March 12, 1953
Dear Max
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
Jim
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.
