23 March 1953
Dr. Alexander L. Dounce
School of Medicine and Dentistry
The University of Rochester
Rochester 20, New York
Dear Dr. Dounce:
I am glad to have your letter, and to see year reprints, and to answer your questions.
First, let me say that Professor Corey and I have found it desirable to make a small change in our structure, in a way pointed
out by Professor Verner Schomaker. We have rotated the phosphate groups through 45° about their vertical axes. This brings
one of the unesterified oxygen atoms out to about 4 A from the axis, and in a position where it might possibly be open to
attack by a reagent, as suggested in your theory.
I do not see (although I have not considered the matter thoroughly) why your mechanism has to be ruled out with a three-chain
helix. I agree that in our original structure the two unesterified phosphate oxygen atoms are pretty well protected from
attack, but in the new structure one of the oxygen atoms is out in the open, and I should think that the three-chain structure
night work just as well as a one-chain structure.
I do not think that there was any difficulty about replacing the phosphate hydrogens by metals, dye molecules, basic proteins,
in the original structure, because it is not necessary that negative charges and positive charges that neutralize them be
on atoms that are in contact with one another.
There is some question as to the cross-sectional area of the DNA molecule. Astbury's estimate was essentially a guess.
Our estimate is supported by a large amount of physical chemical data, and by x-ray data. I cannot be sure that the x-ray
data require three nucleotide residues per 3.4 A. The data do, however, indicate very strongly that there are at least two
residues in this length. I think that the indication for three residues is stronger than for two residues. Probably the
x-ray data that have been gathered recently by the people in King's College, London, are good enough to answer this question
I do not see any serious difficulty in the attack of enzymes on nucleic adds with our structure.
I should think that little change in the structure of the nucleic acid would occur when it is liberated from the protein to
which it is bound.
I know that many people are working on the structure of nucleic acids now - especially people in England. It seems likely
to me that within two or three years the problem will be definitely answered - that is, that the folding of the polynucleotide
chains will be known. Perhaps it will turn out that our structure, with the small revision, is right, but perhaps some other
structure will be found to be right.
Dictated by Linus Pauling
Signed in his absence:W