A Facile Method for the Separation of Methionine Sulfoxide Diastereomers, Structural Assignment, and DFT Analysis

Abstract

Methionine (Met) oxidation is an important biological redox node, with hundreds if not thousands of protein targets. The process yields methionine oxide (MetO). It renders the sulfur chiral, producing two distinct, diastereomerically related products. Despite the biological significance of Met oxidation, a reliable protocol to separate the resultant MetO diastereomers is currently lacking. This hampers our ability to make peptides and proteins that contain stereochemically defined MetO to then study their structural and functional properties. We have developed a facile method that uses supercritical CO2 chromatography and allows obtaining both diastereomers in purities exceeding 99 %. 1H NMR spectra were correlated with X‐ray structural information. The stereochemical interconversion barrier at sulfur was calculated as 45.2 kcal mol−1, highlighting the remarkable stereochemical stability of MetO sulfur chirality. Our protocol should open the road to synthesis and study of a wide variety of stereochemically defined MetO‐containing proteins and peptides.

Investigating the Role of Antioxidant Compounds in Riboflavin-Mediated Photo-Oxidation of Methionine: A 1H-NMR Approach

“…The observed pattern of the γ-CH 2 multiplet corresponds to the overlap of two separate γ-CH 2 spin systems and, indeed, it is identical to the pattern described for a 1:1 mixture of the two diastereoisomers. 36 The assignment was finally confirmed by the addition of the standard compound. During light exposure, the relative intensities of the γ-CH 2 multiplets did not significantly change and were identical to those measured in the commercial compound.…”

Expanding the Tool Kit of Automated Flow Synthesis: Development of In-line Flash Chromatography Purification

“…38 We elected to study the 1 O 2 oxidation of Fmoc-L-methionine (3, Scheme 2A) to the corresponding sulfoxide 4 as the product has significance in peptide synthesis and biological systems. 39 Oxidation of electron-rich heteroatoms by 1 O 2 is well known and, in the case of sulfides, is highly selective for the formation of sulfoxide without overoxidation to the sulfone. 40 This is especially important for the selective oxidation of mustard gas, as the sulfoxide is considered nontoxic, whereas the sulfone is of comparable toxicity to the parent sulfur mustard.…”

“…This is notoriously difficult, and isolation at a significant scale has only been achieved by SCFC with a chiral stationary phase. 39 Our system enables the continuous production of the diastereomer mixture with high purity, which could simplify and facilitate the isolation of the diastereomers by subsequent SCFC purification. System 3: Heterogeneous Photocatalysis.…”

“…We elected to study the 1 O 2 oxidation of Fmoc- l -methionine ( 3 , Scheme A) to the corresponding sulfoxide 4 as the product has significance in peptide synthesis and biological systems . Oxidation of electron-rich heteroatoms by 1 O 2 is well known and, in the case of sulfides, is highly selective for the formation of sulfoxide without overoxidation to the sulfone .…”

“…NMR spectra were compared with literature to confirm identity. 39 5 was recovered separately (10 mg, 53%) as a crystalline yellow solid with identical LC chromatograms, NMR, and mass spectra to the original synthesis procedure (vide inf ra).…”