s-c-i-guy asked: In Rosalind Franklin's crystallography experiment, how did the DNA strand not get destroyed by the high energy X-rays used?
I worked with crystallographers for like seven years, and I’m still convinced it’s 95% magic.
In x-ray crystallography experiments, you don’t shine the x-ray beam on one single DNA strand, or protein, or whatever it is you’re looking at. One DNA strand would certainly be obliterated by the beam, but not necessarily the crystal (although, often, the x-ray does annihilate your sample. That’s why you need more than one, which makes it even harder).
Instead of a single molecule, it’s a solid crystal, precipitated out of a complex solution of sometimes more than a dozen chemicals, with each crystal made of bajillions and bajillions of individual molecules arranged in an organized lattice. Different crystal structures, whether they are cubes, or tetrahedrons, or hexagonal pyramidal pentaglobs, will act differently in the beam. You don’t know in advance what you’re gonna get.
The protein crystals we used to look at back in my Ph.D. lab looked a lot like this:
Actually, if we’re being honest, they usually didn’t look like that. This is the sort of crystal you dream of. Most things aren’t quite this tidy when they crystallize.
When the x-ray beam is directed at the crystal, it diffracts (bounces off of) any atoms in its way. But x-rays have super-short wavelengths, and molecules are mostly empty space, so only a small fraction of the x-ray waves encounter an atom to bounce off of.
It’s the sum of ALL the rare bouncing events, in the entire crystal, organized into its repeating, ordered structure, that creates the x-ray dot pattern. Then the real fun begins, which as any x-ray crystallographer (and I am not one) will tell you, involves lots of math, and a fair bit of magic.
Oh, man. I spent some time as an undergrad trying to crystallize rhodopsin (before I discovered that people had been trying unsuccessfully for decades) and doing SEM measurements. What a nightmarish slog. I would have dreamed of a crystal like that.