10 November 1952

Dr. Barbara W. Low

University Laboratory of Physical Chemistry

Harvard University

25 Shattuck Street

Boston 15, Massachusetts

Dear Dr. Low:

I am pleased that you are publishing a note on the 4.4-residue helix. We evaluated the coordinates for it a short while ago, and have been making some calculations of the strain energy, etc. Also, I tried to find out why we overlooked it in our earlier work, apparently it was simply overlooked, at the time when we were attempting to discover helical structures of this sort by the analytical solution of simultaneous equations (trigonometric ones, of course), defining the bond angles and the distances. We worked very hard on this job, for over a year. It is evident now that this mathematical method is not the best one for finding structures of this kind.

I am sorry that I cannot send you a reprint of my Phi Lambda Upsilon Lecture but our supply of them is exhausted.

As to the Bamford X-ray pattern, the repeating unit with fiber axis 43.2 A. seems to me to be a 29-residue 8-turn unit. I think that the Courtsulds people feel the same way, although, of course, they are not sure about our α helix. I may mention that Dr. Yakel here made a preparation of poly- -methyl-L-glutamate with a different X-ray pattern from the earlier one that we had (which corresponded to 18 residues in 5 turns). The new fiber is not hexagonal, and I am not sure what its identity distance is.

It seems to me to be wrong to emphasize the difference between an 18-residue 5-turn, an 11-residue 3-turn, and a 29-residue 8-turn α helix, as Bamford has done in one of his papers. All of these are included within our original description of the α helix. The coordinates of the 18-residue 5-turn helix were presented, as an example. In our discussion both this structure and the 11-residue structure ware dis-

Dr. B. W. Low: Page 2 November 10, 1952

cussed, and It was pointed out that a little range of values of number of residues per turn could be accepted.

I feel sure that there are at least two, probably three, and perhaps more different modifications of fibrous poly- -methyl-L-glutamate, all, however, based upon the α helix.

Sincerely yours,

Linus Pauling:jb