Donohue published an article in the September 12, 1969, issue of Science (vol. 165, p. 1091), in which he questioned whether
Fourier syntheses, a method for calculating the distribution of electron density from amplitudes (seen in X-ray diffraction
images) and assumed bond angles of molecules in crystals--and thus a method for inferring the structure of such molecules--did
in fact provide evidence for the Watson-Crick structure, specifically the base pairs, as DNA researchers like Maurice Wilkins
claimed. Donohue in his article presented electron density maps of various combinations of base pairs which he claimed fit
Fourier syntheses just as well as the canonical A=T and C=G pairs. He thereby implied that other base pairs and thus other
structures for DNA were conceivable.
Crick, along with Maurice Wilkins and other DNA scientists, refuted Donohue's criticism and arguments in letters published
in the March 27, 1970, issue of Science (vol. 167, p.1693). He did so again in this letter, challenging Donohue to further
explain the results of his Fourier syntheses of his proposed base pairs.
In crystallography, a dyad (mentioned in the letter) is a two-fold symmetry, or a symmetry around two axes. A Patterson function
is a technique developed in the 1920s for estimating the distances between atoms in a molecule by analyzing the intensities--the
darkness--of the spots produced on a photographic plate by X-ray diffraction. This technique thus avoided the so-called phase
problem in crystallography, the problem of measuring the X-rays' peaks and troughs, providing hints (though not conclusive
evidence) of the internal structure of molecules.
Number of Image Pages:
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1970-05-20 (May 20, 1970)
Original Repository: Wellcome Library for the History and Understanding of Medicine. Francis Harry Compton Crick Papers
Thank you for your letter of May 6. I am amused to see from it that, as I suspected, you have not appreciated the point about
the dyads in the DNA model.
Of course, it is quite possible to make errors with centric Fouriers, but it is the general experience of crystallographers,
and I am sure it is yours, that it is much easier to do so with acentric structures. I think you have quite failed to appreciate
the significance of this difference and also that it is far harder to get false structures from polymers than from single
crystals of small molecules. You are only likely to get an "acceptable" false structure if its Patterson is similar
to the Patterson of the real structure, and the much greater restrictions imposed by a polymer make this condition more difficult
Would you please explain to me the following odd circumstances, which must have struck the attentive reader of Science? Whenever
the King's College group do a difference synthesis it shows clearly the direction the groups of atoms should be moved.
Whenever you publish the synthesis of one of your imaginary structures of base pairs, it is difficult to distinguish the false
solution from the true one and the difference synthesis does not show clearly how the groups of atoms should be moved. I
feel that I can explain this rather remarkable difference. Can you?