Increased amounts of D-enantiomer dependent on alkaline concentration in the synthesis of L-[methyl-11C]methionine.

The presence of D-enantiomer in L-[methyl-11C]methionine prepared from [11C]CH3I and L-homocysteine thiolactone, was measured by high performance liquid chromatography using a reverse-phase column with an eluent containing L-proline and cupric acetate. The amount of D-enantiomer increased with concentration of NaOH used. The reaction time, 2-10 min, and the reaction temperature, 40 degrees-80 degrees C, have only minor effect on the formation of D-enantiomer. No significant difference was found for three different lots of L-homocysteine thiolactone. At the highest concentration investigated, 1.0 M NaOH in 50% aqueous acetone, the percentage of D-enantiomer was found to be 7.7%. With 0.025 M NaOH used only 2.1% was measured. When L-[methyl-11C]methionine was incubated in 1.0 M NaOH, no conversion of L- into D-enantiomer was observed.