Monitoring cyclic acyloxonium ion formation in palmitin systems using infrared spectroscopy and isotope labelling technique

To confirm the possible involvement of acyloxonium ions as reactive intermediates in food related lipids, their formation was monitored in triplamitin, 1,2 dipalmitin, 1-monopalmitin and specifically labelled tripalmitin (1,1,1-(13)C(3)) using Fourier-transform IR spectroscopy (FTIR). Reactions were conducted at 100 degrees C using a mixture of ZnCl(2) and the lipids. When tripalmitin or 1,2-dipalmitin samples were heated at 100 degrees C in the presence of ZnCl(2) a new band centred at 1651 cm(-1) was formed and increased over time. When (13)C-labelled tripalmitin (1,1,1-(13)C(3)) was studied the spectrum exhibited an expected 40 cm(-1) shift from 1651 to 1611cm(-1) indicating the involvement of the carbonyl carbon in the formation of the band. The 1-monopalmitin generated a similar but weaker band at higher temperatures and requiring longer times. These observations may indicate that under hydrophobic environment acyloxonium ions are preferentially formed with neighbouring ester groups assisted by the catalytic action of a free hydroxyl group serving as a proton transfer site. In the absence of such a free hydroxyl group tripalmitin undergoes acyloxonium ion formation at a slower rate than 1,2-dipalmitoyl glycerol, whereas, 1-monopalmitoyl glycerol due to the absence of a neighbouring ester shows even slower transformation efficiencies. This order of reactivity may however change in the presence of water.