A sTtvcivee For DW. A,

 

Pauling and Corey" have recently proposed a structure for
nucleic acid, They were kind enough to make their manuscript
available to us in advance of publication. In our opinion their

structure 1s unsatisfactory for two reasons!

de We believe that the material which gives the X-ray diagtem |
4a the salt, not the free acid, | ‘Without the acidic hydrogen.
atoms it is not clear what forces would held the btructure a
together, especially as the negatively charged. phosphates near
the axia will repel each ‘other. oS

@2e Some of the van der Waals Aistances appear, te be too small,

We wish to put forward a radically 4irterent structure for |
the salt of desqxyribose nucleic acid (Doma. )s ‘This structure has
two helical chains each coiled round the. same axis. The: two chains
(but not their bases) are related by a dyad perpendicular to this
axis, ‘Both chains follow right-handed helices, but owing to the dyad —
the sequences of the atoms in the two nia in opposite directions.
Each chain loosely resembles Furberg’ a* model Roels. ‘that is, the
bases are on the inside of the helix and the phosphates | ‘on the outside.
The configuration of the sugar andthe atoms near it: te close to o
Furberg's “standard configuration", the sugar being roughly perpendic-

' ular to the attached base, There is a residue on each chain every.

3.4 R 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, efter au &, The distance of a
phosphorus atom from the fibre axie is 10 8. = The structire is an
open one and its water content will therefore be rather high. Ae
the phosphates are on the outside, cations have easy access to them,
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 ane chain being
hydrogen-bonded to a single fase from the other chain, so that the
two lie side-by-side with identical s co-ordinates, One of the pair
must be a purine and the other a pyrimidine for bonding te ecaur.
The hydrogen bends are made as follows: |

Purine position 1 to pyrimidine position 1
Purine position 6 to pyrimidine position 6

If it ie assumed that the bases only occur in the strueture
in the most plausible tautomeric forme, (that is with the keto rather
than the enel oonfigurations) it 1s found that only specific pairs
of bases can bond together. These pairs aret

Adenine (purine) with Thymine (pyrimidine)
Guanine (purine) with Gytosine (pyrimidine)

In other words, if an adenine forms one member of a pair, on either
_ chain, then gn these assumptions the other member pust be thymine.
Similarly for guanine and cytosine, The sequence of bases on a
single chain dees not appear to be restricted in any way, However,
if only speeifiec pairs of bases can be formed it follows that if the

sequence of bases on one chain is given, then the sequence on the

other chain is automatically determined, ( Svontt—-the—prtring—te—net-
It has been found experimentally Boh that the ratio of the
- amounts of adenine to thymine, and the ratio of guanine to cytosine
are always very close to unity for D.N.A..

It is probably impossible to build this structure with a
ribose sugar in place of the desoxyribose, as the extra oxygen atom
would mike too close a van der Waals contact,

The published X-ray aatad?® on DNA, are inadequate, As
far as we can tell, eur structure is roughly compatible with the
experimental data, It is known’ that there is much unpublished
experimental material, Until this has been used te test the structure
it must be regarded as unproved, |

It has not escaped our notice that the specific pairing we

| have postulated immediately suggests a possible copying mechanism
for the genetio material, ~~~
Pull details of the structure, inoluding the conditions
assumed in building it, together with a set of co-ordinates for the
atoms, will be published elsewhere, ‘

We are heavily indebted to Dr. Jerry Donoghue for constant
advice and eriticiem, especially on inter-atomic distances, We have
also been stimlated by the very beautiful experimental work of Dr,

He F. Wilkins and his co-workers at Kings College, London.

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Pauling, Le and Corey, RB. Nature 171, 346, (1953): ProcwAcads

 Bei.Wash. 39, 84 (1953)
Purberg, S Acta Chem,Scand,. §, 634 (1952)

Chargaff, E. for references see Zamenhof, Se, Yawerman, G, and
Chargeff, E. Biochem, et Biophys. Act. 9, ,02;4.952)

Wyatt, G. Re Jour, Gen, Phys. 36, 201, A952)

Astbury, Wet. Symponitum No. 1 of the Society for Experimental
Biology, 66 (1947)

Wilkins, HHL? and Randall, J.T. Biechem et Biophys.Acta 10,
: 192 (1953).

Wilkins, MHF. - personal communications
 

Pauling, be, and Corey, Ry Bs, nse 124 26 sg ao Kat,

Purberg, 3, Actes Chem, Scand. §, 634, (1952).

ChargafT, Be» Por references see Zawenhof, 8., Brawerman, @, ‘ts
| Chargaf’, K., Biochem, et Biophya. Acta ,9, "402, 19$2)

Ryatt, 3, R,, Journ, Gens Phys. 36, 201, (1952,) -
Astbury, W,T., 8 eniun.. 901 of the Seciety for Exparinentel
olony, (1947).

wanton, w HF, and Randell, Te Biochem et eg GS