DNA Structure: Puzzle Number 2

clive delmonte clived at ndirect.co.uk
Tue Dec 1 05:19:15 EST 1998

It was Franklin's famous diffraction photograph of B-DNA (2), shown to him
by Wilkins, that Watson sketched on the train back to Cambridge to rejoin

One of Wilkins' collaborators, A R Stokes (3), working on the Fourier
transform for a helix, showed that, defining as "delta" the angle between an
arm of the "helical cross" and the Y-axis (meridian),

        tan delta = helical pitch / helical circumference

Applying this result to Franklin's photograph of B-DNA, delta is measured as
40 degrees, and, with a pitch of 3.34 nm, the helical diameter is found to
be some 1.3 nm.

For a supposed double helical diameter of 2 to 2.2 nm, delta should be 28 -
26 degrees respectively, far removed from the actual value of 40.

Interestingly, the helical diameter of 1.3 nm, found from Stokes equation
applied to Franklin's photograph, coincides with the STM value of 1.3 nm
measured by Lee et al. (Puzzle 1, ref. 1) and recorded in their Figure 3(D).

2        Molecular Configuration in Sodium Thymonucleate; R.E. Franklin &
R.G. Gosling; Nature Vol 171 (1953) 740 - 741

3        The Theory of X-ray Fibre Diagrams; A.R. Stokes; Prog. Biophys. &
Biophys. Chem. Vol 5 (1955) 140 - 167
Clive Delmonte

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