Use of ¹⁹F NMR spectroscopy to probe enzymatic oxidation of fluorine-tagged sulfides

A novel ¹⁹F NMR-based method for monitoring the enzymatic oxidation of thia fatty acid analogues is presented. Our approach is based on the observation that methyl ω-monofluorinated 9-thia- and 10-thiaoctadecanoates and their S-oxide and S-dioxide derivatives are easily distinguishable via their ¹H-decoupled ¹⁹F spectra. These long-range substituent effects were used to probe the regio- and chemoselectivity of stearoyl ACP (acyl carrier protein) Δ⁹ desaturase-mediated sulfoxidation. The results clearly demonstrate that mono-oxygenation of a 10-thia analogue ACP Δ⁹ desaturase was more efficient than that of a 9-thia substrate. A product previously undetected by TLC was observed for the first time in the product mixture obtained from 18-fluoro-9-thiaoctadecanoyl-ACP.