Raman scattering investigation of selenomethionine replacement in protein SOUL crystals

The vibrational properties of both wild-type and selenomethionine substituted protein SOUL crystals have been here investigated by Raman spectroscopy. Several Raman peaks, observed in the spectra of methionine and selenomethionine were identified as specific markers. The unambiguous assignment of these peaks has been inferred by comparing the experimental Raman spectra of the pure amino acids, recorded in solid state and in aqueous solution, and the Raman intensities computed using quantum chemical calculations. Moreover, a quantitative evaluation of the relative amount of selenomethionine replacement in the crystals of protein SOUL labelled with selenomethio-nine has been estimated through the ratio between the Raman intensities of marker peaks. These results offer evidence of the potential of Raman microscopy as a reliable and non-invasive tool for novel in-depth structural investigations in biocrystallography.