A novel method for detection of selenomethionine incorporation in protein crystals via Raman microscopy

Multiwavelength anomalous dispersion (MAD) is the most widespread approach in structural biology for the determination of the crystal structure of a novel protein. Mass spectrometry is currently used to evaluate the selenomethionine (SeMet) content in solution, but excluding fluorescence spectroscopy at the absorption edge, no other routine method to check the SeMet incorporation and storage in the crystal state is yet available. Raman microscopy is an increasingly popular tool in physical biochemistry, with applications ranging from studies of ligand binding to secondary-structure analysis. Here, a novel methodological development is presented for the analysis via Raman microscopy of SeMet-labelled protein crystals to be used for MAD crystallography. The method is described and supported by validation and application to two novel proteins (a βγ-crystallin-like protein and a DNA-binding protein). Markers of the SeMet residues are in the range 570–600 cm⁻¹, where proteins do not usually show Raman bands.