Watson Y Crik

No. 4356

April 25, 1953

NATURE

737

equipment, and to Dr. G. E. R. Deacon and the captain and officers of R.R.S. Discovery II for their part in making the observations.
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Young, F. B., Gerrard, H;, and Jevons, W., Phil. Mag., 40, 149 (1920). • Longuet-Higgins, M. S., Mon. Not. Roy. Astra. Soc., Geophys. Supp., 6, 285 (1949). • Von Arx, W. S., Woods Hole Papers in Phys. Oceanog.Meteor., 11 (3) (1950). 'Ekman, Y. W.. Arkiv. Mat. Astron. Fysik. (Stockholm), 2 (11) (1905).

MOLECULAR STRUCTURE OF NUCLEIC ACIDS
A Structure for Deoxyribose Nucleic Acid
E wish to suggest a structure for the salt of deoxyribose nucleic acid (D.N.A,). This structure has novel features which are of considerable biological interest. A structure for nucleic acid has already been proposed byPauling and Corey'. They kindly made their manuscript available to us in advance of publication. Their model consists of three intertwined chains, with the phosphates near the fibre axis, and the bases on the outside. In our opinion, this structure is unsatisfactory for two reasons: (I) We believe that the material which gives the X-ray diagrams is the salt, not the free acid. Without the acidichydrogen atoms it is not clear what forces would hold the structure together, especially as the negatively charged phosphates near the axis will repel each other. (2) Some of the van del' Waals distances appear to be too small. Another three-chain structure has also been suggested by Fraser (in the press). In his model the phosphates are on the outside and the bases on the inside, linked together byhydrogen bonds. This structure as described is rather ill-defined, and for this reason we shall not comment on it. We wish to put forward a radically different structure for the salt of deoxyribose nucleic acid. This structure has two helical chains each coiled round the same axis (see diagram). We have made the usual chemical assumptions, namely, that each chain consists of phosphate diester groupsjoining ~-D-deoxy­ ribofuranose residues with 3',5' linkages. The two chains (but not their bases) are related by a dyad perpendicular to the fibre axis. Both chains follow righthanded helices, but owing to the dyad the sequences of the atoms in the two chains run in opposite directions. Each chain loosely resembles Fur" berg's' model No.1; that is, t he bases are on the inside of the helix and thephosphates on This figure is purely the outside. The configuration diagrammatic. The two of the sugar and the atoms ribbons symbolize the two phosphate-sugar near it is close to Furberg's chains, and the hori'standard configuration', the zontal rods the pairs of bases holding the chains sugar being roughly perpenditogether. The vertical cular to the attached base. There line marks the fibre axisW

is a residue on each chain every 3·4 A. in the z-direetion. We have assumed an angle of 36° between adjacent residues in the same chain, so that the structure repeats after 10 residues on each chain, that is, after 34 A. The distance of a phosphorus atom from the fibre axis is 10 A. As the phosphates are on the outside, cations have easy access to them. The structure is an open one, andits water content is rather high. At lower water contents we would expect the bases to tilt so that the structure could become more compact. The novel feature of the structure is the manner in which the two chains are held together by the purine and pyrimidine bases. The planes of the bases are perpendicular to the fibre axis. They are joined together in pairs, a single base from one chain beinghydrogen-bonded to a single base from the other chain, so that the two lie side by side with identical z-co-ordinates. One of the pair must be a purine and the other a pyrimidine for bonding to occur. The hydrogen bonds are made as follows: purine position 1 to pyrimidine position 1; purine position 6 to pyrimidine position 6. If it is assumed that the bases only occur in the structure in the most...