Structural Information Content at High Resolution: MAD versus Native

Abstract

This chapter discusses special requirements associated with experimental high-resolution phasing, together with its benefits. To obtain such high accuracy, several complete atomic resolution data sets must be collected at different wavelengths. The multiple anomalous dispersion (MAD) method exploits the resonance or anomalous scattering that occurs when the energy of the incident X-rays approaches that of an electronic transition in an atom. The MAD method has become prominent in solving the phase problem. It is similar to the multiple isomorphous replacement (MIR) method, the main difference being that the variations in the diffraction pattern come from the selective absorption of X-rays at given wavelengths and not from the chemical modifications of the protein. MAD phases suffer from measurement errors and the errors in the parameters describing the anomalous dispersing atoms. In the case of the model, errors should be expected in the less-ordered regions that generally are the zones with multiple conformations or high-temperature factors. In the more general case, the prevalence of the MAD method in the solution of the phase problem has a strong impact on the speed and accuracy of obtaining a final model. Because the MAD phases are free of model bias, the interpretation of the electron density maps is more straightforward and can be automated, leading to fast structure determination. In the high-resolution cases, models obtained from MAD maps should not be biased by a priori stereochemical information or subjective interpretations because the atomic positions are taken directly from the electron density map peaks. Therefore, MAD models are more reliable, more accurate, and faster to obtain.