W0146

Using Centric and Symmetry-related Reflections to Evaluate Anomalous Scattering Signal/Noise Ratio. Zheng-Qing Fu, John P. Rose, Bi-Cheng Wang, Southeast Collaboratory for Structural Genomics, Dept. of Biochemistry and Molecular Biology, The Univ. of Georgia, Athens, GA 30602, USA.

Monitoring the signal/noise level in the data set has becoming an important step in our recent efforts of using single-wavelength anomalous scattering data for structure determination. A statistical index Ras is proposed to quantitatively evaluate the signal to noise ratio in the anomalous scattering data by using the statistics from centric and/or symmetry-related reflections as reference for estimating the noise level. This concept is based on the fact that theoretically the Bijvoet difference of centric reflections and difference between symmetry-related reflections should be zero, if there is no error in the data. Thus, these classes of reflections can be used to evaluate the noise level in the data and as references for calculating the signal/noise level in the data set either during the data processing and/or in merging various data sets.

We have found that Ras is more objective and accurate than Rsym, <|ΔI|/σI > and <I/σI > that are commonly used to evaluate anomalous signal and data quality of diffraction experiments of crystals. The applications of Ras and our test results will be presented.

Work is supported in part with funds from the National Institute of Health (GM62407), The Georgia Research Alliance, and The University of Georgia Research Foundation.