March 12, 1953
Thank you very much for your recent letters. We were quite interested in your account of the Pauling Seminar. The day following
the arrival of your letter, I received a note from Pauling, mentioning that their model had been revised, and indicating interest
in our model. We shall thus have to write him in the near future as to what we are doing. Until now we preferred not to
write him since we did not want to commit ourselves until we were completely sure that all of the Van der Waals contacts were
correct and that all aspects of our structure were stereochemically feasible. I believe now that we have made sure that our
structure can be built and today we are laboriously calculating out exact atomic coordinates.
Our model (a joint project of Francis Crick and myself) bears no relationship to either the original or the revised Pauling-Corey-Schomaker
models. It is a strange model and embodies several unusual features. However since DNA is an unusual substance, we are not
hesitant in being bold. The main features of the model are (1) The basic structure is helical - it consists of two intertwining
helices - the core of the helix is occupied by the purine and pyrimidine bases - the phosphates groups are on the outside.
(2) The helices are not identical but complementary so that if one helix contains a purine base, the other helix contains
a pyrimidine - this feature is a result of our attempt to make the residues equivalent and at the same time put the purines
and pyrimidine bases in the center. The pairing of the purine with pyramidines is very exact and dictated by their desire
to form hydrogen bonds - Adenine will pair with Thymine while Guanine will always pair with Cytosine. For example [diagrams
of base pairing] Thymine with Adenine, Cytosine with Guanine.
While my diagram is crude, in fact these pairs form 2 very nice hydrogen bonds in which all of the angles are exactly right.
This pairing is based on the effective existence of only one out of the two possible tautomeric forms - in all cases we prefer
the keto form over the enol[,] the amino over the imino. This is definitely an assumption but Jerry Donohue and Bill Cochran tell us that for all organic molecules so far examined, the keto and amino forms are present
in preference to the enol and imino possibilities.
The model has been derived almost entirely from stereochemical considerations with the only x-ray consideration being the
spacing between the pair of bases 3.4A which was originally found by Astbury. It tends to build itself with approximately 10 residues per tern in 34A. The screw
is right handed.
The x-ray pattern approximately agreed with the model, but since the photographs available to us are poor and meagre (we
have no prototypes of our own and like Pauling must use Astbury's photographs) this agreement in no way constitutes a proof
of our model. We are certainly a long way from proving its correctness. To do this we must obtain collaboration from the
group at Kings College London who possess very excellent photographs of a crystalline phase in addition to rather good photographs
of a paracrystalline phase. Our model has been made in reference to the paracrystalline form, and as yet we have no clear
idea as to how these helices pack together to form the crystalline phase.
In the next day or so Crick and I shall send a note to Nature proposing our structure as a possible model, at the same time
emphasizing its provisional nature and the lack of proof in its favor. Even if wrong I believe it to be interesting since
it promises a concrete example of a structure composed of complementary chains. If by chance, it is right then I suspect
we may be making a slight dent into the manner in which DNA can reproduce itself. For these reasons (in addition to many
others) I prefer this type of model over Pauling's which if true would tell us next to nothing about [the] manner of DNA reproduction.
I shall write you in a day or so about the recombination paper. Yesterday I received a very interesting note from Bill Hayes.
I believe he is sending you a copy.
I have met Alfred Tissieus recently. He seems very nice. He speaks fondly of Pasadena and I suspect has not yet become accustomed
to being a Fellow of Kings.
My regards to Mary
P.S. We would prefer your not mentioning this letter to Pauling. When our letter to Nature is completed we shall send him
a copy. We should like to send him coordinates.