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 definitely.
I do not see any serious difficulty in the attack of enzymes on nucleic adds with
our structure.
Dr. Dounce
Page 2
23/3/53
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.
Sincerely yours,
Dictated by Linus Pauling
Signed in his absence:W
