Direct demodulation method for heavy-atom positions determination in protein crystallography
- Received Date: 2012-03-09
- Accepted Date: 2012-05-07
- Available Online: 2013-01-05
Abstract: The first step of phasing in any de novo protein structure determination using isomorphous replacement (IR) or anomalous scattering (AD) experiment is to find heavy-atom positions. Traditionally, heavy-atom positions can be solved by inspecting the difference Patterson maps. Due to the weak signals in isomorphous or anomalous difference and the noisy background in the Patterson map, the searching of heavy atoms may become difficult. Here, the direct demodulation (DD) method is applied to the difference Patterson maps to reduce the noisy backgrounds and sharpen the signal peaks. The real space Patterson search by using these optimized maps can locate the heavy-atom positions more correctly and accurately. It is anticipated that the direct demodulation method can play an assistant role in the heavy-atom positions determination and facilitate the de novo structure determination of proteins.