MAD phasing - part 6

Expected signal from a MAD experiment

The signal expected from a MAD experiment can be calculated using an approximation similar to that derived by Crick and Magdoff [1956] for an MIR experiment. The equations used are:

perturbation due to = (NA/2NT)1/2(2A/Zeff) perturbation due to = (NA/2NT)1/2(|Ai - Aj|)/Zeff

where

N_A = number of anomalous scatterers, with anomalous scattering factors f' and f"

N_T = total number of atoms in structure

Z_eff = effective normal scattering power for all atoms (6.7e for protein atoms at 2 = 0)

Since the perturbations due to and are orthogonal, we may take the net expected signal to be the root-mean-square of these two quantities.

A remarkable property of anomalous scattering is that it does not fall off with sin()/ . This means that the contribution of anomalous scattering to the total measured intensity actually increases at higher resolution. On the other hand, your data quality normally falls off at higher resolution, so you may or may not be able to take advantage of this behaviour.

To show how this works, here is an interactive form to calculate the approximate signal expected from a MAD experiment. For a more detailed discussion of the signal available from a MAD experiment, see Hendrickson & Ogata (1997) or Bella & Rossmann (1997)

back to index

next topic