Mapping the effects of specific radiation damage and solvent radiolysis in buffers and crystals with online UV-Vis absorption spectroscopy.

Online UV-Vis absorption spectroscopy is often used during crystallographic data collection to characterize chromophores within proteins. This includes the study of electron-rich or redox-active enzyme intermediates and cofactors that are prone to specific radiation damage, necessitating suitable low-dose data-collection strategies. In crystallo spectroscopy enables the monitoring of X-ray-induced changes in the spectroscopic signatures of proteins and ligands; however, a comprehensive approach that also considers buffer components is required. Here, we present a mapping of X-ray-induced spectral changes in common crystallization chemicals and mixtures at cryogenic temperature, for which spectroscopic changes can be detected at doses as low as 1 kGy. A transient increase in absorption between 450 and 700 nm is frequently observed, arising from solvated electron absorption. Below 450 nm, several distinct absorption peaks were detected, for example for halides in buffers. In addition, Rayleigh scattering can lead to an increasing loss of photons from the optical path as the wavelength decreases, and thus to a significant elevation of the baseline at shorter wavelengths. In this study, we demonstrate the use of spectroscopic analyses to investigate the chromophoric enzyme intermediate I320 in vitamin B6 biosynthesis. In this case, X-ray-induced spectral changes were attributed to crystallization agents, cryoprotectants and light scattering, thereby excluding intrinsic alterations to the enzyme intermediate under low-dose conditions. Our study highlights the importance of monitoring spectral changes during diffraction data collection to ensure accurate interpretation of the electronic structure of chromophores.