A new method for predetermining the diffraction quality of protein crystals: using SOAP as a selection tool

A microscope for quantitative analysis of the birefringence properties of samples is introduced. The microscope is used to measure variations in the slow optical axis position (SOAP) across hen egg‐white lysozyme, glucose isomerase and fibronectin crystals. By comparing these variations with indicators of diffraction quality, it is shown that the optical properties of a protein crystal provide a non‐invasive method of determining crystal diffraction quality before any X‐ray data collection is attempted.