Enzyme catalysed synthesis of L‐[4‐11C]aspartate and L‐[5‐11C]glutamate

The nitriles β‐[11C]cyano‐L‐alanine 1 and γ‐[11C]cyano‐α‐amino‐L‐butyric acid 2 were obtained by enzymatic catalysis from O‐acetyl‐L‐serine and O‐acetyl‐L‐homoserine with carrier added hydrogen [11C]cyanide (0.1 µmol), using O‐acetyl‐L‐serine sulfhydrylase [EC.4.2.99.8] and O‐acetyl‐L‐homoserine sulfhydrylase [EC.4.2.99.10], respectively. L‐[4‐11C]Aspartate 3 and L‐[5‐11C]glutamate 4 were then obtained by alkaline hydrolysis of 1 and 2, respectively. Purification by semi‐preparative HPLC yielded 3 and 4 with radiochemical purities higher than 95% and with decay corrected radiochemical yields of 60–70% and 50–60%, respectively based on the total amount of collected hydrogen [11C]cyanide. The enantiomeric purities were determined, after derivatisation with N‐α‐(2,4‐dinitrophenyl‐5‐fluoro‐)‐L‐alaninamide, by HPLC to be higher than 97.5%. The total reaction times for 3 and 4, counted from end of bombardment, were 50–55 min. The specific radioactivity of 3 and 4 were approximately 30 GBq/µmol at end of synthesis starting from 20 GBq hydrogen [11C]cyanide.