Question

Explain the difference between the asymmetric unit and the unit cell of a protein

crystal. (b) Explain the phase problem and one approach to its solution.

Answer 1

a) Assymetric unit: is the smallest portion of a crystal that when duplicated and moved by crystal symmetry operations, can produce the unit cell of the crystal.

Unit cell: is the smallest portion of a crystal that when duplicated and translated can give rise to the entire crystal.

Assymetric unit hence has all the structural information required to produce a unit cell.

b) In a X-ray diffraction experiment, we can measure the intensities and diffraction angles of the diffracted beams. But the relative phases are lost in the process during these experiments. The information regarding this lost phase and amplitude of the diffracted X-rays is essential for solution of crystal structures. This is reffered as phase problem. One of the approaches to its solution is, Molecular Replacement (MR) method. If a related or homologous protein structure is already known, then this method can be used. Find the rotation and translation which position the model structure in the unit cell so as to give the highest correlation between experimental diffraction experiments and those calculated from the model.