Iodinated melatonin: preparation and characterization of the molecular structure by mass and 1H NMR spectroscopy

Synthetic melatonin was iodinated by treatment with potassium iodide in the presence of an oxidizing agent, Iodo-Gen. The iodination products of melatonin were extracted with chloroform and separated by HPLC. The fraction showing immunoreactivity with respect to melatonin antisera was characterized as iodomelatonin by mass spectrometry, so that the substitution of iodine had occurred at a ring carbon atom. 1H NMR spectra showed the iodine to be incorporated at the C-2 position of the indole moiety. The N-[2-(2-iodo-5-methoxy-1H-indol-3-yl)ethyl]acetamide (2-iodomelatonin) reported here is more useful than [3H]melatonin as a tracer in melatonin radioimmunoassay. This method offers also the possibility of preparing iodinated serotonin and other indoleamines for biological studies.