X-ray-excited optical luminescence of protein crystals: a new tool for studying radiation damage during diffraction data collection

Robin L Owen,Briony A Yorke,Arwen R Pearson

doi:10.1107/s0907444912002946

Abstract

During X-ray irradiation protein crystals radiate energy in the form of small amounts of visible light. This is known as X-ray-excited optical luminescence (XEOL). The XEOL of several proteins and their constituent amino acids has been characterized using the microspectrophotometers at the Swiss Light Source and Diamond Light Source. XEOL arises primarily from aromatic amino acids, but the effects of local environment and quenching within a crystal mean that the XEOL spectrum of a crystal is not the simple sum of the spectra of its constituent parts. Upon repeated exposure to X-rays XEOL spectra decay non-uniformly, suggesting that XEOL is sensitive to site-specific radiation damage. However, rates of XEOL decay were found not to correlate to decays in diffracting power, making XEOL of limited use as a metric for radiation damage to protein crystals.

Highlights

Macromolecular crystallography (MX) is a powerful tool for understanding the structure and function of macromolecules
We identify the nature of the contributing amino acids and investigate how X-ray-excited optical luminescence (XEOL) spectra change with increasing absorbed dose
Spectra collected from individual crystalline amino acids suspended in DMSO showed XEOL to arise predominantly from aromatic amino acids (Fig. 1a)

Summary

Introduction

Macromolecular crystallography (MX) is a powerful tool for understanding the structure and function of macromolecules. Luminescence during irradiation is a phenomenon that is common to all molecules and that occurs during all X-ray data collections, in which crystals emit visible light during, and for a short time subsequent to, X-ray exposure. This is known as X-ray-excited optical luminescence (XEOL). McGeehan and coworkers reported the measurement of XEOL from a trypsin crystal using the online single-crystal spectrometer at ID14-2, ESRF (McGeehan et al, 2009) This signal bleached with increasing X-ray dose. We identify the nature of the contributing amino acids and investigate how XEOL spectra change with increasing absorbed dose

Sample preparation and data collection

Results and discussion

Conclusions