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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 by Pauling and Corey (1). They kindly made their manuscript available to us inadvance 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: 1) We believe that the material which gives the X-ray diagrams is the salt, not the free acid. Without the acidic hydrogen 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 der 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 by hydrogen bonds. This structure as described is rather ill-defined, and for thisreason 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 groups joining β-D-deoxyribofuranose 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 right-handed helices, but owing to the dyad the sequences of the atoms in the two chains run in opposite directions. Each chain loosely resembles Furberg’s (2) model No. 1; that is, the bases are on the inside of the helix and the phosphates on the outside. The configuration of the sugar and the atomsnear it is close to Furberg’s ‘standard configuration’, the sugar being roughly perpendicular to the attached base. There is a residue on each chain every 3.4 A. in the z-direction. 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 10A. As the phosphates are on the outside, cations have easy access to them. The structure is an open one, and its 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 thebases are perpendicular to the fibre axis. They are joined together in pairs, a single base from one chain being hydrogen-bonded to a single This figure is purely diagrambase from the other chain, so that the two lie side by side with identical z- matic. The two ribbons symbolize the two phosphate– co-ordinates. One of the pair must be a purine and the other a pyrimidine sugar chains, and thehorizonfor bonding to occur. The hydrogen bonds are made as follows : purine tal rods the pairs of bases position 1 to pyrimidine position 1; purine position 6 to pyrimidine posi- holding the chains together. The vertical line marks the fition 6. bre axis. If it is assumed that the bases only occur in the structure in the most plausible tautomeric forms (that is, with the keto rather than the enolconfigurations) it is found that only specific pairs of bases can bond together. These pairs are: adenine (purine) with thymine (pyrimidine), and guanine (purine) with cytosine (pyrimidine). In other words, if an adenine forms one member of a pair, on either chain, then on these assumptions the other member must be thymine; similarly for guanine and cytosine. The sequence of bases on a single chain...
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