Post-translational modifications (PTMs) of proteins are widespread in human organisms and could be the reason for various civilization diseases. Linking the chemical species to the backbone or sidechains of amino acids residues can significantly change the function and dynamic of proteins, and in consequence, lead to health abnormalities. For instance, cardiovascular and neurodegenerative diseases are associated with acetylation, methylation, and glycosylation, and cancers are related to phosphorylation.[1] The study of proteins' PTMs still arouses the interest of researchers looking for new strategies to treat common civilization diseases.
Chiroptical spectroscopy techniques are promising tools to investigate the proteins' PTMs, however, they are rarely used for that aim. Their potential is mainly due to the high sensitivity to even small structural changes in chiral molecules and supramolecular systems (e.g., carotenoid aggregates, protein fibrils), or biological samples (blood serum and plasma). Raman optical activity (ROA), vibrational circular dichroism (VCD), electronic circular dichroism (ECD), and circularly polarized luminescence (CPL) are the most used chiroptical spectroscopies based on different interactions (Raman scattering, absorption, emission) of chiral molecules with the left- and right circularly polarized light. [2]
ROA-CPL spectroscopy is a method that enables the observation of the CPL signal on the ROA spectrum. It uses a lanthanide-based probe due to its unique luminescent properties and abundance of narrow emission bands. The most suitable compounds for that purpose contain europium(III) ion which gives luminescent bands in the ROA/Raman spectrum range (0-2500 cm-1 which equals 532-610 nm). Additionally, the chiral environment of an achiral Eu(III) probe can induce its chirality which manifests as CPL bands of Eu(III) present on the ROA spectrum. Due to the high power of the excitation laser line used in the ROA spectrometer (532 nm), which is matched to the energy of some electronic transitions of Eu(III), the luminescent bands of europium(III) that appeared in the ROA spectrum are strongly enhanced. This approach allows to register the CPL signals which are hard or impossible to measure using a commercial CPL spectrometer. [3]
In this study, we present the first application of ROA-CPL spectroscopy as an excellent tool to distinguish PTMs of glutathione (GSH). The results unambiguously show different CPL patterns for unmodified peptide and for both modification models, even with their small concentrations. It let us expect that ROA-CPL spectroscopy in the use of the Eu(III) probe will be equally effective for the study of more complex protein systems.