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Unformatted text preview: the helix are separated by a regular
vertical distance P = 3.4 nm (as they are in DNA), and you scatter from these
X-rays with a wavelength of 0.15 nm (as Franklin did), at what angle away
from the undeflected beam will you find the first of the layer lines?
sinmax = n/d, which for this implies max = 2.5
b: (2 points) Explain how the DNA diffraction pattern would change if we squashed the
molecule along its axis, shortening the distance P while keeping the width of the
molecule the same.
If we squashed the molecule, shortening P while keeping the width the same, we
would stretch the whole pattern upward, increasing the layer line spacing, the
spacing of the outer diamond, and tilting the central X upward. The missing
fourth layer line would remain.
c: (3 points) The missing 4th layer line comes about because of the two helices offset by
3P/8. Which layer line would be missing if the separation between the two helices was
P/4 instead? Explain your answer.
The missing 4th layer line is caused by destructive interference between the two
intertwined helices. Each produces a maximum where sin = n/P, but at the
fourth maximum the path length difference between the two helices is 3/2; they
are perfectly out of phase. As a result, though each is sending light out to that
angle, they perfectly cancel one another.
d: (3 points) How would this pattern differ if DNA was just a single helix, st...
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- Spring '08