Pyridine Alkaloids in the Venom of Imported Fire Ants

Venomous imported fire ants cause significant medical problems. Alkaloids are an important component of imported fire ant venom. Piperidine and piperideine alkaloids have been identified in fire ant venom. In this study, we studied the venom alkaloids of the red imported fire ant, Solenopsis invicta Buren, the black imported fire ant, Solenopsis richteri Forel, and the hybrid, S. invicta × S. richteri. Pyridine alkaloids were detected the first time in fire ants using solid-phase microextraction (SPME) coupled with gas chromatography-mass spectrometry (SPME-GC-MS). The thermal desorption process was manipulated to facilitate the isolation and identification of pyridine alkaloids that were coeluted with piperidine or piperideine alkaloids in GC. After SPME extraction of ant venom, we conducted a series of consecutive GC-MS injections, each with a partial desorption. Hidden pyridine alkaloid peaks were revealed after the overlapping piperidine or piperidiene alkaloid peaks had been desorbed. Using this approach, 10 2-methyl-6-alkyl (or alkenyl)pyridines (1-10) were found the first time in the venom of imported fire ants. Structures of three pyridine alkaloids were confirmed by synthesis, including 2-methyl-6-undecylpyridine (1), 2-methyl-6-tridecylpyridine (7), and 2-methyl-6-pentadecylpyridine (10). We also developed a silica gel column chromatography method to separate the pyridine alkaloids from other alkaloids. Using column chromatography and GC-MS with single ion monitoring at 107 m/z, five pyridine alkaloids were quantified for both workers and female alates of S. invicta and S. richteri.