Does glucose enhance the formation of nitrogen containing polycyclic aromatic compounds and polycyclic aromatic hydrocarbons in the pyrolysis of proline?

The gas-phase pyrolysis of proline, glucose, 1-[(2′-carboxy)pyrrolidinyl]-1-deoxy-d-fructose (the proline Amadori compound), and a 1:1 mixture by weight of proline and glucose was investigated at high temperatures (600–840 °C) and short residence time (i.e. 1.0 s) in an inert atmosphere to determine if glucose or Maillard reaction products enhance the formation of nitrogen containing polycyclic aromatic compounds (N-PACs) and polycyclic aromatic hydrocarbons (PAHs) in the pyrolysis of proline. To study the gas-phase formation of N-PACs and PAHs, the substrates were sublimed into the pyrolysis furnace at 460 °C. Thermogravimetric analysis showed that glucose, the proline/glucose mixture, and the proline Amadori compound undergo solid-state decomposition reactions before subliming. Thus, the substrates were pyrolyzed in two stages: at 460 °C during the sublimation and at 600–840 °C. At 800 °C with a residence time of 1.0 s, proline produced low yields of N-PACs, such as quinoline, isoquinoline, indole, acridine, and carbazole, and PAHs, such as phenanthrene, pyrene, benz[a]anthracene, benzofluoranthene isomers, and benzo[a]pyrene. Increasing the temperature and residence time increased the yield of these products. Under similar pyrolysis conditions, the proline Amadori compound produced 2–8 fold more N-PACs and PAHs than proline. A 1:1 mixture of proline and glucose produced a similar slate of pyrolysis products as the proline Amadori compound, but it is unclear whether the proline Amadori compound was an intermediate in the reaction. In general, the proline Amadori compound produced a higher yield of N-PACs and PAHs than the proline/glucose mixture, but glucose clearly enhances the low temperature gas-phase formation of N-PACs and PAHs from the pyrolysis of proline. For example, a 1:1 mixture of proline and glucose was found to produce low yields of quinoline, isoquinoline, and indole at 600 °C while proline alone does not produce these compounds until 800 °C. Thus, glucose provides a low temperature pathway for the decomposition of proline, which leads to formation of N-PACs and PAHs.