Tuesday, June 14, 2011

Crystallography in a very small nutshell

This blog is for all my horsey friends, and anyone else who would like to know what I do, with a minimum of large, gangly sciency words. I have used some convenient analogies and simplifications-The more precise description is in my dissertation- feel free to look that up… I give this explanation at almost any party/social gathering that involves non-science people. I think it works pretty well, but I am always open to constructive criticism.
A chemist makes a new compound*. They don’t know what the molecule looks like, i.e., they don’t know which atoms are bonded with which. They would like to know the 3-dimensional structure of their new baby, so they grow a crystal of it (Crystal growth is a complete whole ‘nother blog, sorry!) They then lovingly send their crystal to me.
I then blast their beloved crystal with 10^10 photons per second of X-ray radiation…I mean, I bathe their crystal in X-rays.  X-rays interact with electrons in a crystal in  predictable ways. The X-rays enter the crystal, run into an electron, and collide elastically, like a billiard ball.  The experiment I do consists of 1) shoot crystal, 2) measure where the X-rays bounced off to, and how intense they are, and 3) rotate crystal slightly, and repeat 3600 times or until crystal dies. Once we know where the X-ray bounce to(also known as the diffraction pattern), and how intense they are, we can do some math** and then do some more math*** which leads to a map of the electron density. Using chemical knowledge, I look at the map and figure out which atoms are where, and what the molecule looks like. I then provide the chemist with what they really wanted the most…

…A picture for their journal article J

*A compound is a generic term for a combination of atoms.  A molecule is a group of atoms bonded together. A bond is formed when atoms share their electrons. Whew!
**Direct methods, which solves the crystallographic “Phase Problem” –which would require many science terms and another blog
***An inverse Fourier Transform, which relates the  diffraction to the electron density

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