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 Unit cell is the smallest unit of volume that contains all of the structural and symmetry information and that by translation can reproduce a pattern in all of space.

Structural information - the pattern (atoms) plus all surrounding space.

Symmetric information - mirrors , glides, axes and inversion centre.

Translation- Motion along a cell edge the length of the cell edge.

An Asymmetric unit is the smallest unit of valume that contains all of the structural information and that by application of the symmetry operations can reproduce the unit cell.

PHASE PROBLEM

when waves are diffracted from a crystal, they give rise to diffraction spots. Each diffraction spot corresponds to a point in the reciprocal lattice and represents a wave with an amplitude and a relative phase.

But what happens is that photons are reflected from the crystal in different directions with a probability proportional to the square of the amplitude of this wave. We count the photons, and we lose any information about the relative phases of different diffraction.

How to solve the phase the problem.

By this method we can solve.

-Guess the sturcture

- Paterson function

-Molecular replacement

-Direct method

- Anamalous dispersion