Overview

How do we know DNA is a double helix? We can’t see it with a microscope (it’s too small). We used Crystallography. We turn a molecule into a crystal (a repeating brick wall), shoot X-rays at it, and look at the shadow it casts. It is the most powerful tool for seeing the invisible.

Core Idea

The core idea is Diffraction. When waves (X-rays) hit a regular grid (atoms), they bounce off and interfere with each other. They create a pattern of bright spots. By doing math on the spots, we can work backward to figure out where the atoms must be.

Formal Definition

The scientific study of crystal structures and their properties. Bragg’s Law: $n\lambda = 2d\sin\theta$. The equation that relates the angle of the X-rays to the distance between atoms.

Intuition

Imagine a chandelier in a dark room. You can’t see the chandelier, but you shine a laser pointer at it. The crystals scatter the light into spots on the wall. By looking at the spots, you can guess the shape of the chandelier.

Examples

  • Photo 51: The most famous X-ray image in history. Rosalind Franklin took a picture of DNA. It showed an “X” shape. Watson and Crick saw it and realized: “It’s a helix!”
  • Salt vs. Sugar: Salt crystals are cubes. Sugar crystals are slanted prisms. This reflects the shape of their molecules.
  • Snowflakes: Always have 6 sides. Why? Because water molecules like to arrange themselves in hexagons when they freeze.

Common Misconceptions

  • Crystals are magic: New Age healing crystals are just rocks (Quartz). Crystallography is hard physics.
  • Everything is a crystal: Glass is an amorphous solid. Its atoms are jumbled (like a liquid frozen in time). It has no pattern, so it doesn’t diffract X-rays well.
  • Unit Cell: The smallest repeating box of atoms. If you stack unit cells, you get the whole crystal.
  • Symmetry: Crystals love symmetry. There are only 230 possible ways to pack things in 3D space (Space Groups).

Applications

  • Drug Design: We crystallize a virus protein, find its shape, and design a drug to plug its active site.
  • Metallurgy: Understanding why steel is hard (crystal grains) and how to make it harder (heat treatment).

Criticism / Limitations

  • The Bottleneck: You have to grow a crystal first. Many proteins (like membrane proteins) refuse to crystallize. They are floppy and messy. This is the hardest part of the science.

Further Reading

  • Rhodes, Gale. Crystallography Made Crystal Clear.
  • Maddox, Brenda. Rosalind Franklin: The Dark Lady of DNA.