Biology Reference
In-Depth Information
203
Comparing this with the integral representation of the Bessel function, we
get that
A(h)
is proportional to (Franklin and Gosling, 1953):
The intensity on the various layer lines,
I(h)
, being the square of the
absolute value of
A(h)
, is thus proportional to It becomes
essential for us to calculate the values of for the various intensity spots
on the X-ray diffraction picture, since the peaks of the Bessel functions are
located at specific values of its arguments. Only then, we can understand
the statement made by Wilkins, Stokes and Wilson (1953).
As pointed out by Franklin and Gosling (1953), the intensity spot on the
equatorial layer line (Fig. 8-1) was due to the side-to-side packing of
helices. Again, using the Bragg's law of diffraction, the value of R for that
spot is roughly equal to 1/2r. Thus, has an approximate value of or
3.14. Among all the only one has a peak roughly at that value of
its argument around 3.14 is Width of the as suggested
by Wilkins, Stokes and Wilson (1953), is not a good measure to differentiate
them. See Table 8-1 and Fig. 8-2 for details.
While the original X-ray diffraction picture obtained by Franklin and
Gosling in 1953 was for sodium salt of DNA fiber at 92% relative humidity,
interestingly enough, Langridge and coworkers obtained another picture in
I960 for the lithium salt of DNA fiber at 66% relative humidity as shown in
Fig. 8-5.
We shall try to use the mathematics developed above to understand how the
DNA double helix can give rise to one of the experimentally obtained
diffraction pictures shown in Fig. 8-1 or 8-5. Then we need to give a
plausible explanation to the other experimental diffraction picture.
