The in crystallo optical spectroscopy toolbox

Over the past ten years, there has been a surge in the demand for in crystallo optical spectroscopy ( ic OS), since optical spectroscopy is one of the few biophysical characterization methods applicable to both protein solutions and crystals. Historically, ic OS has been used to compare the state of proteins in crystals and in solution, and to assess their functionality by determining the redox state of metal ions, cofactors or chromophores. The recent rejuvenation of time‐resolved crystallography experiments has sparked a renewed interest in optical spectroscopy as a bridge between kinetic studies in solution and in the crystalline state. The method of ic OS can be defined as the ensemble of spectroscopic techniques in the UV–visible–infrared range that can be applied to crystals. It has also been instrumental in understanding specific X‐ray radiation damage to redox‐sensitive parts of proteins. Spectra recorded from crystals are affected by crystal orientation, shape or position due to various optical phenomena. Fortunately, these can be modelled and their effect can be corrected. The ic OS laboratory at the European Synchrotron Radiation Facility (ESRF) specializes in recording UV–Vis absorption, fluorescence emission and Raman spectra from protein crystals. Here, we present a suite of utilities that streamline the analysis and correction of UV–Vis absorption ic OS data, encased in a graphical interface. This was originally developed for the ic OS laboratory at ESRF but is available as a standalone package, with the aim of making ic OS more accessible.