Measurement of the intrinsic variability within protein crystals: implications for sample-evaluation and data-collection strategies

Abstract

The advent of micro-focused X-ray beams has led to the development of a number of advanced methods of sample evaluation and data collection. In particular, multiple-position data-collection and helical oscillation strategies are now becoming commonplace in order to alleviate the problems associated with radiation damage. However, intra-crystal and inter-crystal variation means that it is not always obvious on which crystals or on which region or regions of a crystal these protocols should be performed. For the automation of this process for large-scale screening, and to provide an indication of the best strategy for data collection, a metric of crystal variability could be useful. Here, measures of the intrinsic variability within protein crystals are presented and their implications for optimal data-collection strategies are discussed.

Keywords: automation; macromolecular crystallography; microfocus; sample evaluation.

Figure 1

Comparison of variability measures of crystals. Values of V ₁ and V ₂ are plotted against each other and coloured according to sample type. Lines show the values obtained for various ratios N between positions at increasing differences in diffraction power (see §2.4 for an explanation of the model). The red line representing the ratio N = 10 is a reasonable cutoff between variable and homogenous diffraction within crystals.

Figure 2

Values of V ₁ do not necessarily increase with crystal size. Plot of V ₁ values against the largest dimension of the crystal; the colour scheme is the same as in Fig. 1 ▶.

Figure 3

Values observed for users’ crystals in the period June to August 2013. Lines show the values obtained for various ratios N between positions at increasing differences in diffraction power.