A protocol for the synthesis of [35S]-labeled 3-dimethylsulfoniopropionate and dimethylsulfide from L-methionine for use in biogeochemical studies

Radiotracers are highly sensitive tools for quantifying the rates of important biogeochemical processes and the fates of specific atoms and/or compounds within major global elemental cycles, especially those that are requisite for life. Important radiolabeled organosulfur compounds, like dimethylsulfide (DMS) and its precursor 3-dimethylsulfoniopropionate (DMSP), are not commercially available, but their well-documented use has been key in furthering our understanding of the marine sulfur cycle. [³⁵S]-DMSP obtained by chemical synthesis has been used extensively in radiotracer studies involving DMS and DMSP, but its synthesis has been restricted to 2 research groups. Presented here is a protocol for the chemical synthesis of [³⁵S]-DMSP from [³⁵S]-L-methionine, though the method could be used for other radiolabels (e.g. [¹⁴C], [³H]). The synthesis consists of 2 reaction steps, (1) the sequential oxidative deamination and decarboxylation of [³⁵S]-L-methionine to [³⁵S]-3-methylmercaptopropionate and (2) the methylation of [³⁵S]-methylmercaptopropionate to yield the product [³⁵S]-DMSP. The product is purified by liquid chromatography and two cation-resin exchanges. Average final [³⁵S]-DMSP yield was 5.34% (n = 16; range: 1.26% to 14.84%, excluding failures), although updated instrumentation could likely improve final yields. The objective of this work is to standardize the synthesis of [³⁵S]-DMSP to widen its availability and use among the community and hence facilitate increased understanding of the reduced sulfur and carbon cycles.