Choosing your (Friedel) mates wisely: grouping data sets to improve anomalous signal.

Michele Cianci,Nicolas Foos,Max H Nanao

doi:10.1107/s205979831801570x

Abstract

Single-wavelength anomalous diffraction (SAD) phasing from multiple crystals can be especially challenging in samples with weak anomalous signals and/or strong non-isomorphism. Here, advantage is taken of the combinatorial diversity possible in such experiments to study the relationship between merging statistics and downstream metrics of phasing signals. It is furthermore shown that a genetic algorithm (GA) can be used to optimize the grouping of data sets to enhance weak anomalous signals based on these merging statistics.

Highlights

De rigeur before the advent of modern cryocooling methods, until recent years the assembly of a complete data set from multiple incomplete data sets had become less common
We have recently introduced an alternative approach in which a genetic algorithm (GA) is used to partition the pool of data sets into subgroups (Zander et al, 2016; Foos et al, 2018)
We show that the anomalous signal can be improved using the GA approach, that the anomalous correlation coefficient appears to be the best target for GA optimization and that this metric translates into improved downstream metrics such as anomalous difference-map peak heights, the ability to determine substructures and phasing success

Summary

Introduction

De rigeur before the advent of modern cryocooling methods, until recent years the assembly of a complete data set from multiple incomplete data sets had become less common. The use of multiple crystals allows much better data quality for a given X-ray dose because of the potential to use many hundreds or thousands of crystals. While this has previously been explored via microbeams and highprecision sample-manipulation devices (Cusack et al, 1998), the scale and the nature of sample delivery has expanded tremendously. One of the key challenges associated with the use of multiple crystals is non-isomorphism. This is relevant to anomalous phasing, especially in cases where the anomalous signal is very low. It is common for only two groups to exist, but the default

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