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	MR Tutorial Bath - Aim and strategy

1. Aim and strategy

The aim of the X-ray crystallographic experiment:

Locate all the atoms in the unit cell of the crystal.
Over-simplification:

Atoms are not points - information on their spatial/temporal distributions is also required. X-rays are scattered by electrons, atoms vibrate, and crystalline order is never perfect.
Equivalent to the following 2-step thought-experiment:
1. Locate all the atoms in the molecule.
2. Locate all the molecules in the unit cell.
Heavy-atom IR method furnishes the electron density for the molecule; then assignment of density to atoms and atoms to molecules requires input of intelligence. This whole process is very time consuming.
If we have 2 or more different crystals of the same molecule, we need only do step 1 once, but repeat step 2 for each crystal.
Molecules need not be identical; need only have significant features in common, e.g. most (> ~50%) of protein main-chain atoms + fraction (> ~20%) of sidechains.
Atomic model need not be crystal structure: e.g. from NMR or homology modelling.