonucleate column is independent of the water content of the preparation (which we found to be of the order of zo °~, at room humidity). The X-ray photograph of a specimen dried in a vacuum desiccator shows the meridian arc undisturbed, but the diagram as a whole is poor and ill-defined compared with what is obtained at ordinary humidity. The side spacings are seen to be shorter, but it is not possible to be sure in which direction or directions the shrinkage takes