-
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:
Locate all the atoms in the molecule.
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.