Robley Williams was one of many virus researchers that Franklin visited during her 1954 trip to the United States. In this
letter, he responded to her recent article on the probable structural configuration of TMV, as revealed by x-ray diffraction
studies, and mused about the differences between her data and those found by he and his colleagues via different methods.
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2 (117,760 Bytes)
1955-01-06 (January 6, 1955)
University of California, Berkeley
Original Repository: Churchill Archives Centre. The Papers of Rosalind Franklin
Reproduced with permission of Robley C. Williams Jr.
Dr. Stanley has shown me the manuscript of the short paper you have written for Nature on your recent deductions about the
structure of TMV. I found the paper most stimulating reading and was pleased with its lucidity.
I continue to be disturbed about the origin of the hexagonal-shaped platelets of fragmented TMV which I and others have observed.
I conclude that you would say that the external shape of the red is cylindrical, with possibly some grooving, and that the
diameter of the cylinder is 150 A. A difficulty with this conclusion is that the density does not come out right. A cylinder
of length 3000 A, diameter 150 A, and molecular weight 50 x 10 [to the sixth], will yield a density of 1.52 gm/cc. I am under
the impression that a value of 1.37 gm/cc is accepted as being essentially correct for the density of TMV. On the other hand,
an hexagonal contour of 150 A miner diameter and 174 A diameter from corner-to-corner brings the calculated density to 1.56
On the other hand a model in which thin platelets would be hexagonal is not good. If the pitch of the helix is (3n + 1) unite
in three turns, with n = 12, it might be possible to construct a slowly turning hexagonal prism for which, in any one thin
section, there are 12 [corner]-shaped units making up the hexagon. At the end of three turns (69 A) a phase shift of 30 degrees
would have accumulated, thus producing a slowly turning hexagonal prism which would make one complete turn in 36 turns of
the helix. The trouble with this model is that the hexagonal rods could not come closer than a center-to-center distance
of 174 A. As you have stated, the birefringence evidence from wet and dry crystals indicates that there is space between
the packed rods, and so a straight, hexagonal prism for the whole rod is apparently impossible.
There remains the possibility that the rods have a slowly-turning hexagonal contour, but that the corner-to-corner diameter
of the hexagon is 150 A, and the minor diameter is 130 A. The observations of Bernal and Fankuchen yield only the center-to-center
separation of the rods, and give no information about the actual diameter of the rods. This would be a fine model, allowing
a hexagonal cross-section in any thin section, providing for space between the rods, and resulting in a minimal center-to-center
separation of 150 A. The difficulty is that the calculated density would not be entirely out of accord with the observed
So I am afraid this letter is of no use, except that it points out the density difficulty in an assumed cylindrical model,
and it reiterates the fact that numerous hexagonal platelets have been photographed here, at Wisconsin, and at M.I.T. I wish
a consistent contour could be found which would fit together the evidence from X-ray analysis, birefringence, density determinations,
and electron microscopy.