Electron Impact and Chemical Ionization Fragmentation of 5-Methoxytryptamine and Some 6-Methoxy-beta-carbolines.

Abstract

Electron impact (EI) and chemical ionization (CI) mass spectra of 5-methoxytryptamine (5-MT) and seven 6-methoxy-.beta.-carbolines have been recorded and interpreted. Proposed fragmentation pathways are depicted on the basis of spectra of deuterium labelled analogs of 5-MT, 6-methoxy-1,2,3,4-tetrahydro-.beta.-carboline (6-MeO-THBC) and 6-methoxy-1-methyl-1,2,3,4-tetrahydro-.beta.-carboline (6-MeO-1-Me-THBC). In the EI mass spectrum of 5-MT, the base peak m/z 160 is formed by the expulsion of CH4N. The positive ions so formed is suggested to have a quinolinium structure. In chemical ionization with CH4 as the reaction gas, the most abundant ion is produced by cleavage of NH3. The ion m/z 173 yields the base peak in the EI mass spectrum of 6-MeO-THBC. This ion is formed by a retro Diels-Alder reaction, an important fragmentation pathway of 6-MeO-1-Me-THBC, too. 6-MeO-harmalan easily loses one hydrogen atom in EI fragmentation followed by methyl loss from the methoxyl group, this latter process yielding the base peak, m/z 197 in the EI mass spectrum of 6-MeO-harman. M + 1 is the base peak in the CI mass spectra of all 6-MeO-.beta.-carbolines.